U.S. patent application number 14/811065 was filed with the patent office on 2017-02-16 for grafted polymer surfaces for dropwise condensation, and associated methods of use and manufacture.
The applicant listed for this patent is Massachusetts Institute of Technology. Invention is credited to Karen K. Gleason, Andong Liu, Adam T. Paxson, Kripa K. Varanasi, Jose L. Yague.
Application Number | 20170043373 14/811065 |
Document ID | / |
Family ID | 50478538 |
Filed Date | 2017-02-16 |
United States Patent
Application |
20170043373 |
Kind Code |
A1 |
Paxson; Adam T. ; et
al. |
February 16, 2017 |
Grafted Polymer Surfaces for Dropwise Condensation, and Associated
Methods of Use and Manufacture
Abstract
Presented herein are articles and methods featuring substrates
with thin, uniform polymeric films grafted (e.g., covalently
bonded) thereupon. The resulting coating provides significant
reductions in thermal resistance, drop shedding size, and
degradation rate during dropwise condensation of steam compared to
existing coatings. Surfaces that promote dropwise shedding of
low-surface tension condensates, such as liquid hydrocarbons, are
also demonstrated herein.
Inventors: |
Paxson; Adam T.; (Cambridge,
MA) ; Yague; Jose L.; (Somerville, MA) ;
Varanasi; Kripa K.; (Lexington, MA) ; Gleason; Karen
K.; (Cambridge, MA) ; Liu; Andong; (Cambridge,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Massachusetts Institute of Technology |
Cambridge |
MA |
US |
|
|
Family ID: |
50478538 |
Appl. No.: |
14/811065 |
Filed: |
July 28, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14620661 |
Feb 12, 2015 |
9498934 |
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14811065 |
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14181586 |
Feb 14, 2014 |
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14620661 |
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61876195 |
Sep 10, 2013 |
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61874941 |
Sep 6, 2013 |
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61756679 |
Jan 25, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F28F 19/04 20130101;
B05D 5/083 20130101; B32B 7/12 20130101; C09K 5/00 20130101; F28F
21/06 20130101; Y10T 428/24355 20150115; Y10T 428/24851 20150115;
F28F 13/182 20130101; B32B 15/082 20130101; C09K 5/14 20130101;
F28F 13/04 20130101; F28F 2245/04 20130101; Y10T 428/265 20150115;
B32B 2307/302 20130101; B05D 3/0254 20130101; B05D 5/08 20130101;
B05D 3/02 20130101; Y10T 428/1352 20150115; B05D 1/60 20130101;
Y10T 428/3154 20150401 |
International
Class: |
B05D 5/08 20060101
B05D005/08; F28F 19/04 20060101 F28F019/04; C09K 5/14 20060101
C09K005/14; F28F 13/18 20060101 F28F013/18; B05D 1/00 20060101
B05D001/00; B05D 3/02 20060101 B05D003/02 |
Goverment Interests
GOVERNMENT SUPPORT
[0002] This invention was made with government support under
Contract No. W911NF-07-D-0004 awarded by the Army Research Office.
The government has certain rights in the invention.
Claims
1. A method of preparing a surface of a condenser, the method
comprising the step of performing hot wire chemical vapor
deposition (HWCVD) to graft a polymeric film on the surface of the
condenser, wherein the polymeric film has a thickness no greater
than 1500 nm and wherein the polymeric film has a surface with low
contact angle hysteresis of no greater than 50.degree. for
water.
2. The method of claim 1, wherein the step of performing HWCVD
comprises performing initiated chemical vapor deposition (iCVD) to
produce the polymeric film grafted on the surface of the
condenser.
3. The method of claim 1, further comprising the step of annealing
the polymeric film by exposure to heat (e.g., to increase
crosslinking density and/or degree of crystallinity of the
polymeric film).
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. Non-Provisional
application Ser. No. 14/620,661, filed Feb. 12, 2015, which is a
continuation of U.S. Non-Provisional patent application Ser. No.
14/181,586, filed on Feb. 14, 2014, which claims priority to and
the benefit of U.S. Provisional Patent Application No. 61/876,195,
filed Sep. 10, 2013, U.S. Provisional Patent Application No.
61/874,941, filed Sep. 6, 2013, and U.S. Provisional Patent
Application No. 61/765,679, filed Feb. 15, 2013.
FIELD OF THE INVENTION
[0003] This invention relates generally to grafted polymer surfaces
and their use for enhanced heat transfer, improved dropwise
condensation, and/or reduced adhesion of liquids and solids
thereto.
BACKGROUND OF THE INVENTION
[0004] Vapors condense upon a surface if the surface is cooled
below the saturation temperature at a given pressure. The condensed
liquid phase may accumulate on the surface as a film and/or as
droplets or islands of liquid. Condensation is critical in many
industrial applications, although in certain applications, it is
useful to inhibit or prevent the filmwise buildup of condensing
liquid on a surface by promoting droplet shedding and enhancing
dropwise condensation.
[0005] Condensation of water is a crucial process in many
industries, including power generation and desalination. Roughly
85% of the global installed base of electricity generation plants
and 50% of desalination plants worldwide rely on steam surface
condensers, a type of heat exchanger in which a plurality of tubes
flowing coolant contact steam on their outside surface. Given the
widespread scale if these processes, even slight improvements in
cycle efficiencies will have a significant effect on global energy
consumption.
[0006] One useful measure of heat transfer performance for a
condenser is the heat transfer coefficient, defined as the flux per
area in units of kW/m.sup.2K. Heat transfer coefficients
experienced when condensing in the dropwise mode are an order of
magnitude greater than those in the filmwise mode. The presence of
an insulating liquid film during filmwise condensation presents a
significant thermal barrier to heat transfer, whereas the departure
of discrete drops during dropwise condensation exposes the
condensing surface to vapor. The higher heat transfer coefficients
experienced during dropwise condensation make it attractive for
employing in large-scale thermal fluids applications such as steam
power plants and desalination plants, as well as small-area
high-heat flux applications such as electronics cooling. However,
the practical implementation of dropwise condensation in power
generation, desalination, and other applications has been a
significant materials challenge, limited by, among other factors,
durability of existing hydrophobic functionalization for metal heat
transfer surfaces. While metals provide both high thermal
conductivity for maximizing heat transfer and high tensile strength
to minimize the need for structural supports, metals are typically
wetted by water and most other thermal fluids, and, as a result,
metals exhibit filmwise condensation. In order for a metal surface
to exhibit desired dropwise condensation, the surface that is used
for heat transfer needs to be modified. One way to achieve dropwise
condensation on a metal surface where heat transfer takes place is
to modify the metal surface with a hydrophobic coating.
[0007] A number of conventional techniques have been employed
previously to promote dropwise condensation on surfaces, including
the use of monolayer promoters such as oleic acid and stearic acid
(U.S. Pat. No. 2,919,115), noble metals (U.S. Pat. No. 3,289,753
and U.S. Pat. No. 3,289,754 and U.S. Pat. No. 3,305,007),
ion-implanted metal (U.S. Pat. No. 6,428,863), as well as thin
films of polymers applied via sputtering or dip-coating (U.S. Pat.
No. 2,923,640, U.S. Pat. No. 3,899,366, EP2143818 Al, U.S. Pat. No.
3,466,189). However, previous methods suffer from problems such as
low durability and/or high cost. Moreover, most of these
hydrophobic modifiers, and especially the silane-based modifiers
that have been used in some conventional methods, are not robust in
steam environments of industrial interest (in other words, these
modifiers cannot withstand the environments in which they are
used). Previous methods also do not adequately promote rapid
droplet shedding because they do not sufficiently reduce the
contact angle hysteresis. It is possible to have a surface with a
high contact angle but also high adhesion, so even though
condensation would initiate in the dropwise regime, it would
ultimately progress to filmwise condensation because the drops are
not able to shed easily.
[0008] Furthermore, where the condensing liquids are hydrocarbons
or other low-surface tension liquids, the problem of film-wise
condensation is exacerbated. Current surfaces designed for dropwise
condensation of water do not promote dropwise condensation of
low-surface tension hydrocarbon liquids such as n-alkanes (e.g.,
octane, hexane, heptane, pentane, butane) and refrigerants (e.g.,
fluorocarbons, chlorofluorocarbons, hydrochlorofluorocarbons) and
cryogenic liquids (e.g., LNG, O2, N2, CO2, methane, propane).
[0009] Some conventional methods have used nanotextured surface to
improve condensation heat transfer, however, these methods also
rely on silane or thiol modifiers to modify the wettability of a
nanotextured surface from superhydrophilic to superhydrophobic, and
thus these nanotextured surfaces are subject to the same robustness
concerns discussed above. Additionally, because the thermal
conductivities of polymeric materials are typically orders of
magnitude smaller than those of a typical metal substrate, the
thickness of the polymer modifier is extremely important. Hence,
there is currently a need for an ultra-thin robust hydrophobic
modifier that may be applied over a metal surface to enhance heat
transfer.
[0010] There is a need for methods and articles/devices for
improved heat transfer and/or dropwise condensation of low-surface
tension liquids, including hydrocarbon liquids.
SUMMARY OF THE INVENTION
[0011] Presented herein are articles and methods featuring
substrates with thin, uniform polymeric films grafted thereupon.
Techniques such as iCVD allow deposition of precisely-controlled,
extremely thin polymeric films on metal substrates, where the
polymer is covalently bonded to the substrate. Furthermore, the
polymeric film may be crosslinked at or near its exposed surface
and/or throughout the bulk of the film via annealing. The resulting
coating exhibits significant reductions in thermal resistance, drop
shedding size, and degradation rate during dropwise condensation of
steam compared to existing coatings.
[0012] Articles and methods presented herein relate to the use of
ecofriendly monomers (e.g., 1H, 1H, 2H, 2H--perfluorooctyl acrylate
(C6)) for iCVD. C6 monomers undergo surface group reorganization,
which is undesirable. Articles and methods presented herein relate
to overcoming the surface group organization via crosslinking
and/or graded structure. In some embodiments, 1H, 1H, 2H,
2H--perfluorooctyl acrylate as well as C6 monomers with alternative
chemistries are deposited via iCVD as precisely-controlled,
extremely thin polymeric films on metal substrates, where the
polymer becomes covalently bonded to the substrate.
[0013] In some embodiments, the invention relates to an article for
enhanced heat transfer, and/or mitigating phase transition and
nucleation of undesired materials, and/or reducing adhesion of
liquids and solids thereupon, the article comprising a substrate
and a (e.g., thin, uniform) polymeric film grafted (e.g.,
covalently bonded) thereupon.
[0014] In some embodiments, the substrate comprises a metal (e.g.,
steel, stainless steel, titanium, nickel, copper, aluminum,
molybdenum, and/or alloys thereof). In some embodiments, the
substrate comprises a polymer (e.g., polyethylene,
polyvinylchloride, polymethylmethacrylate, polyvinylidene fluoride,
polyester, polyurethane, polyanhydride, polyorthoester,
polyacrylonitrile, polyphenazine, polyisoprene, synthetic rubber,
polytetrfluoroethylene, polyethylene terephthalate, acrylate
polymer, chlorinated rubber, fluoropolymer, polyamide resin, vinyl
resin, expanded polytetrafluoroethylene, low density polyethylene,
high density polyethylene, and/or polypropylene). In some
embodiments, the substrate comprises a semiconductor and/or ceramic
(e.g., SiC, Si, AlN, GaAs, GaN, ZnO, Ge, SiGe, BN, BAs, AlGaAs,
TiO.sub.2, TiN, etc.). In some embodiments, the substrate comprises
a rare earth element or compound comprising a rare earth element
(e.g., a rare earth oxide, carbide, nitride, fluoride, or boride;
e.g., cerium oxide CeO.sub.2).
[0015] In some embodiments, the polymeric film comprises a
fluoropolymer. In some embodiments, the polymeric film is formed
from at least one monomer species comprising one or more pendant
perfluorinated alkyl moieties. In some embodiments, the
fluoropolymer has at least one CF.sub.3 group. In some embodiments,
the fluoropolymer comprises polytetrafluoroethylene (PTFE). In some
embodiments, the fluoropolymer comprises
[C.sub.12H.sub.9F.sub.13O.sub.2].sub.n, where n is an integer
greater than zero.
[0016] In some embodiments, the fluoropolymer comprises a member
selected from the group consisting of
poly(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl methacrylate),
poly(1H,1H,2H,2H-perfluorooctyl acrylate),
poly([N-methyl-perfluorohexane-1-sulfonamide] ethyl acrylate),
poly([N-methyl-perfluorohexane-1-sulfonamide] ethyl (meth)
acrylate), poly(2-(Perfluoro-3-methylbutypethyl methacrylate)),
poly(2-[[[[2-(perfluorohexyl)ethyl]sulfonyl]methyl]-amino]ethyl]acrylate)-
,
poly(2-[[[[2-(perfluoroheptyl)ethyl]sulfonyl]methyl]-amino]ethyl]acrylat-
e),
poly(2-[[[[2-(perfluorooctyl)ethyl]sulfonyl]methyl]-amino]ethyl]acryla-
te), and any copolymer thereof.
[0017] In some embodiments, the fluoropolymer is a C6 analog of
PFDA. In some embodiments, the fluoropolymer comprises
poly(2-(Perfluoro-3-methylbutyl)ethyl methacrylate), or any
copolymer comprising 2-(Perfluoro-3-methylbutyl)ethyl methacrylate,
wherein the fluoropolymer is crosslinked.
[0018] In some embodiments, the polymeric film comprises at least
one member selected from the group consisting of
polytetrafluoroethylene (PTFE), poly(perfluorodecylacrylate)
(PFDA), polymethylmethacrylate (PMMA), polyglycidylmethacrylate
(PGMA), poly-2-hydroxyethylmethacrylate, poly(perfluorononyl
acrylate), poly(perfluorooctyl acrylate), and any copolymer
thereof. In some embodiments, the polymeric film comprises a
copolymer of two or more monomer species.
[0019] In some embodiments, the polymeric film comprises
cross-linked polymer and/or cross-linked copolymer. In some
embodiments, the polymeric film is cross-linked with a crosslinking
agent comprising an organic molecule having at least two vinyl
moieties. In some embodiments, the polymeric film is cross-linked
with a crosslinking agent comprising at least one member selected
from the group consisting of: diethyleneglycol divinyl ether,
diethyleneglycol dimethacrylate, diethyleneglycol diacrylate,
and/or 1H,1H,6H,6H-perfluorohexyldiacrylate. In some embodiments,
the polymeric film is cross-linked with divinyl benzene (DVB). In
some embodiments, the polymeric film is cross-linked with a member
selected from the group consisting of ethylene dimethyacrylate
(EDMA), di(ethyleneglycol)di(methacrylate),
di(ethyleneglycol)di(acrylate), ethyleneglycoldimethyacrylate
(EGDMA), di(ethyleneglycol)di(vinylether) (EDGDVE), and
1H,1H,6H,6H-perfluorohexyldiacrylate.
[0020] In some embodiments, the polymeric film comprises from 0 wt.
% to 99 wt. % crosslinking agent (e.g., from 5 wt. % to 90 wt. %;
from 15 wt. % to 85 wt. %; from 25 wt. % to 75 wt. %; from 35 wt. %
to 65 wt. %; or from 45 wt. % to 55 wt. %).
[0021] In some embodiments, the polymeric film has non-uniform
concentration of crosslinking agent along the thickness of the
film. In some embodiments, the polymeric film is covalently bonded
to the substrate. In some embodiments, the polymeric film is
covalently bonded to the substrate by attachment of a vinyl
precursor to the substrate, thereby forming a surface comprising a
plurality of pendant vinyl moieties. In some embodiments, the vinyl
precursor is a member selected from the group consisting of a vinyl
functional silane, a vinyl functional phosphonic acid, and a vinyl
functional thiol.
[0022] In some embodiments, the vinyl precursor comprises at least
one member selected from the group consisting of trichlorovinyl
silane, bis(triethoxysilylethyl)vinylmethyl-silane,
bis(triethoxysilyl)ethylene, bis(trimethoxysilylmethyl)ethylene,
1,3-[bis(3-triethoxysilylpropyl)poly-ethylenoxy]-2-methylenepropane,
bis[(3-trimethoxysilyl)propyl]-ethylenediamine,
bis[3-(triethoxysilyl)propyl]-di sulfide,
3-mercaptopropyltrimethoxysilane, and vinyl phosphonic acid.
[0023] In some embodiments, the polymeric film is no greater than
500 nm in thickness (e.g., no greater than 400 nm, no greater than
300 nm, no greater than 200 nm, no greater than 100 nm, no greater
than 75 nm, no greater than 50 nm, no greater than 25 nm, or no
greater than 15 nm, e.g., as thin as 10 nm). In some embodiments,
the polymeric film comprises a grafting layer (e.g., where the
polymeric film is covalently bonded to the substrate) and a bulk
film layer (e.g., where the grafting layer has a thickness from
about 0.5 nm to about 5 nm or from about 1 nm to about 3 nm, or
from about 1 nm to about 2 nm). In some embodiments, the polymer
film has a thickness variation of no greater than about 20% (e.g.,
no greater than about 15%, no greater than about 10%, or no greater
than about 5%--e.g., the polymer film is uniform).
[0024] In some embodiments, the polymer film has a texture
comprising micro- and/or nano-scale features (e.g., ridges,
grooves, pores, posts, bumps, and/or protrusions, patterned and/or
unpatterned). In some embodiments, the substrate is textured and
wherein the polymeric film conforms to the textured substrate
surface. In some embodiments, the substrate is textured with micro-
and/or nano-scale surface textures (e.g., posts, ridges, cavities,
pores, posts, protrusions, etc.). In some embodiments, the
polymeric film has a crystalline or semicrystalline surface (e.g.,
formed via annealing, but not necessarily via annealing).
[0025] In some embodiments, the polymeric film has a surface (e.g.,
exposed surface) with low contact angle hysteresis (e.g., no
greater than 50.degree., no greater than 40.degree., no greater
than 30.degree., no greater than 25.degree., no greater than
20.degree., no greater than 15.degree., or no greater than
10.degree., or no greater than 5.degree., or no greater than
1.degree. for water; and no greater than 20.degree., no greater
than 15.degree., no greater than 10.degree., no greater than
5.degree., or no greater than 1.degree. for hydrocarbons,
refrigerants, cryogenic liquids, and other low-surface tension
liquids, where contact angle hysteresis is the difference between
advancing contact angle and receding contact angle).
[0026] In some embodiments, the polymeric film has a surface (e.g.,
exposed surface) with high advancing contact angle (e.g., no less
than 70.degree., no less than 80.degree., no less than 90.degree.,
no less than 100.degree., no less than 120.degree., no less than
130.degree., no less than 140.degree. for water; and no less than
30.degree., no less than 40.degree., no less than 50.degree., no
less than 60.degree., no less than 70.degree., no less than
80.degree., no less than 90.degree., no less than 100.degree. for
hydrocarbons, refrigerants, cryogenic liquids, and other
low-surface tension liquids) and/or high receding contact angle
(e.g., no less than 60.degree., no less than 70.degree., no less
than 80.degree., no less than 90.degree., no less than 100.degree.,
no less than 110.degree., or no less than 120.degree. for water;
and no less than 20.degree., no less than 30.degree., no less than
40.degree., no less than 50.degree., no less than 60.degree., no
less than 70.degree., no less than 80.degree., no less than
90.degree. for hydrocarbons, refrigerants, cryogenic liquids, and
other low-surface tension liquids).
[0027] In some embodiments, the article is a condenser (e.g., where
dropwise condensation is promoted on the surface of the polymeric
film for enhanced heat transfer). In some embodiments, the article
is a cooling device for an electronic and/or photonic component
(e.g., where heat transfer is promoted from the electronic or
photonic component to the surface of the polymeric film, wherein
the polymeric film is in contact with the component, and/or wherein
the polymeric is in contact with a fluid that is in contact with
the component).
[0028] In some embodiments, the article is flexible. In some
embodiments, the substrate and the polymeric film grafted thereupon
is flexible. In some embodiments, the article is retrofitted to
form the grafted polymeric film.
[0029] In another aspect, the invention is directed to a method for
using the article described in any of the above embodiments,
wherein the method comprises contacting an exposed surface of the
polymeric film with a Thermal Interface Material (TIM) (e.g., a
thermally conductive material used between microprocessors and
heatsinks to increase thermal transfer efficiency).
[0030] In some embodiments, the polymeric film comprises a polymer
and/or copolymer, the polymer and/or copolymer comprising at least
one perfluorinated pendant chain (e.g., a perfluorinated acrylate
and/or a perfluorinated cyclic group, e.g., with 4 to 6 carbons in
the ring), a spacer group, and a vinyl-based backbone group.
[0031] In some embodiments, the method includes contacting an
exposed surface of the polymeric film with a Thermal Interface
Material (TIM) (e.g., a thermally conductive material used between
microprocessors and heatsinks to increase thermal transfer
efficiency).
[0032] In some embodiments, the invention is directed to a method
of preparing an article (e.g., the article described in any of the
above embodiments), the method including the step of performing hot
wire CVD (HWCVD) to produce the polymeric film grafted on the
substrate. In some embodiments, the step of performing HWCVD
comprises performing initiated chemical vapor deposition (iCVD) to
produce the polymeric film grafted on the substrate.
[0033] In some embodiments, the method further includes the step of
annealing the polymeric film by exposure to heat (e.g., to increase
crosslinking density and/or degree of crystallinity of the
polymeric film). In addition, in some embodiments, annealing can
reduce hysteresis, increase crystallinity at interface, and
increase crosslinking at the exposed interface.
[0034] In some embodiments, the HWCVD step is performed to retrofit
an existing article (e.g., a condenser, boiler or other heat
transfer surface in an HVAC device, a power plant, a desalination
plant, a natural gas liquefaction ship, etc.) by grafting the
polymeric film upon a surface thereof.
[0035] In some embodiments, the article is a Thermal Interface
Material (TIM).
[0036] In some embodiments, the polymeric film has an exposed
surface with critical surface energy no greater than 18 mN/m. In
some embodiments, the polymeric film has an exposed surface with
critical surface energy no greater than 6 mN/m.
[0037] In some embodiments, the polymeric film has an exposed
surface with contact angle hysteresis no greater than 25.degree.
for water, hydrocarbons, refrigerants, cryogenic liquids, and other
heat transfer fluids. In some embodiments, the exposed surface has
contact angle hysteresis no greater than 1.degree. or no greater
than 5.degree. for water, hydrocarbons, refrigerants, cryogenic
liquids, and other heat transfer fluids.
[0038] In some embodiments, the polymeric film has RMS roughness no
greater than 100 nm (e.g., no greater than 100 nm, no greater than
75 nm, no greater than 50 nm).
[0039] In some embodiments, the polymeric film provides dropwise
condensation and shedding of a hydrocarbon, refrigerant, cryogenic
liquid, water, or other low-surface tension liquid. In some
embodiments, the hydrocarbon liquid is a member selected from the
group consisting of alkanes, alkenes, alkynes, and fuel mixtures
(e.g., gasoline, kerosene, diesel, fuel oil); the refrigerant is a
member selected from the group consisting of chlorofluorocarbons,
hydrofluorocarbons, and hydrochlorofluorocarbons; and the cryogenic
liquid is selected from the group consisting of N.sub.2, O.sub.2,
CO.sub.2, He, LNG, methane, butane, propane, and isobutene. In
certain embodiments, the hydrocarbon liquid is a member selected
from the group consisting of hexane, toluene, isopropanol, ethanol,
octane, pentane, and perfluorohexane.
[0040] In some embodiments, the hydrocarbon liquid has surface
tension no greater than 30 mN/m (e.g., no greater than 28 mN/m, no
greater than 21 mN/m, no greater than 18 mN/m, no greater than 16
mN/m, or no greater than 12 mN/m, or no greater than 6 mN/m).
[0041] In some embodiments, the article is a component (e.g.,
vessel, pipe, fin, etc.) of a condenser that comes into contact
with a condensing liquid (e.g., working fluid). In some
embodiments, the article is a component of an oil and/or gas
processing apparatus (e.g. fractionation column, liquefaction
device). In some embodiments, the article is (or is a component of)
a power line, a turbine, an aircraft, a pipeline, a boiler, a
windshield, a solar panel, industrial machinery, cookware, a
consumer electronic device, a printed circuit board, an electronic
component, or a medical device.
[0042] Another aspect discussed herein relates to a method for
manufacturing a surface for promoting dropwise condensation and/or
shedding of a liquid, the method including the steps of: providing
a substrate; and controllably depositing a polymeric film on the
substrate via initiated chemical vapor deposition (iCVD).
[0043] In some embodiments, the method includes depositing a vinyl
precursor on the substrate prior to, or concurrently with,
depositing the polymeric film. In some embodiments, the method
includes modulating an average roughness of the deposited layer
(e.g., such that roughness is no greater than 100 nm, or no greater
than 75 nm, or no greater than 50 nm). In some embodiments, the
modulating includes monitoring a degree of crystallization of the
deposited polymeric film; or controlling the proportion of
crosslinker; or controlling a temperature of the substrate during
deposition; or any combination thereof.
[0044] In some embodiments, the deposited polymeric film has an
average thickness from 1 nm to 1 micron. In some embodiments, the
deposited polymeric film has an average thickness from 1 nm to 100
nm.
[0045] In some embodiments, the substrate comprises one or more
materials selected from the group consisting of a metal (e.g.,
copper, brass, stainless steel, aluminum, aluminum bronze, nickel,
iron, nickel iron aluminum bronze, titanium, scandium, and any
alloys thereof), polymer, glass, rubber, silicon, polycarbonate,
PVC, ceramic, semiconductor, and any combinations thereof. In some
embodiments, the substrate comprises one or more materials selected
from the group consisting of plastic, silicon, quartz, woven or
non-woven fabric, paper, ceramic, nylon, carbon, polyester,
polyurethane, polyanhydride, polyorthoester, polyacrylonitrile,
polyphenazine, polyisoprene, synthetic rubber,
polytetrfluoroethylene, polyethylene terephthalate, acrylate
polymer, chlorinated rubber, fluoropolymer, polyamide resin, vinyl
resin, expanded polytetrafluoroethylene, low density polyethylene,
high density polyethylene, and polypropylene.
[0046] In some embodiments, the polymeric film has an exposed
surface with critical surface energy no greater than 18 mN/m. In
some embodiments, the polymeric film has an exposed surface with
critical surface energy no greater than 6 mN/m.
[0047] In some embodiments, the polymeric film has an exposed
surface with contact angle hysteresis no greater than 25.degree..
In some embodiments, the exposed surface has contact angle
hysteresis no greater than 5.degree. for water, hydrocarbons,
refrigerants, cryogenic liquids, and other heat transfer fluids, or
any combination thereof.
[0048] In some embodiments, the polymeric film has roughness no
greater than 100 nm (e.g., no greater than 100 nm, no greater than
75 nm, no greater than 50 nm).
[0049] In some embodiments, the polymeric film provides dropwise
condensation and shedding of a hydrocarbon, refrigerant, cryogenic
liquid, water, other low-surface tension liquids, or any
combination thereof. In some embodiments, the hydrocarbon liquid is
a member selected from the group consisting of alkanes, alkenes,
alkynes, and fuel mixtures (e.g., gasoline, kerosene, diesel, fuel
oil); the refrigerant is a member selected from the group of
chlorofluorocarbons, hydrofluorocarbons, and
hydrochlorofluorocarbons; and the cryogenic liquid is selected from
the group consisting of N.sub.2, O.sub.2, CO.sub.2, LNG, methane,
propane, isobutene, and any combination thereof. In some
embodiments, the hydrocarbon liquid has surface tension no greater
than 30 mN/m (e.g., no greater than 28 mN/m, no greater than 21
mN/m, no greater than 18 mN/m, no greater than 16 mN/m, or no
greater than 12 mN/m, or no greater than 6 mN/m).
[0050] Another aspect discussed herein relates to a method of
manufacturing the polymeric film on the article of any of the
aspects or embodiments discussed above.
[0051] Elements of embodiments described with respect to a given
aspect of the invention may be used in various embodiments of
another aspect of the invention. For example, it is contemplated
that features of dependent claim depending from one independent
claim can be used in apparatus, articles, systems, and/or methods
of any of the other independent claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] The foregoing and other objects, aspects, features, and
advantages of the invention will become more apparent and may be
better understood by referring to the following description taken
in conjunction with the accompanying drawings, in which:
[0053] FIGS. 1a-1c illustrate iCVD reactor geometries and reaction
processes, in accordance with certain embodiments of the invention.
FIG. 1b illustrates a schematic of a lab-scale 200 mm diameter iCVD
reactor system. For a vinyl homopolymerization, a constant flow of
monomer and initiator are metered into the `pancake` style vacuum
reaction chamber. An array of resistively heated wires, suspended a
few centimeters above the substrate, heats the vapors. Laser
inteferometry provides real time monitoring of the iCVD polymer
thickness. The pressure of the chamber is controlled by a
throttling value. An unreacted species and volatile reaction
by-products are exhausted to a mechanical pump. For
copolymerization, an additional monomer feed line would need to be
added to the system. FIG. 1c shows a schematic cross-section of the
iCVD reactor showing decomposition of the initiator by the heated
filaments. Surface modification through polymerization of the
monomer occurs on the actively cooled substrate.
[0054] FIGS. 2a-2e illustrate comparison of water condensation on
p(PFDA-co-DVB) and fluorosilane coatings deposited on silicon
substrates, in accordance with some embodiments of the present
invention. Environmental scanning electron micrograph of
condensation of pure saturated water vapor at 800 Pa and a
supersaturation of 1.16.+-.0.05, showing pre-coalescence behavior
on copolymer (FIG. 2a) and comparing to condensation behavior on
fluorosilane (FIG. 2b) surfaces, indicating higher nucleation
density on copolymer surface. Photographs of condensation of water
vapor in air at 40% R.H. on copolymer (FIG. 2c) and fluorosilane
surfaces (FIG. 2d) immediately before and after a shedding event
(top and middle photographs, respectively) and 15 seconds after the
shedding event (bottom photograph), indicating smaller departing
drop diameter on copolymer surface. FIG. 2(e) illustrates
time-averaged normalized droplet diameter distributions. Smaller
drop sizes on copolymer surface indicate better shedding
behavior.
[0055] FIGS. 3a-3e illustrate surface topology and water vapor
condensation on p(PFDA-co-DVB) coating deposited on an aluminum
substrate, in accordance with some embodiments of the present
invention. FIG. 3a illustrates 50.times.50 .mu.m AFM height scan of
surface topology. Dashed box indicates region of the image shown in
FIG. 3b, 10.times.10 .mu.m AFM height scan of surface topology.
Photographs during condensation of saturated steam at 100.degree.
C. and 101 kPa of prolonged dropwise condensation on grafted
coating over a period of 48 hours (FIG. 3c) and degradation of
fluorosilane coating over a period of 30 min (FIG. 3d). FIG. 3e
illustrates heat transfer coefficient of aluminum substrates with
no coating, with a fluorosilane coating, and with a grafted
p(PFDA-co-DVB) coating, plotted vs. time.
[0056] FIG. 4a illustrates dropwise condensation of saturated steam
at 6.9 kPa on a copper tube coated with p(PFDA-co-DVB). FIG. 4b
illustrates snapshots immediately before and after a droplet
shedding event (left and center photographs, respectively) and 4
hours after shedding event (right photograph).
[0057] FIG. 5 (left) illustrates high resolution angle-resolved XPS
spectra taken at 0.degree. takeoff angle. Peaks corresponding to
--CF.sub.2-- and --CF.sub.3 environments are highlighted. FIG. 5
(right) illustrates 10.times.10 .mu.m AFM height scan of surface
topology showing spherulitic texture. Dashed box indicates region
of (e), 1.times.1 AFM phase scan of single roughness feature
(bottom) and line height scan (top).
[0058] FIG. 6 illustrates the experimental chamber that was used in
Dropwise Condensation Experiments.
[0059] FIG. 7 illustrates the flow loop of the experimental setup
shown in FIG. 6.
[0060] FIG. 8a illustrates grafted PFDA samples after 1 hour of
condensation in saturated steam at 90.degree. C. and 70 kPa. FIG.
8b illustrates ungrafted PFDA sample after 1 hour of condensation
in saturated steam at 90.degree. C. and 70 kPa. Condensate drops on
grafted (FIG. 8c) and ungrafted (FIG. 8d) PFDA surfaces after 10
minutes of condensing saturated steam. The distorted drop shape on
the ungrafted sample indicates severe contact line pinning.
Departing drop sizes on ungrafted sample were 3.1 mm, compared to
2.3 mm for the grafted surface. Heat transfer coefficient was
measured at 31.+-.2 kW/m.sup.2K at the beginning of the experiment,
and 23.+-.2 kW/m.sup.2K after deterioration of ungrafted
surface.
[0061] FIG. 9 shows AFM images of the
poly(1H,1H,2H,2H-perfluorodecyl Acrylate) (pPFDA)homopolymer and
different poly(1H,1H,2H,2H-perfluorodecyl
Acrylate-copolymer-divylbenzene) (p(PFDA-co-DVB)) copolymers before
and after annealing. The flow rate for each monomer is provided
between the brackets.
[0062] FIG. 10 is a graph of FT-IR of the pPFDA homopolymer, the
pDVB homopolymer and a P(PFDA-co-DVB) copolymer in accordance with
some embodiments presented herein.
[0063] FIG. 11 shows contact angle measurements using water and
mineral oil of the pPFDA homopolymer, the pDVB homopolymer and a
series of p(PFDA-co-DVB) copolymers, for non-annealed (left) and
annealed (right) samples.
[0064] FIG. 12 is a water contact angle graph for pPFDA homopolymer
and a series of p(PFDA-co-DVB) copolymers, for non-annealed (solid)
and annealed samples (open) in accordance with some embodiments
presented herein.
[0065] FIG. 13 is a mineral oil contact angle graph for pPFDA
homopolymer and a series of p(PFDA-co-DVB) copolymers, for
non-annealed (solid) and annealed samples (open) in accordance with
some embodiments presented herein.
[0066] FIG. 14 illustrates XPS analysis of different p(PFDA-co-DVB)
copolymers in accordance with some embodiments presented
herein.
[0067] FIG. 15 illustrates XRD analysis of the pPFDA homopolymer,
the pDVB homopolymer and diverse P(PFDA-co-DVB) copolymer in
accordance with some embodiments presented herein.
[0068] FIG. 16 illustrates XRD analysis of the pPFDA homopolymer,
the pDVB homopolymer and diverse P(PFDA-co-DVB) copolymer after the
annealing process in accordance with some embodiments presented
herein.
[0069] FIG. 17 shows XRD Comparison of pPFDA homopolymer and a
series of p(PFDA-co-DVB) copolymer films before and after thermal
annealing in accordance with some embodiments presented herein.
[0070] FIG. 18 schematically illustrates embodiments employing
variation in degree of crosslinking and/or variation in
concentration of crosslinking agent as a function of position of
the crosslinking agent along the thickness of the polymeric film,
in accordance with some embodiments presented herein.
[0071] FIG. 19 is a plot showing the effect of contact angle
.theta. on heat transfer coefficient h, where maximum h is at
0.about.90.degree. (hexane 66.5/54.6.degree.), in accordance with
some embodiments presented herein.
[0072] FIGS. 20a and 20b are photographic stills from a video
showing dropwise condensation and shedding of n-hexane on a
PFDA-co-DVB on silicon substrate, where P=15 kPa,
T.sub.s=10.+-.1.degree. C., T.sub.sat=18.3.degree. C., and
.DELTA.T=8.3.+-.1.degree. C., in accordance with some embodiments
presented herein.
[0073] FIG. 21 is a plot showing FT-IR spectra of eco-friendly
pC6PFA homopolymer (a), the pDVB homopolymer (b), and the
p(C6PFA-co-DVB) copolymer.
[0074] FIG. 22 is a plot showing the water contact angles on PTFE,
PVDF, and diverse compositional ranges from pC6PFA homopolymer to
pDVB homopolymer.
[0075] FIG. 23 shows the reorientation, or lack thereof, of pendant
perfluorinated pendant groups upon exposure with water: (a)
demonstrates how amorphous chains of C6 polymer reorient into the
bulk and contribute to high CAH; and (b) shows how steric hindrance
afforded by DVB crosslinking restricts rearrangement of pendant
groups into the bulk of the film, reducing CAH.
[0076] FIG. 24 is a plot of the advancing and receding contact
angles, along with the contact angle hysteresis, of eco-friendly
small-chain perfluorinated films, in accordance with some
embodiments presented herein.
[0077] The features and advantages of the present disclosure will
become more apparent from the detailed description set forth below
when taken in conjunction with the drawings, in which like
reference characters identify corresponding elements throughout. In
the drawings, like reference numbers generally indicate identical,
functionally similar, and/or structurally similar elements.
DESCRIPTION
[0078] It is contemplated that compositions, mixtures, systems,
devices, methods, and processes of the claimed invention encompass
variations and adaptations developed using information from the
embodiments described herein. Adaptation and/or modification of the
compositions, mixtures, systems, devices, methods, and processes
described herein may be performed by those of ordinary skill in the
relevant art.
[0079] Throughout the description, where articles, devices and
systems are described as having, including, or comprising specific
components, or where processes and methods are described as having,
including, or comprising specific steps, it is contemplated that,
additionally, there are articles, devices, and systems of the
present invention that consist essentially of, or consist of, the
recited components, and that there are processes and methods
according to the present invention that consist essentially of, or
consist of, the recited processing steps.
[0080] Similarly, where articles, devices, mixtures, and
compositions are described as having, including, or comprising
specific compounds and/or materials, it is contemplated that,
additionally, there are articles, devices, mixtures, and
compositions of the present invention that consist essentially of,
or consist of, the recited compounds and/or materials.
[0081] It should be understood that the order of steps or order for
performing certain actions is immaterial so long as the invention
remains operable. Moreover, two or more steps or actions may be
conducted simultaneously.
[0082] The mention herein of any publication, for example, in the
Background section, is not an admission that the publication serves
as prior art with respect to any of the claims presented herein.
The Background section is presented for purposes of clarity and is
not meant as a description of prior art with respect to any
claim.
[0083] Presented herein are articles and methods featuring
substrates with thin, uniform polymeric films grafted thereupon. An
exposed surface of the film is configured for contact with a
liquid, another solid, a vapor, and/or a combined vapor and
liquid--that is, there is either a solid-liquid interface, a
solid-solid interface, solid-vapor interface, or a
solid-vapor/liquid interface at the surface of the grafted
polymeric film. The polymeric film may be tuned to have a precise
thickness and uniformity. For example, a thickness of less than
about 200 nm, less than about 150 nm, less than about 100 nm, less
than about 80 nm, less than about 50 nm, less than about 20 nm, or
even less than about 10 nm, and the variation of the film thickness
of the surface may be less than 20%, less than 15%, less than 10%,
or less than 5%.
[0084] Methods are provided herein to graft this uniform polymeric
film onto a wide variety of substrate materials. For example,
traditional engineering materials such as stainless steel,
titanium, nickel, copper, aluminum, magnesium and/or oxides and/or
alloys thereof may be coated by a thin conformal film of polymer to
obtain a surface that exhibits robust dropwise condensation.
According to some embodiments of the present invention,
semiconductors such as Si, SiC, AN, GaAs, ceramics such as TiN,
TiC, Sic, SiN, TiO2, and rare-earth oxides can be coated as well.
Methods are also provided to provide a film with controllable
thickness and morphology. For example, in certain embodiments, the
film is a conformal film on a textured substrate. In other
embodiments, the film is a conformal film on a smooth surface. In
other embodiments, the film is a textured film on a smooth
surface.
[0085] A film may include or be a thin hydrophobic
polymer/copolymer film. Techniques such as initiated chemical vapor
deposition (hereafter, "iCVD") allow deposition of
precisely-controlled, extremely thin (e.g., as thin as 10 nm)
polymeric films on metal substrates, where the polymer is
covalently bonded to the substrate. Furthermore, the polymeric film
may be crosslinked at or near its exposed surface and/or throughout
the bulk of the film via introduction of a crosslinking agent to
the gas stream, and may be followed subsequently by annealing. The
resulting film or coating exhibits significant reductions in
thermal resistance, drop shedding size, and/or degradation rate
during dropwise condensation of steam compared to existing
coatings. Certain advantages of the described compositions and
methods thereof are detailed as follows.
[0086] Variability of Film and Substrate Composition
[0087] In some embodiments, compositions and methods described
herein may have a wide variability of film and substrate materials.
Exemplary film materials include, but are not limited to
fluoropolymers, including poly-tetrafluoroethylene (PTFE),
poly-perfluoroacrylates, poly-perfluormethacrylates, and copolymers
thereof. Other exemplary film materials include, but are not
limited to, poly-methylmethacrylate (PMMA), poly-glycidyl
methacrylate (PGMA), and poly-2-hydroxyethyl methacrylate. In
certain embodiments, the polymeric film includes a fluoropolymer,
e.g., PFDA, along with a crosslinker species, e.g., divinylbenzene
(DVB). Some embodiments of the present invention utilize a
fluorinated polymer, e.g. PTFE or PFDA, or combination thereof. For
example, Teflon by DuPont, a PTFE, may be used. Some commercialized
films of PTFE are available from GVD (http://www.gvdcorp.com/).
Such films are described, for example, in U.S. Patent Application
Publication No. 2013/0280442, U.S. Patent Application Publication
No. 2013/0171546, and U.S. Patent Application Publication No.
2012/0003497, although these films as-described would not be
suitable for dropwise condensation owing to high contact angle
hysteresis and lack of crosslinking or other means of inducing
steric hindrance.
[0088] In certain embodiments, the polymeric film includes
exemplary eco-friendly C6-type fluoropolymer materials including,
but not limited to 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl
methacrylate, 1H, 1H, 2H, 2H-perfluorooctyl acrylate,
2-(perfluorohexyl) ethyl methacrylate,
[N-methyl-perfluorohexane-1-sulfonamide] ethyl acrylate,
[N-methyl-perfluorohexane-1-sulfonamide] ethyl (meth) acrylate,
2-(Perfluoro-3-methylbutyl)ethyl methacrylate,
2-[[[[2-(perfluorohexyl) ethyl] sulfonyl] methyl]-amino] ethyl]
acrylate, and copolymers thereof.
[0089] In addition, 2-(Perfluoro-3-methylbutyl)ethyl methacrylate
(C5PFMA), combined with the crosslinking strategy or
graded-structure strategy, can be explored via iCVD polymerization.
This monomer has enriched CF.sub.3 end groups, which lowers surface
energy and promotes hydrophobicity.
[0090] In certain embodiments, the polymeric film comprises at
least one member selected from the group consisting of
polymethylmethacrylate (PMMA), polyglycidylmethacrylate (PGMA),
poly-2-hydroxyethylmethacrylate, polyperfluoroacrylate (PFDA), and
copolymers thereof. In certain embodiments, the polymeric film
comprises a fluoropolymer. In certain embodiments, the
fluoropolymer comprises polytetrafluoroethylene (PTFE). In certain
embodiments, the fluoropolymer comprises
[C.sub.12H.sub.9F.sub.13O.sub.2].sub.n, where n is an integer
greater than zero (e.g.,
poly(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl methacrylate), or
`C6`-analog of PFDA). In certain embodiments, the fluoropolymer
comprises a copolymer of divinylbenzene (DVB) and one or both of:
PFDA and PTFE.
[0091] In one aspect, the invention is directed to an article for
enhanced heat transfer, the article comprising a substrate and a
(e.g., thin, uniform) polymeric film grafted (e.g., covalently
bonded) thereupon. In certain embodiments, the substrate comprises
a metal (e.g., steel, stainless steel, titanium, nickel, copper,
and/or alloys thereof). In certain embodiments, the substrate
comprises a semiconductor (e.g., SiC, Si, AlN, GaAs, GaN, ZnO, Ge,
SiGe, BN, BAs, AlGaAs, TiO.sub.2, etc.). In certain embodiments,
the substrate comprises a rare earth element or compound comprising
a rare earth element (e.g., a rare earth oxide, carbide, nitride,
fluoride, or boride; e.g., cerium oxide CeO.sub.2).
[0092] In some embodiments, methods are provided herein to graft
this uniform polymeric film onto a wide variety of substrate
materials. In certain embodiments, the film is a conformal film on
a textured substrate. For example, in some embodiments, traditional
materials such as stainless steel, titanium, nickel, copper,
aluminum and/or their alloys may be coated by a thin conformal film
of polymer to obtain a surface that exhibits robust dropwise
condensation. According to some embodiments of the present
invention, semiconductors such SiC, AN, GaAs can be coated as
well.
[0093] In some embodiments, the substrate on which the film is
deposited includes plastic, silicon, quartz, woven or non-woven
fabric, paper, ceramic, nylon, carbon, polyester, polyurethane,
polyanhydride, polyorthoester, polyacrylonitrile, polyphenazine,
polyisoprene, synthetic rubber, polytetrafluoroethylene,
polyethylene terephthalate, acrylate polymer, chlorinated rubber,
fluoropolymer, polyamide resin, vinyl resin, expanded
polytetrafluoroethylene, low density polyethylene, high density
polyethylene, or polypropylene. In some embodiments, the substrate
is homogeneous. In some embodiments, the substrate is
heterogeneous. In some embodiments, the substrate is planar. In
some embodiments, the substrate is non-planar. In some embodiments,
the substrate is concave. In some embodiments, the substrate is
convex. In some embodiments, the substrate possesses a
micro/nanoscale hierarchical texture.
[0094] Covalent Grafting
[0095] In some embodiments, compositions and methods described
herein may have a covalently bonded interface between a film and a
substrate. The film-substrate interfaces obtained by other methods
of deposition, such as sputtering or casting, suffer from weak
bonds between substrate and film. When stressed by the large
mismatch in coefficient of thermal expansion
(.DELTA..alpha..about.1.times.10.sup.-4), hydrolysis in the
presence of steam, or the shear stresses encountered during droplet
coalescence, these interfaces have been shown to be highly prone to
delamination. The covalently bonded interface used in accordance
with some embodiments described herein may be shown to resist
delamination for prolonged periods. The covalent bonding between
the film and the substrate can also lower the thermal interface
resistance, thereby improving the overall heat transfer
coefficient.
[0096] Many different chemistries exist for covalently attaching a
vinyl or other reactive group to a substrate, and specifically a
metal substrate. Silanes, thiols, carboxylic acids, and
phosphonates (or phosphonic acids) are examples of such well-known
chemistries. Under some conditions, such as alkaline conditions
with pH>7, the hydrolytic stability of phosphonates exceeds that
of silanes. Under other conditions, such as under solar
irradiation, silanes are more stable than phosphonates. Both
phosphonates and silanes can possess one or more vinyl functional
group. Silanes with more than one anchor point, referred to as
dipodal silanes, result in greater stability and substrate
adhesion.
[0097] Tunable Thickness & Morphology
[0098] Previous attempts at promoting dropwise condensation, for
example with self-assembled monolayers, have usually resulted in
films that degrade over time. Monolayers will inevitably have holes
in the film that will act as degradation initiation sites. For
example, the silane-metal bonds of a silanized substrate are
susceptible to hydrolysis by steam. Other promoters, such as oleic
acids, have been shown to function only on copper substrates, and
are incompatible with the more industrially-relevant materials used
in heat exchangers such as stainless steel and titanium alloys. A
thicker film, e.g., more than a monolayer, will help ensure that
there are no regions of exposed substrate.
[0099] However, since the thermal conductivity of polymers are much
lower than that of a metal tube (for example, the thermal
conductivity of bulk PTFE is approximately 0.25 W/mK as compared to
approximately 20 W/mK for stainless steel), previous attempts at
obtaining a dropwise promoter surface via polymer films were many
microns thick. The additional thermal resistance posed by such
thick films was enough to offset any benefits of the higher heat
transfer coefficient during dropwise condensation, making these
films unusable for promoting dropwise condensation.
[0100] To optimize the film thickness and to ensure that the
conduction resistance of the polymer film contributes no more than
1% of the total resistance, the thickness, in some embodiments,
must be less than 1 .mu.m. The total thermal resistance includes
the following resistances in series: the resistance from the
condensing vapor to the substrate, the conduction resistance
through the film and the substrate, and the convection resistance
of the coolant:
R.sub.T=Rs+R.sub.f+R.sub.m+R.sub.w=(1/h.sub.S)+(1/k).sub.f+(1/k).sub.m+(-
1/h).sub.w (1)
where the subscripts s, f, m, and w represent the steam
condensation, film conduction, metal conduction, and water
convection, respectively. Typical orders of magnitude of the
variables are as follows:
h.sub.s.apprxeq.10.sup.4Wm.sup.-2K.sup.-1,
k.sub.f.apprxeq.10.sup.-1Wm.sup.-1K.sup.-1,
l.sub.f.apprxeq.10.sup.-3m, k.sub.m.apprxeq.10.sup.23
Wm.sup.-2Wm.sup.-1K.sup.-1,
h.sub.w.apprxeq.10.sup.3Wm.sup.-2K.sup.-1. Thus, the total
resistance of the condenser is on the order of 10.sup.-3
KmW.sup.-1, whereas the conduction resistance due to the film is on
the order of 10.sup.-8KmW.sup.-1. Since the present coating is so
thin (e.g., on the order of 10 nm, 20 nm, 30 nm, 40 nm), it
represents only about 0.5% of the condensation resistance and
.about.0.001% of the total thermal resistance. This is in contrast
to the polymer films in conventional systems that were typically
many microns thick.
[0101] In some embodiments, a film described herein can be
sufficiently thick enough to provide complete coverage, but thin
enough to minimize any added thermal resistance. The thickness of a
film may be precisely controlled in real time, for example, by
laser interferometry (or other suitable methods) to obtain films as
thin as 10 nm. The thermal resistance of a 10 nm film of PTFE is
negligible: 4.times.10.sup.-8K/W, corresponding to a thermal
conductance of 25 MW/m.sup.2K.
[0102] In certain embodiments, the deposited polymeric film has an
average thickness from 1 nm to 1 micron. In certain embodiments,
the deposited polymeric film has an average thickness from 1 nm to
100 nm.
[0103] In certain embodiments, the polymeric film is no greater
than 500 nm in thickness (e.g., no greater than 400 nm, no greater
than 300 nm, no greater than 200 nm, no greater than 100 nm, no
greater than 75 nm, no greater than 50 nm, no greater than 25 nm,
or no greater than 15 nm, e.g., as thin as 10 nm). In certain
embodiments, the polymeric film has a thickness variation of no
greater than about 20% (e.g., no greater than about 15%, no greater
than about 10%, or no greater than about 5%--e.g., the polymer film
is uniform). In some embodiments, the thickness of the polymeric
film is about 10 nm, about 20 nm, about 30 nm, about 40 nm, about
50 nm, about 60 nm, about 70 nm, about 80 nm, about 90 nm, about
100 nm, about 125 nm, about 150 nm, about 175 nm, about 200 nm,
about 225 nm, about 250 nm, about 275 nm, about 300 nm, about 325
nm, about 350 nm, about 375 nm, about 400 nm, about 425 nm, about
450 nm, about 475 nm, about 500 nm, about 525 nm, about 550 nm,
about 575 nm, about 600 nm, about 625 nm, about 650 nm, about 675
nm, about 700 nm, about 725 nm, about 750 nm, about 775 nm, about
800 nm, about 825 nm, about 850 nm, about 875 nm, about 900 nm,
about 1000 nm, about 1100 nm, about 1200 nm, about 1300 nm, about
1400 nm, or about 1500 nm.
[0104] Minimizing Contact Angle Hysteresis
[0105] In certain embodiments, a film is particularly useful for
enhancing dropwise condensation. The dropwise heat transfer
coefficient is strongly influenced by the size of the departing
drops. Since a condensate drop begins to present a thermal
resistance as soon as it forms, it would be desirable to shed
condensate drops as soon as possible. A typical surface will be
able to support a drop as it grows to the capillary length, which
is approximately 2.7 millimeters for water. At this size, the drops
present a significant thermal barrier. If instead the drops can be
shed at a much smaller size, the overall heat transfer coefficient
will be increased significantly. External forces such as gravity or
vapor shear may be utilized to remove condensate droplets, but they
will have to overcome the forces due to surface tension that pin
the contact line of the drop to the condensing surface. A useful
measure of the pinning strength of a surface is the contact angle
hysteresis (CAH)--the difference between the advancing and receding
contact angles. A lower CAH will result in easier shedding of
condensate drops. For smooth surfaces, CAH is minimized when the
surface is free of morphological and chemical inhomogeneities.
Thus, a smooth, chemically homogeneous surface is desirable for
minimizing CAH and maximizing the heat transfer coefficient.
[0106] Additionally, the molecular rearrangement of pendant
moieties upon exposure to a wetting fluid such as water gives rise
to increased CAH, as explained, for example, in A. Synytska, D.
Appelhans, Z. G. Wang, F. Simon, F. Lehmann, M. Stamm, K. Grundke,
Macromolecules 2007, 40, 1774. This rearrangement may be prevented
by increasing the molecular rigidity via adjusting the degree of
crystallinity and/or the degree of crosslinking to minimize the
contact angle hysteresis. In some embodiments, compositions and
methods described herein may have a tunable molecular rigidity. By
altering the deposition parameters, the molecular rigidity may be
adjusted at any position (e.g., at any depth or location) within
the film, including the film-substrate interface and throughout
bulk of the film. At the free surface of the film that is exposed
to liquids, it is particularly desirable to obtain rigid films.
[0107] In some embodiments, a film is treated (e.g., annealed)
after deposition. Without being bound to any particular theory,
annealing can reduce hysteresis, by increasing the degree of
crystallinity and/or increasing the degree of crosslinking of the
film as explained, for example, in J. L. Yague, K. K. Gleason,
Macromolecules, 2013, 46, 6548.
[0108] For example, a film described according to some embodiments
discussed herein may be thermally annealed to improve both
durability and contact angle hysteresis (CAH). In experiments
described in more detail herein below, copolymer films of
poly-perfluorodecyl acrylate and divinylbenzene (PFDA/DVB) were
annealed at 60.degree. C. to improve crosslinking, resulting in a
surface with lower CAH and improved durability in the presence of
high-temperature steam.
[0109] The smaller, eco-friendly C.sub.6-type perfluorinated chains
are more difficult to restrain from reorientation upon contact with
water, but by carefully choosing an appropriate spacer group
(located between the acrylate backbone and the fluorinated
functional group), this reorientation may be mitigated. For
example, a spacer consisting only of an ethyl group, such as
(1H1H2H2HC.sub.n+2F.sub.2n+1) acrylate, crystallization at room
temperature is possible only for n.gtoreq.8, (for example such as
(1H1H2H2H perfluorodecyl) acrylate with n=8), since interactions
between adjacent monomers only occur only between their
perfluorinated pendant groups. However, by substituting a
[[sulfonyl]methyl]-amino] spacer for the ethyl spacer as an
example, additional dipole-dipole interactions between the spacer
groups of adjacent monomers are able to further restrain pendant
groups and promote crystallization of smaller perfluorinated
chains. Referring now to FIG. 23, we find that films of
[N-methyl-perfluorohexane-1-sulfonamide] ethyl (meth) acrylate
(C6PFSMA) exhibit significantly smaller contact angle hysteresis
compared to films of [N-methyl-perfluorohexane-1-sulfonamide] ethyl
acrylate (C6PFSA) and Poly(2-(Perfluorohexyl)ethyl methacrylate)
(pC6PFMA).
[0110] A polymeric film may be crosslinked to improve rigidity and
minimize CAH. Exemplary crosslinkers include, but are not limited
to, divinylbenzene (DVB), ethylene dimethyacrylate (EDMA),
di(ethyleneglycol) di(methacrylate), di(ethyleneglycol)
di(acrylate), ethyleneglycoldimethacrylate (EGDMA) and
di(ethyleneglycol)di(vinyl ether) (DEGDVE), and/or 1H, 1H, 6H,
6H-perfluorohexyldiacrylate.
[0111] The contact angle hysteresis of iCVD films with various
liquids is given in Table 1 below. The contact angle hysteresis may
be measured with a goniometer by injecting liquid into a drop to
measure the advancing contact angle, and withdrawing liquid from
the drop to measure the receding contact angle.
TABLE-US-00001 TABLE 1 Contact angle hysteresis of iCVD films
described herein with various liquids iCVD film Liquid
.DELTA..theta. [.degree.] PFDA homopolymer Water 5 PFDA homopolymer
Mineral oil 22 PFDA-co-DVB copolymer Hexane 11 PFDA-co-DVB
copolymer Pentane 8
[0112] In certain embodiments, the polymeric film has a surface
(e.g., exposed surface) with low contact angle hysteresis (e.g., no
greater than 50.degree., no greater than 40.degree., no greater
than 30.degree., no greater than 25.degree., no greater than
20.degree., no greater than 15.degree., or no greater than
10.degree. for water, where contact angle hysteresis is the
difference between advancing contact angle and receding contact
angle). In certain embodiments, the polymeric film has a surface
(e.g., exposed surface) with high advancing contact angle (e.g., no
less than 70.degree., no less than 80.degree., no less than
90.degree., no less than 100.degree., no less than 120.degree., no
less than 130.degree. for water) and/or high receding contact angle
(e.g., no less than 60.degree., no less than 70.degree., no less
than 80.degree., no less than 90.degree., no less than 100.degree.,
no less than 110.degree., or no less than 120.degree. for water).
In some embodiments, the advancing water contact angle is greater
than about 150.degree.. In some embodiments, the advancing water
contact angle is about 150.degree., about 155.degree., about
160.degree., about 165.degree., or about 170.degree.. In some
embodiments, the receding water contact angle is greater than about
150.degree.. In some embodiments, the receding water contact angle
is about 150.degree., about 155.degree., about 160.degree., about
165.degree., or about 170.degree..
[0113] Preferably, the contact angle hysteresis is <25.degree..
More preferably, the contact angle hysteresis is <5.degree.. If
the contact angle hysteresis is higher, it may be compensated for
by a lower surface energy, which would result in a larger contact
angle and a larger gravitational body force per length of contact
line acting to shed the drop.
[0114] In some embodiments, the water contact angle hysteresis is
about 10.degree., about 9.degree., about 8.degree., about
7.degree., about 6.degree., about 5.degree., about 4.degree., or
about 3.degree.. In some embodiments, the water contact angle
hysteresis is between about 3.degree. and about 10.degree..
[0115] In some embodiments, the advancing mineral oil contact angle
is greater than about 100.degree.. In some embodiments, the
advancing mineral oil contact angle is about 100.degree., about
105.degree., about 110.degree., about 115.degree., about
120.degree., about 125.degree., or about 130.degree.. In some
embodiments, the advancing mineral oil contact angle is between
about 100.degree. and about 130.degree..
[0116] In some embodiments, the receding mineral oil contact angle
is greater than about 100.degree.. In some embodiments, the
receding mineral oil contact angle is about 100.degree., about
105.degree., about 110.degree., about 115.degree., about
120.degree., about 125.degree., or about 130.degree.. In some
embodiments, the receding mineral oil contact angle is between
about 100.degree. and about 130.degree..
[0117] In some embodiments, the static mineral oil contact angle is
greater than about 100.degree.. In some embodiments, the static
mineral oil contact angle is about 100.degree., about 105.degree.,
about 110.degree., or about 115.degree.. In some embodiments, the
static mineral oil contact angle is between about 100.degree. and
about 115.degree..
[0118] iCVD Coating Process
[0119] Coating typically involves the deposition of films or layers
on a surface of a substrate. One manner of effecting the deposition
of such films or layers is through chemical vapor deposition (CVD).
CVD involves a chemical reaction of vapor phase chemicals or
reactants that contain the constituents to be deposited on the
substrate. Reactant gases are introduced into a reaction chamber or
reactor, and are decomposed and reacted at a heated surface to form
the desired film or layer.
[0120] In some embodiments, CVD used in accordance with the present
invention is an initiated CVD (iCVD). iCVD Deposition Example in
Cylindrical Reactor below discusses a typical experimental set-up
for iCVD. In an iCVD process, thin filament wires are heated, thus
supplying the energy to fragment a thermally-labile initiator,
thereby forming a radical at moderate temperatures. The use of an
initiator not only allows the chemistry to be controlled, but also
accelerates film growth and provides control of molecular weight
and rate. The energy input is low due to the low filament
temperatures, but high growth rates may be achieved. The process
progresses independent of the shape or composition of the
substrate, is easily scalable, and easily integrated with other
processes.
[0121] In certain embodiments, iCVD takes place in a reactor. In
certain embodiments, a variety of monomer species may be
polymerized and deposited by iCVD. In certain embodiments, the
surface to be coated is placed on a stage in the reactor and
gaseous precursor molecules are fed into the reactor; the stage may
be the bottom of the reactor and not a separate entity. In certain
embodiments, a variety of carrier gases are useful in iCVD.
[0122] In certain embodiments, the iCVD reactor has automated
electronics to control reactor pressure and to control reactant
flow rates. In certain embodiments, unreacted vapors may be
exhausted from the system.
[0123] The iCVD process is a single-step, solvent-free, low-energy,
vapor-phase method used to deposit conformal films with precisely
controllable thickness and in which grafting to the substrate
provides enhanced durability, as discussed, for example, in M. E.
Alf, A. Asatekin, M. C. Barr, S. H. Baxamusa, H. Chelawat, G.
Ozaydin-Ince, C. D. Petruczok, R. Sreenivasan, W. E. Tenhaeff, N.
J. Trujillo, S. Vaddiraju, J. Xu, K. K. Gleason, Adv. Mater. 2010,
22, 1993. The large choice of suitable monomers that may be used
allows for precise design and modulation of surface properties.
[0124] Certain embodiments presented herein relate to films
exhibiting a combination of durability and low contact angle
hysteresis. Copolymerization with a crosslinker is an additional
method that aids in both further reduction of contact angle
hysteresis and also rendering the films more stable to chemical and
mechanical degradation--making the films more robust and extending
the useful life of those films.
[0125] Certain embodiments presented herein relate to the use of
vapor synthesis for copolymerization, which in some embodiments
does not require that the two monomers being copolymerized have a
common solvent. This characteristic will be recognized by those
skilled in the art as a significant advantage over wet-chemistry
synthesis techniques, as a common solvent does not exist for PFDA
and DVB. In some embodiments, iCVD allows a non-fluorinated
crosslinker, DVB, to be readily copolymerized with the fluorinated
monomer, PFDA, over its entire compositional range.
[0126] Copolymerization also disrupts crystallization. Since
crystallites are one source of roughness, copolymer films in some
embodiments may be made to be smoother than crystalline iCVD
p(PFDA) homopolymer layers. Such smooth surfaces may be desired to
reduce the contact angle hysteresis of low-surface tension fluids
such as hydrocarbons, refrigerants, and/or cryogens. Additionally,
the perfluorinated side chains of the PFDA units segregate to the
interface under dry conditions in order to minimize surface energy.
Surface reconstruction in which the perfluoro chains orient away
from the interface can occur when the surface becomes wet.
[0127] The iCVD of homopolymers p(PFDA) and p(DVB) results in
highly conformal thin films, and superhydrophobic and
superoleophobic surfaces have been demonstrated with iCVD films of
p(PFDA).
[0128] Certain embodiments described herein prevent the
reorientation of CF.sub.3 groups via crosslinking. A crosslinking
agent provides a controllable means of steric hindrance, because
the proportion (e.g., concentration in particular location) of
crosslinking agent may be varied along the film thickness. FIG. 18
schematically illustrates embodiments employing variation in degree
of crosslinking and/or variation in concentration of crosslinking
agent as a function of position of the crosslinking agent along the
thickness of the polymeric film. The polymeric film includes a
grafting layer (e.g., where the grafting layer has a thickness from
about 0.5 nm to about 5 nm, or from about 1 nm to about 3 nm, or
from about 1 nm to about 2 nm), and a bulk film layer making up the
majority (e.g., more than 50%, more than 55%, more than 60%, more
than 70%, more than 80%, more than 90%, more than 95%) of the
polymeric film. In certain embodiments, the polymeric film has at
thickness no greater than 400 nm, no greater than 300 nm, no
greater than 200 nm, no greater than 100 nm, no greater than 75 nm,
no greater than 50 nm, no greater than 25 nm, or no greater than 15
nm, in thickness. In some embodiments, the polymeric film may be as
thin as 10 nm or have a thickness on the order of 10 nm.
[0129] One of the main difficulties in obtaining a surface that
exhibits dropwise condensation of hydrocarbons and other
low-surface tension liquids has been obtaining a surface with a
sufficiently low critical surface tension. The condensate will
spread to form a film unless the critical surface tension of the
surface is below that of the condensing liquid. Table 2 below lists
the surface tension values for water and a variety of other
liquids, including n-alkanes (octane, hexane, pentane) and a
fluorocarbon similar to a typical refrigerant. Table 3 lists
refrigerants, e.g., hydrofluorocarbons, chlorofluorocarbons, and
hydrochlorofluorocarbons.
[0130] The n-alkanes have surface tensions that are considerably
lower than water, and also lower than most common industrial
materials (including polymers) whose critical surface tensions are
shown in Table 4. For example, Teflon has a surface energy of 19
mN/m, since it is composed principally of CF.sub.2 groups, and is
not sufficient to condense hexane or lower alkanes. Even
trichloro(1H,1H,2H,2H-perfluorooctyl)silane (commonly referred to
as fluorosilane, a low-surface energy fluorinated silane surface
modifier), has a critical surface energy of 10 mN/m. Although
fluorosilane is terminated by CF.sub.3 groups, the lack of
crosslinking or other steric hindrance allows these CF.sub.3 group
to reorient in the presence of water or another wetting liquid. As
a result, it is difficult to obtain a surface with a critical
surface energy low enough to promote dropwise condensation of these
liquids.
TABLE-US-00002 TABLE 2 Surface tensions of water and various
low-surface tension fluids. .sigma..sub.iv @25.degree. C. liquid
[mN/m] water 72.71 toluene 27.93 isopropanol 20.92 ethanol 24.77
octane 21.08 hexane 17.98 pentane 15.47 perfluorohexane 11.47
TABLE-US-00003 TABLE 3 List of refrigerants. Chlorofluorocarbons
R-11 Trichlorofluoromethane R-12 Dichlorodifluoromethane R-13
Chlorotrifluoromethane R-13B1 Bromotrifluoromethane R-14
Tetrafluoromethane R-113 Trichlorotrifluoroethane R-114
1,2-Dichloro-1,1,2,2-Tetrafluoroethane R-500
Dichlorodifluoromethane, Difluoroethane R-502
Chlorodifluoromethane, Chloropentafluoroethane R-503
Chlorotrifluoromethane, Trifluoromethane Hydrochlorofluorocarbons
R-12 1-Chloro-1,2,2,2-tetrafluoroethane, 1,1,1,2-Tetrafluoroethane
R-22 Chlorodifluoromethane R-123 Dichlorotrifluoroethane R-124
1-Chloro-1,2,2,2-Tetrafluoroethane R-401A Chlorodifluoromethane,
Chlorotetrafluoroethane R-401B Chlorodifluoromethane,
Chlorotetrafluoroethane R-402A Chlorodifluoromethane,
Pentafluoroethane R-402B Chlorodifluoromethane, Pentafluoroethane
R-408A Trifluoroethane, Chlorodifluoromethane R-409A
Chlorodifluoromethane, Chlorotetrafluoroethane R-412A
Chlorodifluoromethane, 1-Chloro-1,1-Difluoroethane and
Octafluoropropane R-414B Chlorodifluoromethane,
Chlorodifluoroethane, Chlorotetrafluoroethane R-416A
1-Chloro-1,2,2,2-tetrafluoroethane, 1,1,1,2-Tetrafluoroethane
Hydrofluorocarbons R-23 Trifluoromethane R-116 Hexafluoroethane
R-134a 1,1,1,2-Tetrafluoroethane R-404A Pentafluoroethane,
1,1,1,2-Tetrafluoroethane, Trifluoroethane R-407A Difluoromethane,
Pentafluoroethane, 1,1,1,2-Tetrafluoroethane R-407B
Difluoromethane, Pentafluoroethane, 1,1,1,2-Tetrafluoroethane
R-407C Difluoromethane, Pentafluoroethane,
1,1,1,2-Tetrafluoroethane R-410A Pentafluoroethane, Difluoromethane
R-417A 1,1,1,2-Tetrafluoroethane and Pentafluoroethane R-422A
1,1,1,2-Tetrafluoroethane and Pentafluoroethane R-422D
1,1,1,2-Tetrafluoroethane and Pentafluoroethane R-423A
Tetrafluoroethane, Heptafluoropropane R-427A
1,1,1,2-Tetrafluoroethane, Pentafluoroethane R-438A
Difluoromethane, Pentafluoroethane, 1,1,1, 2- Tetrafluoroethane,
n-Butane, Isopentane R-507 Pentafluoroethane, Trifluoroethane
R-508A Trifluoromethane, Hexafluoroethane R-508B Trifluoromethane,
Hexafluoroethane
TABLE-US-00004 TABLE 4 Critical surface energy of industrial
polymers. Surface Contact Polymer Energy Angles Abbr. Polymer Name
(dynes/cm) (degrees) PES Polyethersulfone 46 90 Styrene butadiene
rubber 48 PPO Polyphenylene oxide 47 75 Nylon 6/6
(polyhexamethylene adipamide) 46 PC Polycarbonate 46 75 Nylon-6
(polycaprolactam) 38 PET Polyethylene terephthalate 42 76 PMMA
Polymethylmethacrylate 41 82 SAN Styrene acrylonitrile 40 74
Polyimide 40 83 PCV r Polyvinyl chloride, rigid 39 90 Polyester 41
70 Acetal 36 85 ABS Acrylonitrile butadiene styrene 35 82 PPS
Polyphenylene sulfide 38 87 PVA Polyvinyl alcohol 37 10
Polyacrylate (acrylic film) 35 PVC p Polyvinyl chloride,
plasticized 35 89 PS Polystyrene 34 72 Nylon-12 36 Surlyn ionomer
33 80 PBT Polybutylene terephthalate 32 88 CTFE
Polychlorotrifluoroethylene 31 PP Polypropylene 30 88 PU
Polyurethane 38 85 PE Polyethylene 30 88 PVF Polyvinyl fluoride 28
PVDF Polyvinylidene fluoride 25 80 Natural rubber 24 PDMS
Polydimethyl sioloxane (silicone elastomer) 23 98 FEP Fluorinated
ethylene propylene 20 98 PTFE Polytetrafluoroethylene 19 120
[0131] Even if a surface can be found with sufficiently low surface
energy to avoid spreading of the condensate, a second difficulty in
obtaining dropwise condensation of low surface tension liquids is
reducing the contact angle hysteresis (and thus the drop adhesion).
If the adhesion of the condensate drops to the surface is high,
then the drops will be unable to shed from the surface, and the
initial dropwise condensation will proceed until the individual
drops merge to form a continuous film. This is an especially
difficult problem in the case of low-surface tension fluids. Since
the contact angle of a condensate drop will inevitably be low (in
the range of about 10.degree. to 30.degree.), the ratio of the body
force due to gravity acting to shed the drop will be small compared
to the force acting to pin the drop to the surface. A plot showing
the effect of contact angle .theta. on heat transfer coefficient h
is shown in FIG. 19, where maximum h for water is at
0.about.90.degree. and for octane .about.50.degree..
[0132] Surfaces that promote dropwise shedding of low-surface
tension condensates, such as liquid hydrocarbons, are demonstrated
in the experimental examples presented herein. For example,
demonstrated herein is the dropwise condensation of hexane on a
surface comprising iCVD copolymer of PFDA-co-DVB. FIGS. 22A and 22B
are photographic stills from a video showing dropwise condensation
and shedding of n-hexane on a PFDA-co-DVB on silicon substrate,
where P=15 kPa, T.sub.s=10.+-.1.degree. C., T.sub.sat=18.3.degree.
C., and .DELTA.T=8.3.+-.1.degree. C.
[0133] Surfaces such as the ones shown in FIGS. 20a and 20b have
valuable applications in a wide variety of industries, for example,
in applications of refrigeration, dehumidification, and HVAC, which
condense a refrigerant, generally a low-surface tension
fluorocarbon fluid. Condensers that promote dropwise shedding of
such fluids result in higher overall efficiencies and/or lower
device footprint. Further applications include power plants
utilizing organic Rankine cycles, e.g., with isobutene, pentane, or
propane as the working fluid, which may allow for smaller
condensers to be used, and lower capital costs for such power
plants. Other applications include the fractionation of hydrocarbon
crude streams into constituent components, allowing for smaller
fractionation columns with fewer stacks.
[0134] Also presented herein is the finding that surfaces with both
(1) low critical surface energy and (2) low contact angle
hysteresis promote dropwise shedding of low-surface tension
condensates such as liquid hydrocarbons. Furthermore, owing to the
grafting (e.g., covalent bonding) of the film to the substrate,
these surfaces display a high degree of robustness. They are seen
to survive prolonged condensation in 100.degree. C. steam with no
noticeable degradation.
[0135] For example, the critical surface energy of an iCVD-grafted
PFDA homopolymer has been determined to be 5.6 mN/m, as compared to
18.5 mN/m for the ungrafted homopolymer. Critical surface energy
may be determined by plotting 1-cos(.theta..sub.a), where
.theta..sub.a is the cosine of the advancing contact angle of a
homologous series of liquids, e.g. n-heptane, n-octane, n-decane,
etc., and finding the x-intercept. In certain embodiments, the
critical surface energy is <18 mN/m. If the critical surface
energy is higher, the surface may become flooded by the low-surface
tension fluid. In some embodiments, the critical surface energy is
<6 mN/m.
[0136] There is a wide array of industrial applications for iCVD
coatings for dropwise condensation and shedding of low-surface
tension liquids. Condensation of low-surface tension liquids in
industrial applications has occurred in the filmwise mode due to
the aforementioned difficulties in achieving dropwise condensation
and shedding. As a result, since the thermal conductivities of
low-surface tension fluids (typically <0.2 W/mK) are worse even
than water (0.6 W/mK), these condensers suffer from considerable
thermal inefficiencies due to the thermal resistance of the
condensate film. By implementing a dropwise condenser, e.g., with
iCVD coating as described herein, the heat transfer coefficient
[W/m.sup.2K] can increase tenfold. Thus, in certain embodiments,
for a given coolant temperature, ten times the heat can be
transferred, or the same amount of heat can be transferred by a
heat exchanger that is smaller than the original size or by a
temperature difference that is smaller than the original
temperature difference.
[0137] The coatings/surfaces described herein have numerous
important uses in oil and gas processing (e.g., LNG, propane,
etc.); refrigerants, condenser coils in dehumidification systems,
commercial/residential HVAC, consumer packaging, medical devices,
water recovery from cooling towers, dew/fog collection, organic
Rankine cycles, steam based power generation (e.g., solar thermal,
geothermal, etc.), liquefaction (including LNG, CO.sub.2, N.sub.2,
liquid oxygen, etc.), and phase transition applications involving
mitigation of icing, hydrates, and scale formation.
[0138] In applications of refrigeration, dehumidification, and HVAC
which condense a refrigerant (typically a low-surface tension
fluorocarbon fluid), dropwise condensers would result in higher
overall efficiencies and lower device footprint. In power
applications utilizing organic Rankine cycles, e.g. with isobutene,
pentane, or propane as the working fluid, condensers must be used
to pull the working fluid through the turbine and condense back to
liquid to be pumped back through the cycle. Implementing dropwise
condensers would allow for smaller equipment size, which would
significantly reduce the capital cost of such plants; and would
also allow for better overall cycle efficiencies. In the
fractionation of crude streams into constituent components, e.g.
kerosene, alkanes, fuel oils, and diesel and heavier fuels,
dropwise condensing surfaces would allow for smaller fractionation
columns with fewer stacks. In applications such as the liquefaction
of natural gas, oxygen, and nitrogen, cold boxes are used to
condense a gas stream into a liquid. The cooling flux of the cold
box is provided by a portion of the liquefied product, and so by
increasing the heat transfer coefficient of the condensers, the
liquefaction plant would be able to produce a larger amount of
valuable liquid product instead of less-valuable gaseous product.
Furthermore, with the advent of ship-based liquefaction plants, the
heat transfer equipment becomes severely space-constrained. A
dropwise condenser would provide the same heat flux in a much
smaller footprint than the current filmwise condensers.
[0139] Industrial applications of the surfaces described herein
include phase change applications, wherein the surfaces minimize
adhesion of solid phases nucleating and growing on the surfaces,
e.g., where there is ice formation on power lines, wind turbines,
aircraft, and municipal pipelines; where there is hydrate formation
on oil and gas equipment (e.g., undersea); and where there is scale
formation on equipment in power plants and boilers, in desalination
plants, and municipal pipelines. The low hysteresis of the
coatings/surfaces described herein can be exploited for shedding
(e.g., dropwise shedding) of unwanted liquid drops, as in water
from car windshields, solar panels, and industrial machinery; oil
contaminants from household cookware, consumer electronics, and
industrial machinery; and blood and other biological fluids from
medical devices. The low surface energy of the coating/surfaces
described herein can also be exploited for their low solid-solid
frictional properties, e.g., sliding linear bearings, bushings, and
non-stick household implements.
[0140] In certain embodiments, a film described herein is used in
power plants, desalination condensers,
humidification-dehumidification systems, or heating, ventilation,
and air conditioning (HVAC). In certain embodiments, a film is used
in a thermal interface material (TIM) because of its covalent
bonding and flexibility. In certain embodiments, a film is used for
cooling of electronics and photonics.
[0141] In certain embodiments, the surface energy of a thin film
(e.g., a film of fluorinated polymer) is sufficiently low to be
oleophobic, which would allow it to be used for dropwise
condensation of hydrocarbons.
[0142] Some examples below discuss sustained dropwise condensation
of steam on a thin film of poly-(1H,1H,2H,2H-perfluorodecyl
acrylate)-co-divinyl benzene (p(PFDA-co-DVB).
[0143] It is found that roughness can be precisely specified and
designed so that it is high enough to enhance nucleation density
but low enough such that it does not adversely affect hysteresis.
Roughness may be designed by numerous methods, including, for
example, degree of crystallization, extent of crosslinking,
composition of crosslinker, and substrate temperature during
deposition.
[0144] Also described herein are findings regarding variables of
the described coatings/surfaces, including surface energy,
roughness, and substrate bonding.
[0145] Regarding surface energy, it is found that the surface
energy of the surface/coating should be lower than the condensate
liquid. For example, for the PFDA-co-DVB copolymer described
herein, surface energy may be determined from a ratio of CF.sub.3
groups to CF.sub.2 groups at the surface, where .sigma..sub.CF3=6
mN/m and .sigma..sub.CF2=18 mN/m. It is found that on a
non-crosslinked surface (e.g., fluorosilane), CF.sub.3 groups
re-orient away from the surface when exposed to water. It is also
found that DVB crosslinking rigidifies the CF.sub.3 groups of the
PFDA and prevents reorientation. Furthermore, it is found that
grafting forces orientation of CF.sub.3 groups toward the
surface.
[0146] Regarding roughness, it is found that roughness should be
low enough to avoid contact angle hysteresis. For example,
roughness features small than .about.100 nm are "weak" defects and
do not contribute to hysteresis. It is also found that some small
amount of roughness is beneficial for providing nucleation sites.
Moreover, roughness can be controlled by crosslinking. For example,
PFDA homopolymer (non-crosslinked) crystallizes into large
hemispherical agglomerations. Crosslinking prevents crystallization
and lowers roughness. Copolymer films of p(PFDA-co-DVB) exhibit a
much smaller degree of crystallinity than PFDA homopolymer, however
still exhibit semicrystalline agglomerations that enhance the
nucleation density.
[0147] Regarding substrate bonding, it is found that covalent bonds
of the present coatings/surfaces are stronger than van der Waals
bonds of typical Teflon coatings. Moreover, the vinyl group of PFDA
is found to bond covalently with an initiated vinyl group on the
surface.
Experimental Examples
iCVD Deposition Experiment A--p(PFDA-co-DVB)
[0148] In this Example, polymerizations were conducted in a
custom-design cylindrical reactor (diameter 24.6 cm and height 3.8
cm). On top of the reactor laid a quartz top that allowed laser
interferometry (633-nm He--Ne laser, JDS Uniphase) for in-situ film
thickness monitoring. Inside the reactor, 14 parallel ChromAlloy
filaments (Goodfellow) were resistively heated at 230.degree. C.
and the stage was back-cooled at a constant temperature of
30.degree. C. by water using a recirculating chiller/heater (Neslab
RTE-7). Reactor pressure was maintained at 200 mTorr using a
throttle valve (MKS Instruments). The radical initiator, and the
gas carrier were delivered inside the reactor through mass flow
controllers (MKS Instruments). The fluorinated PFDA monomer and the
DVB cross-linker were heated in a glass jar to a temperature of
80.degree. C. and 60.degree. C. respectively, and their flows were
controlled by needle valves. The flow rates of initiator and
monomer were kept constant at 3.2 and 0.2 sccm. For the different
experiments, the flow rate of crosslinker was varied to 0, 0.2, 0.6
and 1 sccm, and a patch flow of gas carrier was introduced to keep
a total flow of 5 sccm. Thickness samples ranged from 10 nm to 3
.mu.m. FIG. 10 shows incorporation of DVB in the copolymer
film.
iCVD Deposition Experiment B--Grafted p(PFDA-Co-DVB)
[0149] To deposit a silane adhesion layer prior to grafted iCVD
polymerization, substrates were first cleaned by sonication in
acetone for 5 minutes, followed by rinsing in DI water (18 MOhm),
followed by sonication in isopropanol for 5 minutes, and finally a
rinse with DI water. The surfaces were treated with oxygen plasma
for 10 minutes for further cleaning and for creating surface
hydroxyl groups. After plasma treatment, the surfaces were
immediately placed in a vacuum desiccator along a small open vial
containing 100 .mu.L of either trichlorovinylsilane (97%, Sigma
Aldrich) as a grafting precursor for the polymer films. The chamber
was pumped down to 200 mTorr, and the chamber was isolated to allow
the silane to vaporize. The chamber was purged twice more, then
isolated. The silane was allowed to vaporize and react with the
substrate for 2 hours. After deposition, the surfaces were
sonicated in toluene to remove excess unreacted silane and rinsed
with DI water.
[0150] iCVD polymerizations were conducted in a custom-design
cylindrical reactor (diameter 24.6 cm and height 3.8 cm),
supporting an array of 14 parallel chromoalloy filaments
(Goodfellow) suspended 2 cm from the stage. Tert-butyl peroxide
(TBPO) (98%, Aldrich), PFDA (97%, Aldrich), and DVB (80%, Aldrich)
were used as received. The peroxide initiator, TBPO, was delivered
into the reactor through a mass flow controller (MKS Instruments)
at a constant flow rate of 3.2 sccm. PFDA and DVB were vaporized in
glass jars that were heated to 80 and 60.degree. C., respectively.
The flow rates were controlled using needle valves and kept
constant at 0.2 and 0.6 sccm. The filaments were resistively heated
to 230.degree. C. using a DC power supply (Sorensen), and the
temperature was measured by a K-type thermocouple (Omega
Engineering). The sample stage was backcooled at 30.degree. C.
using a recirculating chiller/heater (Neslab RTE-7). The working
pressure was maintained at 200 mTorr using a throttle valve (MKS
Instruments). The reactor was covered with a quartz top (2.5 cm)
that allowed for in-situ thickness monitoring by interferometry
with a 633 nm HeNe laser source (JDS Uniphase). Final thickness of
the copolymer deposited on the metal substrate corresponded to 40
nm. Afterwards, a thermal annealing process was performed by
introducing the sample in an oven (VWR) at 80.degree. C. for 30
min. The full width at half-maximum (FWHM) was fixed at 2-3 eV to
take into account the broadening of the 1 eV electron beam, while
using XPS Scienta Database F1s peaks with FWHM of 2 eV.
iCVD Deposition Experiment C--Annealing
[0151] This Example characterizes samples that were prepared via
iCVD deposition of p(PFDA-co-DVB) on silicon substrates before and
after annealing. iCVD films were prepared in the same manner
described in iCVD Deposition Example--p(PFDA-co-DVB, and then
further characterized by AFM. After iCVD deposition, samples were
annealed in a furnace at 80.degree. C. for 30 min and characterized
again by AFM. Referring now to FIG. 9, we observe that after
annealing, the quadratic mean roughness of all surfaces decreases,
indicating an increase in the degree of crystallinity in the case
of the PFDA homopolymer and an increase in the degree of
crosslinking in the cases of the DVB-crosslinked copolymers.
Referring now to FIG. 17 showing a comparison of XRD spectra of
PFDA homopolymer and p(PFDA-co-DVB) films before and after thermal
annealing, we also observe an increase in the degree of
crystallinity of PFDA homopolymer as evidenced by the increased
area under the curve corresponding to intensity vs. 20, and a
decrease in degree of crystallinity of the crosslinked polymers as
evidenced by a decrease in the area under the curve corresponding
to intensity vs. 20.
iCVD Deposition Experiment D--Eco-Friendly pC6PFA-Co-DVB
[0152] In this Example, films of varying compositional ranges of
1H,1H,2H,2H-perfluorooctyl acrylate) (pC6PFA; C6) and
divinylbenzene (DVB) were deposited via iCVD on silicon wafer
substrates. Flowrates of monomer and initiator species and nitrogen
patch flow are indicated in Table 5 below.
TABLE-US-00005 TABLE 5 Nomenclature and flow rates of precursors
Flow rate (sccm) Sample C6PFA DVB TBPO N.sub.2 C0 0.2 0 1.2 1.6 C1
0.2 0.2 1.2 1.4 C2 0.2 0.4 1.2 1.2 C3 0.2 0.6 1.2 1 C4 0.2 1 1.2
0.6
[0153] The Fourier transform infrared spectroscopy (FT-IR) spectra
of the films are shown in FIG. 21. pC6PFA homopolymer gives a sharp
band due to carbonyl group at 1743 cm.sup.-1. The two bands at 1237
and 1204 cm.sup.-1 are caused by the asymmetric and symmetric
stretching of the --CF.sub.2-- moiety, respectively. The sharp band
at 1146 cm.sup.-1 is caused by the --CF.sub.2--CF.sub.3 end group.
The pDVB homopolymer FT-IR spectrum shows the --CH.sub.2--
stretching bands at 2871 cm.sup.-1, confirming the formation of
backbone. The aromatic --CH-- contribute to bands between 3000 and
3100 cm.sup.-1. The bands between 700 and 1000 cm.sup.-1 are
characteristics of substituted phenyl groups. The band at 903
cm.sup.-1 results from unreacted vinyl groups. The copolymer
presents all the characteristic bands associated with its
components. The FT-IR results show the incorporation of the two
monomers into the copolymer film and the retention of the chemical
functionality from both reactants after the polymerization.
[0154] The effects of DVB crosslinking on CAH were studied by WCA
measurements (FIG. 22). pC6PFA homopolymer film presents high
static WCA and advancing WCA, but low receding WCA. This behavior
of pC6PFA surface has been well explained: in its dry state, the
fluoroalkyl side chains orient to the outermost surface layer due
to phase segregation between hydrogenated and fluorinated moieties.
Surface reorganization occurs in presence of water, leading to
surface exposure of hydrophilic moieties. The econstruction easily
happens because pC6PFA is unable to form crystalline structure. In
contrast, p(C6PFA-co-DVB) films show improved dynamic water
repellency. The receding WCA of all copolymer films are
significantly enhanced. The movement of water front can be affected
by surface roughness, heterogeneity, reorientation, and mobility.
The AFM observation of films shows that the differences in
roughness are not significant enough to influence the WCA
hysteresis. Therefore the results suggest that the crosslinking of
DVB units hinders the reorientation of surface fluorine groups. It
is hypothesized here that the DVB units have two effects, on main
chain and side chain respectively, contributing to the restrain of
fluorine groups (Fig. X): first, the rigid crosslinker can reduce
the flexibility of main chain, reducing the T.sub.g; second, the
planar crosslinker can sterically mitigate side chain
reconstruction by reducing free volume.
Film Deposition Experiment--Effect of Spacer Groups
[0155] Do demonstrate the ability of the spacer group to affect the
rigidity and thus CAH of deposited films, thin films of
1H,1H,2H,2H-perfluorooctyl acrylate
(C6PFA)Poly(2-(Perfluorohexyl)ethyl methacrylate) (pC6PFMA)
[N-methyl-perfluorohexane-1-sulfonamide] ethyl acrylate (C6PFSA)
and [N-methyl-perfluorohexane-1-sulfonamide] ethyl (meth) acrylate
(C6PFSMA) were spin-coated onto silicon substrates. Advancing and
receding contact angles and CAH are shown in FIG. 24, indicating
that the additional dipole-dipole interactions afforded by the
spacer group of pC6PFSMA act to significantly reduce the CAH as
compared to pC6PFA as shown in FIG. 23.
Film Characterization Experiment a--XPS Spectra
[0156] FIG. 5 (left) shows the high-resolution C1s X-ray
photoelectron spectra (XPS) of the iCVD p(PFDA-co-DVB) copolymer
surface. The pendant groups from the PFDA consist of --CF.sub.2--
and --CF.sub.3-- and these two bonding environments can be readily
resolved at 290.8 and 293.1 eV, respectively. In aggregate, these
fluorinated carbon groups account for 61.8.+-.0.4% of the area of
the spectrum. The assignments at lower binding energies represent
carbon items directly bonded only to oxygen, hydrogen, or other
carbon atoms. However, the precise assignments of the peaks at
lower binding energy is ambiguous due to the multitude of
environments arising from the main acrylate portion of the PFDA and
from the DVB.
[0157] The --CF.sub.2-- and --CF.sub.3-- bonding environments were
previously observed in C1s XPS spectrum of the iCVD PFDA
homopolymer, representing a combined area of 61.4.+-.0.3% and in
agreement with the structural formula for PFDA which gives a
theoretical value of 61.5%. The similarity with homopolymer results
suggests the degree of DVB crosslinker incorporation in the
copolymer in the near-surface region probed by XPS is quite low.
Thus, the surface properties of the copolymer in the dry state,
such as the advancing contact angle, will be dominated by the PFDA
units. When examined by Fourier transform infrared spectra (FTIR),
which penetrates the entire film thickness, sp.sup.2C--H stretching
modes between 2810 and 2890 cm.sup.-1 were observed, confirming the
incorporation of the DVB in the bulk of the film. These underlying
crosslinking units are anticipated to reduce the ability of the
surface layer to reconstruct between the dry and wet states,
potentially reducing this contribution to contact angle hysteresis.
By following a deposition of PFDA:DVB 0.2:0.6 sccm with a thermal
annealing step, the advancing and receding water contact angles on
the resultant thin film are 132.degree..+-.1.degree. and
127.degree..+-.1.degree., respectively, with a CAH of 5.degree..
Average film thicknesses were measured by ellipsometry, AFM, and
contact profilometry to be 41.5.+-.2.4 nm. AFM scans (FIGS. 1d and
1e) illustrate that the surface is covered by structures with a
height of ca. 100 nm and an average spacing of 1.3.+-.0.7 .mu.m,
resulting in an RMS roughness of 75 nm. These rough features are
semicrystalline aggregates formed at nucleation sites during the
condensation polymerization reaction of the monomers.
[0158] Previous literature has shown that --(CF.sub.2)--CF.sub.3
chains with n.gtoreq.leads to aggregates in a smectic B structure
that arrange into a rotationally symmetric fiber texture. On the
other hand, the fluorosilane surface, which is composed of larger,
less sterically-hindered functional groups with a thickness of 2.5
nm and an RMS roughness of 1.5.+-.0.3 nm, exhibited a CAH of
25.degree..+-.3.degree.. Since the roughness of the silanized
surface is lower than that of the copolymer surface, morphology
alone cannot explain the lower hysteresis of the copolymer surface.
Instead, this may be attributed to the steric hindrance induced by
the crosslinking that prevents the --CF.sub.3 groups from shifting
away from their low-energy unwetted state.
Film Characterization Experiment B--Film Thickness Measurements
[0159] Film thicknesses were measured with variable-angle
ellipsometric spectroscopy (VASE, M-2000, J. A. Woollam) and by
measuring scratch step height with atomic force microscopy (AFM,
MP3D-SA, Asylum) and contact profilometry (Model 150, Dektak). All
VASE thickness measurements were performed at a 70.degree.
incidence angle using 190 different wavelengths from 315 to 718 nm.
A nonlinear least-squares minimization was used to fit
ellipsometric data of dry films to the Cauchy-Urbach model. The
thickness was obtained upon convergence of the algorithm. FTIR
measurements were performed on a Nicolet Nexus 870 ESP spectrometer
in normal transmission mode equipped with a MCT (mercury cadmium
tellurium detector and KBr beamsplitter. Spectra were acquired over
the range of 400 to 4000 cm.sup.-1 with a 4 cm.sup.-1 resolution
for 256 scans. All AFM thickness measurements were performed in
tapping mode over an area of 20 .mu.m.times.20 .mu.m using a
cantilever with a tip radius of 9.+-.2 nm (AC200TS, Asylum). The
film thickness was calculated as the difference between the average
heights of the rough film surface and the trough of the scratch;
the rough built-up edge of the scratch was masked from analysis.
The profilometry measurements were performed with a stylus having a
radius of 12.5 .mu.m. The film thickness was similarly calculated
as the difference in the average height of the rough film and the
smooth scratch trough. AFM and profilometry measurements were
repeated on at least four locations. Film thickness is reported as
the mean and standard deviation of all measurements.
Film Characterization Experiment C--Surface Roughness
Measurements
[0160] Surface roughness was measured using atomic force microscopy
(AFM, MP3D-SA, Asylum) in tapping mode. The advancing and receding
contact angles were measured using a goniometer (Model 590
Advanced, rame-hart). The hysteresis was also measured during
condensation on the grafted polymer sample as the difference
between the receding and advancing ends of a drop immediately
before departure. Contact angles during condensation on the
silanized sample could not be measured due to the film covering the
surface.
Dropwise Condensation Experiment A--Nucleation and Shedding
Comparison
[0161] In addition to CAH, the dropwise condensation heat transfer
depends on a number of complex factors including nucleation site
density and population distribution. To investigate the behavior of
these surfaces during condensation, saturated pure water vapor at
800 Pa was condensed while cooling the surface with a Peltier
device to a supersaturation of 1.16.+-.0.05 and imaging with an
environmental scanning electron microscope (ESEM). 2 mm.times.2 mm
sample substrates were secured to an aluminum stub with
double-sided carbon adhesive and instrumented with a K-type
thermocouple embedded into the tape. The aluminum stub was clamped
into a Peltier cooling stage (Coolstage Mk 2, Deben) which was
attached to the stage of an environmental scanning electron
microscope (EVO 55, Zeiss). The chamber was purged with water vapor
three times up to 3 kPa and down to 10 Pa to remove non-condensable
gases. After purging, the pressure was held at 800 Pa, and the
temperature was slowly decreased at a rate of 0.5 K min.sup.-1
until formation of observable water droplets (>1 .mu.m
diameter). Accelerating voltage was 20 kV and beam current was 100
nA. Images were recorded at ca. 1 Hz, and the stage was moved to
different areas to avoid charging effects on nucleation. Nucleation
densities were measured as the mean and standard deviation of at
least five different locations on each surface. During the
pre-coalescence growth regime, it was observed that the nucleation
density on a p(PFDA-co-DVB) surface (173.+-.19 mm.sup.-2, as shown
in FIG. 2a) was significantly higher than that on a fluorosilane
surface (110.+-.10 mm.sup.-2, as shown in FIG. 2b)--owing at least
in part to the rougher surface providing a larger number of
concavities that act as nucleation sites. During condensation of an
air stream saturated with water vapor under ambient conditions
(21.degree. C., 40% relative humidity), the departing diameter was
2.0.+-.0.3 mm (as illustrated in FIG. 2c). This is considerably
smaller than the departing drop sizes on other common hydrophobic
modifiers such as gold (3.3 mm) and oleic acid (4.3 mm). When
compared to a silanized silicon surface with a departing diameter
of 2.9.+-.0.2 mm (as shown in FIG. 2d), a shift was also observed
in the distribution of droplet diameters to smaller sizes (as shown
in FIG. 2e). The increased nucleation density, lower departure
diameter, and droplet size distribution of the copolymer surface on
a smooth silicon substrate indicate an improved condensation heat
transfer coefficient according to widely-accepted models.
Dropwise Condensation Experiment B--Aluminum Substrate
Experiment
[0162] Commercial condensers are typically constructed using alloys
of metals such as titanium, stainless steel, copper, and aluminum.
To test a prototype that was most similar to an industrial
condenser, a 40 nm film of p(PFDA-co-DVB) was grafted onto 50 mm
diameter aluminum substrates via iCVD. The additional roughness
imparted by the metal surface (RMS=118.+-.33 nm) was apparent in
the AFM height scans (shown in FIGS. 3a and 3b). As expected on a
rougher surface in a Wenzel state, the CAH measured during
condensation at 6.9 kPa was similar (37.degree..+-.5.degree.) and
accordingly, the size of a departing drop (4.2.+-.0.1 mm) was
larger than that on a silicon substrate (as shown in FIG. 3c).
Dropwise Condensation Experiment C--Effect of Grafting
[0163] In this Example, coated substrates were tested for
condensation performance in the apparatus described below and shown
in FIG. 6. The flow loop of the test apparatus is shown in FIG. 7.
Saturated steam is produced by an electric boiler using deionized
feedwater with a resistivity of 5 MOhm that is further passed
through a membrane vacuum degassifier to reduce dissolved oxygen to
below 1 ppm. The steam is produced at 380 kPa and passes through a
pressure regulator and a separator to the condensing chamber, which
is evacuated before each test by a rotary vane vacuum pump. The
sample is cooled by a heat exchanger operating at 60 psig with 1
MOhm deionized chilled water at 4.degree. C.
[0164] Condensing specimens coated with p(PFDA-co-DVB) were secured
in a chamber with the coated side exposed to saturated steam and
the other side cooled by running water, in FIG. 6. The chamber was
initially evacuated to remove non-condensable vapors, and steam was
introduced at a variable rate to maintain pressures ranging from 10
kPa to 100 kPa. Saturated steam was produced by an electric boiler
using deionized feedwater with a resistivity of 5 MOhm that was fed
through a degassifier to reduce dissolved oxygen to below 1 ppm.
The rear side of the sample was cooled by a forced chilled water at
4.degree. C. Temperature gradients within the specimens were
measured by thermistors embedded at precise locations within the
specimen. The heat transfer coefficient could be determined from
the temperature gradient and the surface temperature. After several
hours of operation, the coated specimens exhibited an improved heat
transfer coefficient.
[0165] FIGS. 8a-8b show (a) Grafted PFDA and (b) ungrafted PFDA
samples after 1 hour of condensation in saturated steam at
90.degree. C. and 70 kPa. FIGS. 8c-8d also show condensate drops on
(c) grafted and (d) ungrafted PFDA surfaces after 10 minutes of
condensing saturated steam. The distorted drop shape on the
ungrafted sample indicates severe contact line pinning following
delamination of the polymer film. Departing drop sizes on ungrafted
sample were 3.1 mm, compared to 2.3 mm for the grafted surface.
Heat transfer coefficient was measured at 31.+-.2 kW/m.sup.2K at
beginning of test, 23.+-.2 kW/m.sup.2K after deterioration of
ungrafted surface. This example illustrates how covalent grafting
can significantly improve the adhesion of the polymer films on
metal substrates and increase their durability in the presence of
condensing steam.
Dropwise Condensation Experiment D--Film Thickness & Heat
Transfer Coefficient
[0166] Referring now to FIG. 1a, monomer and initiator species are
flowed into a reactor at controlled rates, where the monomer and
initiator species encounter heated filaments and a cooled
substrate, as shown in FIG. 1b. The locally heated zone around the
filaments thermally cleaves the initiator species (tert-butyl
peroxide, TBPO). The produced radical fragments initiate vinyl
polymerization of the monomers absorbed on the surface, which is
held at a lower temperature. The functional groups, such as the
perfluorinated side chain of PFDA, are fully preserved after
polymerization.
[0167] The film thickness is measured in-situ during deposition, so
that the process can be stopped when the thickness reaches the
desired value. In some embodiments, the iCVD copolymer layers are
ultra-thin (.about.40 nm), leading to an estimated contribution to
total thermal resistance of less than 0.001%. To verify that the
film thickness did not have an effect on the condensation heat
transfer coefficient, two different thicknesses of films were
measured, with the results being provided in Table 6 below. As seen
in Table 6 below, the condensation heat transfer coefficients of
the two film thicknesses are nearly identical.
TABLE-US-00006 TABLE 6 Effect of Film Thickness on Heat Transfer
Coefficient Thickness (nm) h (k W m.sup.-2 K.sup.-1) 41.5 .+-. 2.4
38.1 .+-. 4.0 59.2 .+-. 6.6 39.5 .+-. 4.2
Dropwise Condensation Experiment E--Prolonged Exposure
Experiment
[0168] Accelerated endurance tests were conducted by condensing
pure saturated steam at 100.degree. C., Coatings of p(PFDA-co-DVB)
were compared to fluorosilane coatings, both on aluminum substrates
(shown in FIGS. 3c and 3d). FIG. 3e shows a comparison of these two
surfaces, along with an uncoated aluminum surface that undergoes
filmwise condensation for reference, under prolonged condensation
at 103.4 kPa. Although the silanized surface initially displayed a
larger heat transfer coefficient of 61.+-.2 kW m.sup.-2 K.sup.-1
due to the lower hysteresis (31.degree..+-.3.degree.) and departing
droplet size (3.6.+-.0.4 mm), it quickly degraded in a matter of
minutes and exhibited dropwise condensation with a heat transfer
coefficient of 4.6.+-.0.4 kW m.sup.-2 K.sup.-1. The grafted polymer
coating exhibited dropwise condensation with a departing droplet
size of 4.2.+-.0.1 mm and a heat transfer coefficient greater than
35 kW m.sup.-2 K.sup.-1, which was more than 7 times greater than
the steady-state filmwise heat transfer coefficient of the degraded
silanized surface, with no noticeable degradation after 48 hours of
condensation.
[0169] Grafted polymers deposited via iCVD lead to robust dropwise
condensing surfaces that can sustain prolonged exposure (e.g.,
>48 hours) to steam at 100.degree. C., significantly
outperforming a fluorosilane treatment tested under identical
conditions. A simple first-order exponential to fit to the
degradation of the heat transfer coefficients results in
degradation time constants of ca. 2 minutes and O.apprxeq.10.sup.4
hours for fluorosilane and grafted copolymer surfaces,
respectively. Thermal degradation of films deposited using the iCVD
process has previously been tested and described by a logistic
model. Since degradation under a steam environment is an entirely
different process, and fitting to the logistic model would require
knowledge of the time required to degraded to 50%, there is a
further need for longer-duration endurance tests. The unique
composition of the copolymer achievable iCVD is essential for
achieving low contact angle hysteresis, which results from the
combination of low roughness and limited reorientation of the
surface fluorinated groups between the wet and dry states. iCVD
surfaces exhibit heat transfer coefficients that are more than 7
times greater than filmwise condensation when deposited on
practical engineering heat transfer substrates, such as aluminum
and copper. A successful industrial prototype has been demonstrated
and successfully tested, indicating scalability to industrial
processes.
Dropwise Condensation Experiment F--Tubing Coil Experiment
[0170] As a further demonstration of the versatility of
iCVD-deposited copolymers to coat complex shapes such as heat
exchanger tubing, a 40 nm thin film of p(PFDA-co-DVB) was grafted
conformally onto the outer surface of a copper tubing coil. It
would have been exceedingly difficult to achieve such an
ultra-thin, uniform layer by common surface modification techniques
such as spray coating, spin casting and/or doctor blade
application, and/or with vacuum techniques such as sputtering
and/or evaporation. As shown in FIGS. 4a-4b, the tubing coil
exhibited prolonged dropwise condensation after a single-step
deposition.
Dropwise Condensation Experiment G--Hydrocarbon Condensation
Experiment
[0171] To demonstrate the ability of a grafted iCVD surface to
promote dropwise condensation of low-surface tension fluids, a
silicon substrate coated with a thin film of p(PFDA-co-DVB) was
fixed in a custom-built vacuum chamber shown in FIG. 6 such that
the surface was held vertically. Hydrocarbon vapors were supplied
by a container filled with 30 mL of either pentane or hexane, and
immersed in a water bath (Julabo FP-25) heated to 40.degree. C. The
vacuum chamber was purged three times below 0.1 kPa and above 50
kPa with pentane vapor to remove non-condensables. After purging,
the rear side of the surface was cooled with forced chilled water
to a temperature of around 10.degree. C. The hydrocarbon vapor
pressure was increased by opening a needle valve until the
corresponding saturation temperature was greater than 10.degree.
C., thus initiating condensation of hydrocarbon vapor onto the
chilled copolymer surface. FIGS. 20a and 20b show snapshots of
dropwise condensation of hexane on a copolymer film. Hexane CAH and
departing diameter are also shown in FIG. 20b. Heat transfer
coefficients during condensation of pentane vapor were measured by
thermistors embedded behind the surface. The condensation heat
transfer coefficient of pentane was 22.5 kW/m.sup.2K, condensing at
a pressure of 52.0 kPa saturation temperature 17.7.degree. C.,
surface temperature 17.4.degree. C., and a heat flux of 7.3
kW/m.sup.2.
EQUIVALENTS
[0172] While the invention has been particularly shown and
described with reference to specific preferred embodiments, it
should be understood by those skilled in the art that various
changes in form and detail may be made therein without departing
from the spirit and scope of the invention as defined by the
appended claims.
* * * * *
References