U.S. patent application number 16/854576 was filed with the patent office on 2020-08-20 for passive anti-frosting surface comprised of microscopic wettability patterns containing sacrificial ice.
This patent application is currently assigned to Virginia Tech Intellectual Properties, Inc.. The applicant listed for this patent is Virginia Tech Intellectual Properties, Inc. UT-BATTELLE, LLC. Invention is credited to Caitlin Bisbano, Jonathan B. Boreyko, C. Patrick Collier, Ryan Hansen, Grady J. Iliff, Saurabh Nath.
Application Number | 20200262568 16/854576 |
Document ID | 20200262568 / US20200262568 |
Family ID | 1000004808687 |
Filed Date | 2020-08-20 |
Patent Application | download [pdf] |
United States Patent
Application |
20200262568 |
Kind Code |
A1 |
Boreyko; Jonathan B. ; et
al. |
August 20, 2020 |
Passive Anti-frosting Surface Comprised of Microscopic Wettability
Patterns Containing Sacrificial Ice
Abstract
A method and device for reducing ice and frost on a surface
comprising a wettable pattern on a surface. The pattern is wetted
with water which is frozen into ice to create overlapping
hygroscopic that cover the surface.
Inventors: |
Boreyko; Jonathan B.;
(Christiansburg, VA) ; Nath; Saurabh; (Blacksburg,
VA) ; Bisbano; Caitlin; (Blacksburg, VA) ;
Iliff; Grady J.; (Hamilton, VA) ; Hansen; Ryan;
(Manhattan, KS) ; Collier; C. Patrick; (Oak Ridge,
TN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Virginia Tech Intellectual Properties, Inc.
UT-BATTELLE, LLC |
Blacksburg
Oak Ridge |
VA
TN |
US
US |
|
|
Assignee: |
Virginia Tech Intellectual
Properties, Inc.
Blacksburg
VA
UT-BATTELLE, LLC
Oak Ridge
TN
|
Family ID: |
1000004808687 |
Appl. No.: |
16/854576 |
Filed: |
April 21, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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15724995 |
Oct 4, 2017 |
10661908 |
|
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16854576 |
|
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62403924 |
Oct 4, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B05D 1/38 20130101; F28F
19/02 20130101; F28F 19/006 20130101; B64D 15/06 20130101; B05D
2350/33 20130101; F25D 21/04 20130101; B05D 7/24 20130101; F25C
1/06 20130101; F25B 47/006 20130101; F28F 2245/02 20130101 |
International
Class: |
B64D 15/06 20060101
B64D015/06; F28F 19/02 20060101 F28F019/02; F28F 19/00 20060101
F28F019/00; B05D 7/24 20060101 B05D007/24; B05D 1/38 20060101
B05D001/38; F25C 1/06 20060101 F25C001/06; F25B 47/00 20060101
F25B047/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &
DEVELOPMENT
[0002] This invention was made with government support under
Contract No. DE-AC05-000R22725 awarded by the U.S. Department of
Energy. The government has certain rights in the invention.
Claims
1. (canceled)
2. (canceled)
3. (canceled)
4. (canceled)
5. (canceled)
6. (canceled)
7. (canceled)
8. (canceled)
9. (canceled)
10. (canceled)
11. (canceled)
12. (canceled)
13. (canceled)
14. (canceled)
15. A method of reducing ice and frost on a surface comprising the
steps of: providing a pattern of stripes on said surface; filling
said stripes with water; freezing said water in said stripes to
form ice in said stripes; said ice in said stripes creates
overlapping hydroscopic zones; and said hydroscopic zones overlap
to cover the entire surface of said substrate.
16. The method of claim 15 wherein said hydroscopic zones are
in-plane.
17. The method of claim 16 wherein said hydroscopic zones keep
siphoning nearby water vapor, keeping the rest of the surface
completely dry from condensation and frost.
18. The method of claim 17 wherein said stripes are spaced apart,
said spacing between stripes is less than twice the value of a
hydroscopic zone.
19. The method of claim 17 wherein said stripes are formed into
linear arrays.
20. The method of claim 19 wherein said linear arrays include fins
having grooves thereon, said fins having varying heights and
varying spaced apart distances.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. Ser. No.
15/724,995 filed on Oct. 4, 2017, which claims the benefit of U.S.
Provisional Application No. 62/403,924, filed Oct. 4, 2016, both of
which are herein incorporated by reference.
INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT
DISC
[0003] Not applicable.
BACKGROUND OF THE INVENTION
[0004] Accretion of ice and frost on infrastructure is a
multi-billion-dollar problem that adversely affects multiple
industries worldwide, including aviation, electrical transmission,
hydropower and almost all modes of transportation. For instance,
the dynamic characteristics of aircraft flight can be significantly
affected by ice accumulating on the airplane wings, resulting in
severe damage and even plane crashes.
[0005] Frost also accumulates on refrigerators and heat exchangers.
It has been found that this may reduce their heat transfer
efficiency by as much as 50-75%.
[0006] Ice accretion on wind turbines can cause significant
reduction in aerodynamic efficiency, with power losses up to
50%.
[0007] However, despite unprecedented advances in the fields of
surface chemistry and micro/nanofabrication, no engineered surface,
to date, has been able to passively suppress the in-plane growth of
frost occurring in humid, subfreezing environments.
BRIEF SUMMARY OF THE INVENTION
[0008] In one embodiment, the present invention creates an
anti-frosting surface which maximizes the dry region where no
condensation and frost forms, but would also be cost-effective and
environment-friendly.
[0009] In another embodiment, the present invention, using ice
itself, creates stable dry zones that are free from supercooled
condensation and frost even in humid environments. Ice has a
depressed vapor pressure relative to supercooled liquid water,
which creates a dry zone around ice where no condensation or frost
can grow. However, unlike other hygroscopic materials which get
increasingly diluted with condensed water, ice is composed solely
of water molecules and therefore its low vapor pressure remains
stable as it harvests water vapor from the ambient. By spacing
microscopic arrays of ice in such a way that the in-plane dry zone
about each ice strip overlap, even macroscopically, the surface can
remain largely frost-free over time.
[0010] In another aspect, the present invention creates microscopic
arrays of sacrificial ice by chemical micropatterning to create a
wettability pattern that creates arrays of water stripes.
[0011] In another aspect, the present invention creates microscopic
arrays of sacrificial ice using physical microgrooves. These
embodiments may be used in combination or separately.
[0012] In other embodiments, the present invention provides a
wettability pattern that creates arrays of water strips.
[0013] In other embodiments, the present invention provides a
wettability pattern that creates arrays of water stripes. The water
stripes can subsequently be frozen by chilling beneath 0.degree.
C., by electrofreezing or by contact with an external piece of ice.
The ice stripes serve as intermittent humidity sinks on the
substrate creating overlapping dry zones that keep the surface
frost-free.
[0014] In another embodiment, the present invention provides a
passive anti-frosting surface technology, where chilled substrate
stays dry from dew and frost under highly supersaturated
conditions.
[0015] In other embodiments, the present invention comprises an
array of small metallic fins that run along the surface, where the
top edges of the fins are roughened to enable preferential wicking
of water `stripes` along each fin.
[0016] In other aspects, the embodiments of the present invention
prevent condensation and frost from forming elsewhere on the
surface. The enabling mechanism is the depressed (hygroscopic)
saturation vapor pressure of ice compared to supercooled liquid
water at the same temperature such that the embodiments act as
humidity sinks that may be overlapping, and function to siphon
nearby moisture from the air.
[0017] In other embodiments, the present invention provides
sacrificial ice stripes that may grow over time on the surface in
the out-of-plane direction.
[0018] In other embodiments, the present invention provides methods
that passively suppress frost indefinitely on a chilled surface
under supersaturated conditions.
[0019] In other embodiments, the present invention provides
passive-anti frosting methods that keep a surface predominantly dry
from condensation and frost without requiring the active input of
chemicals, heat, mechanical forces, wind, or electricity.
[0020] The embodiments of the present invention provide methods and
devices that exploit the hygroscopic nature of ice for
anti-frosting applications itself. The fact that ice itself has
hygroscopic properties that can be tapped into for anti-frosting
itself.
[0021] In certain aspects, the embodiments of the present invention
utilize dilute arrays of hygroscopic ice stripes to mitigate the
use of salts and other harmful chemicals by the fact that: (1) ice
is environmentally benign, and (2) as ice harvests water vapor it
remains pure ice, so it is the only hygroscopic material that does
not degrade over time.
[0022] In certain aspects, the embodiments of the present invention
provide a method of reducing ice and frost on a surface comprising
the steps of: providing a pattern of channels on said surface;
filling said channels with water; freezing said water in said
channels to form ice in said channels; said ice in said channels
creates overlapping hygroscopic zones; and said hygroscopic zones
overlap to cover the entire surface of said substrate.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0023] In the drawings, which are not necessarily drawn to scale,
like numerals may describe substantially similar components
throughout the several views. Like numerals having different letter
suffixes may represent different instances of substantially similar
components. The drawings illustrate generally, by way of example,
but not by way of limitation, a detailed description of certain
embodiments discussed in the present document.
[0024] FIG. 1A is a top-down and side-view of a microscopic pattern
of interconnected hydrophilic stripes for an embodiment of the
present invention.
[0025] FIG. 1B is a top-down and side-view of the embodiment shown
in FIG. 1A that has been wetted with water.
[0026] FIG. 1C is a top-down and side-view of the embodiment shown
in FIG. 1B where the water has subsequently frozen into ice.
[0027] FIG. 1D is a top-down and side-view of the embodiment shown
in FIG. 1C showing that the entire surface area between the ice
stripes remain substantially dry from condensation and frost when
the spacing between stripes is less than twice the value of a
single dry zone for an embodiment of the present invention.
[0028] FIG. 2 is an isometric view of the microgrooves connected to
the water reservoir which would fill the grooves to enable the
freezing of the stripes together by forming contact with an
external ice for an embodiment of the present invention.
[0029] FIG. 3 shows a side view of the elevated microgrooves, some
being flush with the surface and some as high as 1 mm in height for
an embodiment of the present invention.
[0030] FIG. 4 is an isometric view of the entire aluminum plate
with microgrooves of different elevations and inter-stripe
distances for an embodiment of the present invention.
[0031] FIG. 5 depicts frozen 10 .mu.m water stripes with 1 mm
inter-stripe distances for an embodiment of the present
invention.
[0032] FIG. 6A shows micro-grooves along the top of each fin for an
embodiment of the present invention.
[0033] FIG. 6B shows how water preferentially wicks along the tops
of the fins.
[0034] FIG. 6C shows how water freezes into ice.
[0035] FIG. 6D shows the hygroscopic ice stripes siphoning water
vapor, keeping the rest of the finned surface dry from condensation
and frost.
[0036] FIG. 7A is a micrograph of how a millimetric frozen droplet
creates a dry zone around itself on a substrate kept at
Tw=-12.5.degree. C., air temperature being T.infin.=17.4.degree. C.
and humidity RH=21%.
[0037] FIG. 7B shows a saturation vapor pressure of water and ice
plotted against temperature. The saturation vapor pressure
difference peaks at T=-12.5.degree. C. It is this vapor pressure
difference that causes ice to behave as a humidity sink, creating
the dry zone.
[0038] FIG. 8A depicts sacrificial fin tops on an aluminum surface
with the fins acting to keep the aluminum surface completely dry
from condensation and frost even after 24 hours of exposure to
supersaturated conditions for an embodiment of the present
invention.
[0039] FIG. 8B shows how a smooth aluminum surface is completely
frosted over in under 1 hour under the identical environmental
conditions.
[0040] FIG. 8C is a plot of frost-free area over time for regular
aluminum surface (iii) and an embodiment of the present invention
(i) and (ii). The data points of (i) correspond to flat bare
aluminum surface between the ridges, which is a critical region of
interest, as shown in (i). Note that this part of the surface
develops no frost whatsoever since the data points are constant at
100%. The (ii) data points correspond to the top-down projected
area including the ridges. The decreasing nature of the (ii) data
points is because the frost atop of the ridges coarsen over time.
However, this frost could not reach the aluminum floors at the
bottom between the ridges, that is, region (i) at any point of
time.
DETAILED DESCRIPTION OF THE INVENTION
[0041] Detailed embodiments of the present invention are disclosed
herein; however, it is to be understood that the disclosed
embodiments are merely exemplary of the invention, which may be
embodied in various forms. Therefore, specific structural and
functional details disclosed herein are not to be interpreted as
limiting, but merely as a representative basis for teaching one
skilled in the art to variously employ the present invention in
virtually any appropriately detailed method, structure or system.
Further, the terms and phrases used herein are not intended to be
limiting, but rather to provide an understandable description of
the invention.
[0042] As shown in FIGS. 1A and 2, in one embodiment, the present
invention may utilize photolithography to pattern in substrate 100
arrays of microscopic hydrophilic (bare silicon oxide) stripes
110-112 onto a hydrophobic (silane monolayer) backdrop. The pattern
may also be exposed to an oxygen plasma to restore the full
hydrophilicity of the silicon oxide features, followed by a dry
peel-off of the patterned parylene coating that was protecting the
hydrophobic monolayer. The hydrophilic stripes may have a width of
around 10-15 .mu.m and may be connected to a long hydrophilic water
pad 130 at the edge of the substrate as shown which holds water 150
as shown in FIG. 1B.
[0043] The water pad 150 serves two purposes: a) it can be used to
deposit a water drop which would then spread onto the hydrophilic
stripes and b) when the water pad is frozen by touching it with a
piece of ice it would also freeze all the water stripes together at
the same time. Another possible way of obtaining ice stripes is to
cool the substrate below the dew point so that condensation fills
the hydrophilic stripes. Freezing may also be induced in multiple
ways for the supercooled water pattern: by touching with ice,
electrofreezing, or by spontaneous heterogeneous ice
nucleation.
[0044] As shown in FIGS. 1A-1D, a microscopic pattern of
interconnected hydrophilic stripes or channels 110-112 are wetted
with water 150 and subsequently frozen into ice 170. The entire
surface area between the ice stripes 171-173 should remain dry from
condensation and frost when the spacing between stripes is less
than twice the value of a single dry zone.
[0045] Physical microgrooves patterns are known to be more robust,
durable, easy to fabricate and less expensive than chemical
microfabrication and can be done in a number of other ways such as
3D printing, molding, etc. The distance between two microgrooves is
varied with the maximum being 1 mm. These grooves are 15 .mu.m in
width and 25 .mu.m in depth and are connected to a water reservoir.
Some of these grooves are flush with the dry zone surface while
others may be elevated off the ground by different heights ranging
from 100 .mu.m to 1 mm. Elevating the microgrooves off the ground
serves several purposes. First, elevating the microgrooves off the
ground assists in preventing the ice stripes from growing in-plane
over the dry regions. Also, elevating assists in pushing the
boundary layer thickness higher, which in turn, serves to increase
the dry zone length about each ice stripe.
[0046] As shown in FIG. 3, elevated microgrooves 310-312, some
being flush with the surface (310) and some as high as 1 mm in
height (312). It is expected that the taller the pillars are with
the frost on top, the more pronounced would be the effect of dry
zone.
[0047] In an alternate embodiment, as shown in FIG. 4, a substrate
400 is provided which may be an aluminum plate. For this embodiment
of the present invention, a plurality of microgroove sets 410-416
in the form of linear arrays of channels is provided. The channels
may have different elevations and varying inter-stripe, channel, or
groove distances.
[0048] FIG. 5 depicts frozen 15 .mu.m water stripes with 1 mm
inter-stripe distances for an embodiment of the present invention.
The ice stripes keep the intermittent distances dry because of
overlapping dry zones. Experiment shows near 90% dry surface with
no frost or condensation at around T=-8.degree. C. and a
supersaturation S=1.2.
[0049] In a preferred method, the nucleation energy barrier for
condensation is lower for microgrooves than for flat surfaces which
causes preferential condensation in the grooves. An alternative way
for doing the same is by filling the water reservoir connected to
the grooves with water. The next step is to freeze the water
stripes all at the same time. This creates parallel arrays of ice
stripes than can have overlapping dry zones that keep the surface
macroscopically frost free.
[0050] A sample was put on a Peltier stage and placed inside a
humidity chamber. In order to obtain microscopic arrays of ice on
the hydrophilic regions, the temperature of the substrate was
brought down to -10.degree. C. Once supercooled condensation
completely wetted the hydrophilic stripes, the temperature was
sharply brought down to -30.degree. C. Approximately 5 s later, all
the hydrophilic stripes were frozen. The stage was brought to
around -8.degree. C., and the humidity was set to 21%.
Corresponding to air temperature of 16.6.degree. C., the
supersaturation was S=p.infin./pw=1.2, where P.infin. is the vapor
pressure in the ambient and Pw is the saturation vapor pressure
corresponding to the substrate temperature. The fact that S>1
implies that the substrate temperature was significantly below that
the dew point and hence the surface should exhibit condensation and
subsequent freezing. However, the entire sample barring the
sacrificial ice stripes was observed to be frost free for 12 mins.
The hydrophilic stripes were .about.20 .mu.m in width, while the
edge to edge separation between two ice stripes was 1 mm. This
implies that despite being in a subfreezing humid environment close
to 90% of the substrate was completely dry without any observable
condensation or frost whatsoever
[0051] FIGS. 6A-6D shown an alternate embodiment of the present
invention. For this embodiment, microgrooves 600-603 may be
machined into substrate 610. Reservoir 620 is also provided. By
machining micro-grooves along the top of each fin 630-633, water
preferentially wicks along the tops of the fins. Upon freezing into
ice 650 in chilled conditions, these hygroscopic ice stripes siphon
all nearby water vapor, keeping the rest of the finned surface
completely dry from condensation and frost as shown in FIGS. 6C and
6D.
[0052] In other embodiments, the fins may have the same height and
be equally spaced apart or be not equally spaced apart. In other
embodiments, the fins may have varying heights and be equally
spaced apart or be not equally spaced apart. The fins may also be
arranged in linear arrays.
[0053] The embodiments of the present invention follow directly
from the discovery that ice can evaporate liquid water droplets
around itself, creating a dry zone 700, where no condensation or
frost can grow as shown FIG. 7A. The underlying mechanism is that
the saturation vapor pressure of ice is lower than that of water at
the same subzero temperature, causing ice to behave as a humidity
sink as shown in FIG. 7B.
[0054] FIG. 8A depicts sacrificial fin tops on an aluminum surface
staying completely dry from condensation and frost even after 24 hr
of exposure to supersaturated conditions for an embodiment of the
present invention. FIG. 8B shows a smooth aluminum surface is
completely frosted over in under 1 hr under the identical
environmental conditions.
[0055] FIG. 8C is a plot of frost-free area over time for regular
aluminum surface (iii) and an embodiment of the present invention
(i) and (ii). The data points of (i) correspond to flat bare
aluminum surface between the ridges, which is a critical region of
interest, as shown in (i). Note that this part of the surface
develops no frost whatsoever since the data points are constant at
100%. The (ii) data points correspond to the top-down projected
area including the ridges. The decreasing nature of the (ii) data
points is because the frost atop of the ridges coarsen over time.
However, this frost could not reach the aluminum floors at the
bottom between the ridges, that is, region (i) at any point of time
because ice 811 and 813 create hygroscopic zones 815 and 817 that
form overlap 819 to reduce ice formation on substrate 823.
[0056] In yet other embodiments of the present invention,
micro-milling was employed to create an array of fins on an
aluminum substrate that were 1 mm tall, about 200 .mu.m wide, and
spaced 1 mm apart from each other. Subsequently, 15 .mu.m
micro-grooves were cut into the middle of the top of each fin. By
having all these grooves feed into a connecting mini reservoir, the
array of water stripes could be easily produced by simply filling
the reservoir with water. The surface was then chilled down to
Tw=-10+/-1.degree. C. on a Peltier stage to freeze the water into
ice stripes, and the resulting anti-frosting behavior in a humid
environmental chamber was observed using top-down and side-view
microscopes. The air was both warm (T.infin.=17+/-1.degree. C.) and
humid: both 30% and 16% relative humidities were tried, which
corresponds to supersaturations of 1.5 and 1.1, respectively,
relative to the saturation pressure of the Tw=-10+/-1.degree. C.
surface. It was observed that no matter how much time elapsed, all
of the frost growth occurred solely atop the ice stripes, leaving
the rest of the substrate completely dry from both supercooled
condensation and frost.
[0057] While the foregoing written description enables one of
ordinary skill to make and use what is considered presently to be
the best mode thereof, those of ordinary skill will understand and
appreciate the existence of variations, combinations, and
equivalents of the specific embodiment, method, and examples
herein. The disclosure should therefore not be limited by the above
described embodiments, methods, and examples, but by all
embodiments and methods within the scope and spirit of the
disclosure.
* * * * *