Levitator

Abstract

A levitator for use with a light scattering photometer unit including a spaced pair of plate electrodes to provide an electric field for producing a first electrostatic force on a charged particle located between the spaced plate electrodes and with the levitator additionally including a pin electrode extending through and insulated from one of the plate electrodes to provide an electric field for producing a second electrostatic force and with the combination of the first and second electrostatic forces suspending the charged particle between the plate electrodes at a location spaced from but adjacent to the pin electrode. An automatic servo system includes an optical detector for detecting the position of the charged particle to produce a control signal to adjust the electric fields to maintain the charged particle in the proper position.

Parent Case Text

This is a division of application Ser. No. 191,373 filed Oct. 21, 1971, now U.S. Pat. No. 3,754,830.

Description

The present invention is directed to a levitator for use with a light scattering photometer unit. As one specific example, the light scattering photometer unit may operate in the following manner. Microparticles, whose light scattering properties are to be investigated, are located in a beam of light produced by a light source, preferably the light energy produced by the light source is polarized and at a single frequency, such as the light energy produced by various lasers. The microparticles may be of many types, such as bacteria or latex spheres of about 1 micrometer in diameter, but the levitator of the present invention could also be used with particles of greater or lesser size. The light scattered by the microparticle is intercepted by a detector mounted beneath a periscope which moves in an arc around the particle. The signal from the detector is amplified to drive a recorder to record a plot of the scattered light intensity as a function of the angle of the detector relative to the incident beam of light, this plot being a differential light scattering pattern.

A clearer understanding of the operation of a light scattering photometer may be had with reference to U.S. Pat. application Ser. No. 777,837, filed on Nov. 21, 1968, in the name of Philip J. Wyatt, and U.S. Pat. Ser. No. 34,243, filed on May 4, 1970, in the names of Philip J. Wyatt, et al., and both assigned to the same assignee as the instant application.

In the present invention, a stream of microparticles whose light scattering properties are to be investigated is introduced into a scattering chamber such as by nebulizing a liquid suspension or by any other convenient means. Most microparticles upon being nebulized from a liquid suspension or when collected from an aerosol source naturally tend to have a small positive or negative charge. An individual microparticle is isolated and positioned in the center of a laser beam through the use of pneumatic and electrical controls. Once the particle is suspended in the laser beam, the particle may be automatically held in position by means of an automatic servo mechanism so that the light scattering properties of the particle may be investigated.

Generally, the particle is maintained in the proper position through the use of a levitator which includes a pair of parallel plate members serving as electrodes and with a third pin electrode extending through but insulated from one of the parallel plate electrodes. The combination of the three electrodes provides for electric fields to maintain the particle suspended between the parallel plates and in a position for interception with the light beam.

In the prior art it is known to use a pair of parallel plate electrodes to suspend a particle between the electrodes. For example, Millikan, in the early part of this century, used parallel plate electrodes to suspend small oil drops between the plates. Generally, with the prior art apparatus, the plates were charged to provide an electric field interacting with the charge of the particles to counterbalance the force of gravity to maintain the particle between the plates. The difficulty with this type of apparatus is that the particle would not necessarily be centered and would tend to move off from between the plates. In addition, the electric field would have to be very accurately maintained and parallel to the gravitational field which required that the plates be very accurately aligned and perpendicular to the local gravitational field or the particle would tend to move within the space between the plates. These above difficulties made the use of the Millikan-type apparatus impractical other than for demonstrations.

In order to overcome some of the difficulties of the Millikan apparatus, Fletcher, in 1914, added a third electrode which took the form of a small plate located within and insulated from one of the parallel plates, which third electrode was used to produce an electric field to pull the particle in toward the center, and also to counterbalance the gravitational forces on the particle. By using this center electrode at selected times to produce an electric field between the center electrode and the pair of parallel plate electrodes, the particle could be returned to the center position. The difficulty with the Fletcher apparatus is that the size of the radial field produced by the small third electrode and used for centering is limited by the fact that, in addition to producing the radial field, the third electrode also produces a vertical field. The vertical field is limited to a level to counterbalance gravity which, in turn, limits the size of the radial field. The Fletcher apparatus, therefore, provides for a very weak radial field which is often not effective.

In order to overcome the shortcomings of both the Millikan and Fletcher apparatus, the present invention uses a pair of spaced parallel plate electrodes and with a pin electrode extending through and insulated from one of the plate electrodes. In a preferred embodiment of the invention the pin electrode extends through the top electrode and wherein the two parallel plate electrodes are charged to provide an electric field to produce a force which is "down" to add to the gravitational force on the particle. In order to compensate for this electric field produced by the parallel plate electrodes, the center pin electrode is charged relative to each of the plate electrodes to provide a field to produce a force which is "up" to overcome both the effects of gravity and the electric field pushing down thereby balancing the particle between the plates. This "up" field is therefore relatively large since it must overcome both the "down" field and gravity.

In addition to the "up" field produced by the pin electrode, a radial field is also produced, which is quite large since it is produced by the same charge as the "up" field. This combination of fields provides a strong radial field to maintain the particle in a centered position while the pair of fields, one "up" and one "down," is used to overcome the effects of gravity to maintain the particle between the plates. Once the particle is in the center position, a servo system with an optical detector may adjust the magnitude of the fields to keep the particle suspended in the proper position.

The above arrangement of two parallel plates forming a pair of electrodes and with a third electrode extending through and insulated from one of the parallel plates so as to provide for a strong radial field while still balancing the particle may be energized in a number of different ways. For example, the electrodes may be energized alternately so that the "up" and "in" field can be produced alternately with the "down" field to produce the strong net radial field. The alternation must be rapid relative to any rate of motion of the particle. In addition, both fields may be produced simultaneously so as to produce the strong net radial field while still providing the balancing of the particle. The invention will be described with reference to an arrangement wherein the fields are produced simultaneously, but it is to be appreciated that the fields could be applied alternately, as explained above.

A clearer understanding of the invention will be had with reference to the following description and drawings, wherein:

FIG. 1a, 1b, and 1c illustrate in schematic form the operation of the three-electrode levitator of the present invention;

FIG. 2 illustrates in detail a particular construction for a scattering cell including a levitator for receiving and suspending the microparticles;

FIG. 3 illustrates a general arrangement of the scattering cell 1 of FIG. 2 in combination with the various pneumatic controls for introducing microparticles to the cell;

FIG. 4 illustrates a view of the control panel used to provide the electrical and pneumatic control of the microparticles within the scattering cell of FIG. 2;

FIG. 5 illustrates the arrangement of the optical detector and other optics for use with scattering cell of FIG. 2; and

FIG. 6 illustrates a block diagram of a servo for controlling the position of the microparticles within the scattering cell.

Referring first to FIGS. 1a, 1b, and 1c, a schematic representation of the levitator of the present invention is shown. Specifically, the levitator includes a pair of parallel plate electrodes 10 and 12 and a third pin electrode 14 extending through but insulated from the electrode 10. As shown in FIG. 1c, the various electrodes may have potentials applied thereto, the values of which, shown in FIG. 1c, are representative only. For a given particle, the electrode 10 may have a reference potential or be at ground, the electrode 12 may have a potential on the order of +50 volts, and the electrode 14 may have a potential on the order of +100 volts. These positive potentials for the electrical energy applied to the electrodes 12 and 14 are based on an assumption that the microparticle to be suspended has a negative charge as shown by charged particle 16 located in a suspended position. If the particle had a positive charge, instead, then the corresponding voltages on electrodes 10, 12 and 14 would be reversed.

Referring now to FIG. 1a, it can be seen that the difference in potential between the electrodes 12 and 14 provides for strong electric field producing a force which is "up" and "in" as shown by arrows 18. The difference in potential between the electrodes 10 and 14 provides for strong electric field producing a force which is radial and "in," as shown by the arrows 20. If the electrode 12 had no potential, a negatively charged particle would therefore be drawn toward the center pin electrode 14.

Referring now to FIG. 1b, the difference in potential between the electrodes 10 and 12 provides for weak electric field producing a force which is "down," as shown by the arrows 22, and if no potential were applied to electrode 14, a negatively charged particle would be pulled downward toward the electrode 12. Since the particle also is pulled downward by the force of gravity, the electric field as shown by the arrows 22 is in a direction to increase this downward movement.

FIG. 1c shows the combination of these electric fields. Specifically, the strong radial electric field 20 is still present to hold the particle 16 in a center position. A combined vertical field 24 is a combination of the fields 18 and 22 shown in FIGS. 1a and 1b and is weaker than the field shown by the arrows 20. This weak vertical field is used to counterbalance the force of gravity of the charged particle 16. It can be seen, therefore, that by the use of this three-electrode structure, and specifically in the particular manner in which this three-electrode structure is energized, the gravitational forces acting on the charged particles may be counterbalanced by a weak vertical field, while at the same time a strong radial field is used to pull the charged particle into a central position.

It is to be appreciated that the pin electrode 14 may extend through the bottom electrode instead of the top electrode and with the electric fields arranged to provide for the charged particle properly positioned between the electrodes. Also, the plane of the plate electrodes need not be perpendicular to the force of gravity and the plate electrode may be disposed in any angular relationship to the force of gravity and with the charged particle still held suspended between the plate electrodes.

FIG. 2 is an exploded view of a scattering cell including a levitator of the present invention to receive microparticles and to provide for the detection of the differential light scattering properties of these particles. Since air currents may exert a substantial force on a suspended particle, preferably all parts of the scattering cell are pneumatically sealed to one another, as by O-rings, and the inlet and outlet also include means permitting them to be selectively pneumatically sealed from exterior pressure fluctuations. The scattering cell includes a cover member 100, having an inlet connector 102 and a flush connector 104. The cover fits over a settling chamber 106 which receives the microparticles. The cover and settling chamber are sealed by an O-ring 107. The base 10 of the settling chamber 106 is the first upper electrode 10 shown in FIG. 1. The pin electrode 14 extends through an insulating plug 108 which is received in an opening 110 in the base 10.

An electrical connector 112 is mounted on the outside wall of the settling chamber 106 to provide for the application of electrical potential through the wire 114 to the pin electrode 14. The plate electrode 10 and the settling chamber 106 are grounded through the electrical connector 112. A bellows connector 116 extends through the wall of the settling chamber 106. An opening 118 in the insulator 108 allows microparticles to pass from the settling chamber 106 into the light scattering area of the scattering cell of FIG. 2.

The light scattering area is formed by a transparent cell member 120 which includes a masked section 122. Light energy, such as from a laser beam, passes into the transparent cell 120 through an entrance mask 124. A light trap 126 receives the light energy after it passes through the transparent cell. A light exit 128 may be provided through the masked portion 122 so that the interior of the cell may be visually observed with a microscope in a manner to be described in a later portion of this specification. This sealed light exit is wedged at a slight angle to the axis of the scattering cell in the preferred embodiment to avoid reflecting light back into the cell in the horizontal plane in which scattered light is viewed. The settling chamber 106 and the transparent cell 120 are sealed by an upper O-ring 130.

The light energy, when appropriately directed through the transparent cell 120 from the entrance port 124, intersects any microparticle located at the proper position in the transparent cell and provides for differential light scattering in accordance with the differential light scattering properties of the microparticle. This differential light scattering may be detected by viewing the differentially scattered light at different angular positions through the transparent portion of the transparent cell 120.

A base member 132 supports the lower plate electrode 12. The base member 132 and the transparent cell 120 are sealed using a lower O-ring 134. An opening 136 extends through the lower plate electrode and is connected to an exhaust connector 138 to provide the exhausting of microparticles within the transparent cell 120. Finally, an electrical connection for applying potential to the lower plate electrode 12 is provided by electrical connector 140 which may be an opening to receive a plug, such as a banana plug.

FIG. 3 illustrates a typical manner in which the scattering cell of FIG. 2 may be interconnected with other elements to introduce microparticles to the transparent cell. Specifically, a nebulizer 200 may be used to provide individual microparticles to the scattering cell through a hose 202 connected to the inlet connector 102. The nebulizer 200 operates in a known manner using a supply of a filtered gas, such as air to atomize portions of a liquid suspension of microparticles and provide a stream of the gas containing individual microparticles. An another example, microparticles may be introduced by drawing an aerosol of microparticles into a syringe, thus connecting the outlet of the syringe to line 202 and selectively injecting the entrained microparticles by collapsing the syringe.

When a nebulizer is employed, a supply of air is coupled through a hose 204 and is connected to an air control valve 206. The air control valve 206 has two outputs, one of which is through a hose member 208 directly to the flush connector 104. An air control switch 210 controls the application of air from the hose 204 through the hose 208 to flush out the scattering cell. A second output from the air control valve 206 is through a hose 210 to a nebulizer control valve 212. The output from the nebulizer control valve is through a hose member 214 which is connected to the nebulizer 200. A nebulizer button control member 216 is used to control the application of air to the nebulizer 200. When the button 216 is pushed, air is supplied to the nebulizer 200 to provide for the introduction of microparticles contained in the stream of air through the hose 202 to the inlet connector 102.

A bellows assembly 218 is used to supply a gentle supply of air through a hose member 220 to the bellows connector 116. Finally, an exhaust hose 222 is connected to the exhaust connector 138 to provide for an exhaust of any microparticles within the scattering cell. The interior of the scattering cell is pneumatically isolated from any external air pressure changes and the flow through the scattering cell is controlled by the bellows assembly 218 and the nebulizer 200.

FIG. 4 illustrates the control panel of the levitator of the present invention. As shown in FIG. 3, the control panel includes an air control switch 210 and a nebulizer button switch 216. In addition, the control panel of FIG. 4 includes a polarity control knob 224, a voltage control knob 226, a balance control knob 228, an automatic-manual-position knob 230, a bellows control knob 232 and a focus control knob 234. Also, an eyepiece 236 of a microscope extends from the control panel.

The use of the various control knobs shown on the control panel of FIG. 4 are as follows:

The air control switch 210 in its forward position provides for the flushing of the scattering cell with a clean gas, such as filtered air. In the rear position of the switch 210, the air control valve 206 shown in FIG. 3 is connected to have the air supplied to the nebulizer control valve 212. The nebulizer control button 216, when pushed, supplied air to the nebulizer to spray particles from the nebulizer into the scattering cell. The polarity knob 224 controls the electrode voltages to be either plus or minus so that particles of either charge polarity can be levitated within the scattering cell.

The voltage knob 226 sets the maximum voltage which may be applied to the scattering cell electrodes. The balance knob 228 is set to balance the input to the servo system for the desired position of the particle. The position knob 230 may be in one of two modes, depending upon whether the knob is pulled out or whether the knob is pushed in. When the knob 230 is pulled out, the levitator is in the manual mode. Rotating the knob 230 adjusts the voltages provided to the electrodes so as to move the particle within the scattering cell. When the knob is pushed in, the levitator is in the automatic mode, and the levitator servo controls the electrode voltages to maintain the particle in the proper position within the scattering cell.

The bellows control knob 232 is used to flex a small bellows to move air and particles slowly through the scattering cell. Finally, the focus control knob 234 is used to focus the microscope so as to visually observe the center of the scattering cell.

FIG. 5 illustrates the optical arrangement for visually observing the center of the scattering cell and for providing for an optical detection of the position of the particle within the scattering cell. The light energy scattered by the particle near the center of the scattering cell, as shown by arrows 300, leaves the scattering cell through the exit 128 and is focused by a lens structure 302 onto a beam splitter 304. A portion of the light energy passes through the beam splitter 304 to a diagonal mirror 306. A second portion of the light energy is reflected by the beam splitter 304 to a viewing mirror 310 and may be viewed through a microscope 308. When the eyepiece 236 of the microscope 308 is properly focused, a visual observation may be had of the central region of the scattering cell.

The light energy which impinges on the diagonal mirror 306 is directed upward toward a second diagonal mirror 312. A portion of the light energy is reflected by the second diagonal mirror 312 to a first photomultiplier 314. In addition, a portion of the light energy from the diagonal mirror 306 passes by the second diagonal mirror 312 to impinge on a second photomultiplier 316. The combination of thd two photomultipliers 314 and 316 with the second diagonal mirror 312 may be used to provide an optical detection of the vertical position of the particle within the scattering cell. For example, if there are no particles in the scattering cell in the path of the laser beam, then no light energy would be scattered out of the exit 128 and no light energy would therefore pass to the diagonal mirror 306. If there is a particle in the path of the light beam within the scattering cell, then the position of this particle determines the intensity distribution of the scattered light passing through the exit which in turn determines the amount of light energy which is received by each of the two photomultipliers 314 and 316. In this way, the vertical position of the particle determines the relative output of the photomultipliers 314 and 316.

FIG. 6 illustrates the operation of the automatic servoing of the levitator so as to maintain the microparticle in the proper position within the scattering cell. In FIG. 6 the three-electrode levitator of the present invention, including the parallel plate electrodes 10 and 12 and the center pin electrode 14 is shown with the plate electrode 10 grounded and with voltages applied to the electrodes 12 and 14. A laser beam supplies light energy to the scattering cell and a scattered portion of the light energy is directed toward the mirror 312. The scattered light strikes the edge of the mirror 312 which divides this scattered light between the two photomultiplier tubes 314 and 316. The intensity of the light received by the two photomultiplier tubes 314 and 316 is equal when the particle is in the proper position within the scattering cell.

The output from the photomultipliers 314 and 316 are applied to a pair of matched characteristic logarithmic input amplifiers 350 and 352, which logarithmic amplifiers convert the photomultiplier output current to a voltage proportional to the logarithm of the current from the photomultiplier. The use of logarithmic amplifiers maintains constant servo gain and provides servo stability for a wide range of particles. The balance control 228 is adjusted to provide equal gain for equal signals into the input amplifiers 350 and 352. The outputs from the amplifiers 350 and 352 are applied to a difference circuit 354 and the difference between the two voltages is amplified by isolation amplifier 356 and applied to a sample and hold circuit 358. The output of the sample and hold circuit 358 is controlled by a one-shot multivibrator 360 which controls a switch 362. Since a pulsed laser is used in a preferred embodiment, the output signals from the input amplifiers 350 and 352 are used to control the multivibrator 360. If a continuous wave laser is used, the sample and hold circuit may be ommitted, or replaced by an averaging circuit.

The position control 230(a) which is the resistive control portion of the positioning control switch 230 shown in FIG. 4 is used to control an input signal to an isolation amplifier 364. In addition, the position control switch 230 may be either in a manual or an automatic mode as controlled by the switch portion 230(b) of the position control. The manual mode is when the switch 230(b) is closed, which controls the multivibrator 360 to maintain the switch 362 in an open position. At this time only the position control 230(a) effects the voltages applied to the electrodes of the levitator. When the switch 230(b) is in the open position, the servoing is automatic. At this time input signals are provided from the differencing circuit 354 to the sample and hold circuit 358 so that the output of the sample and hold circuit is an error signal in accordance with the position of the particle.

As indicated above, the output from the sample and hold circuit 358 is applied to the isolation amplifier 364 and then to a pair of d-c to d-c converters 366 and 368. The maximum output from the d-c to d-c converters 366 and 368 is adjusted by the voltage control 226, but below that maximum value the output from the d-c to d-c converters are control signals which are applied through the polarity switch 224 to the electrodes 12 and 14 to maintain the particle in the proper position within the scattering cell.

The operation of the levitator in isolating and positioning an individual particle within the laser beam with the structure shown in FIGS. 2-5 is as follows:

First, the nebulizer 200 is filled with a suspension of the microparticles to be studied and the hoses are connected to the scattering cell in the manner shown in FIG. 3. The various electrical power requirements are coupled to the instrument so that the laser (or other light source) is energized and the electrodes in the levitator are also energized. Initially the polarity control 224 may be positioned to the plus position. The voltage control 226 is adjusted to zero volts which is normally in the full counterclockwise position. The position control 230 is in the manual mode and turned counterclockwise which normally means that the particle is pulled downward.

In order to clear the scattering cell, the air toggle switch 210 is switched forward to flush the scattering cell with clean air. This may be visually observed by watching through the eyepiece 236 of the microscope 308 until no particles can be seen passing through the laser beam. When this has occurred, toggle switch 210 is returned to the rear position to connect up the nebulizer 200. The nebulizer button 216 may then be pushed briefly a few times until several particles are observed passing through the laser beam with each pulse of the nebulizer button 216. Now the bellows knob 232 may be moved back and forth to move particles slowly through the beam. The larger particles may be easily seen and for smaller particles the defraction images of the particles appear as sharply defined concentric rings will be visible through the microscope. The focus knob 234 may be adjusted for maximum image sharpness.

When there is a bright image in the field of view of the microscope, representing a particle, the voltage control 226 may be turned up until the image can be observed moving in response to the voltage control 226. The motion of the particle should be in the downward direction. If the motion is not downward, this means that the polarity of the particle is reversed and the polarity control 224 should be switched to the minus position. The position knob 230 may be moved to arrest the downward motion of the particle and bring the particle back toward the center of the field of view of the microscope. Actually, both the voltage and position controls may be simultaneously manipulated so as to stop the particle's motion before it leaves the region within which it can be seen.

If the particle is lost, then the voltage and position controls may be returned to zero and the nebulizer button pushed again to bring new particles in the field of view. Since the particles may have different polarities, it is important to remember that the polarity control may have to be reversed in order to provide the downward motion. As an alternative to the above, it may be more convenient to begin with the polarity control 224 at the zero position and with the voltage control 226 set high enough to produce relatively fast motion. The positioning of the particle may then be accomplished by simultaneously manipulating the polarity and position controls instead of the voltage and position controls.

When a particle has been positioned in the center of the field of view and remains nearly stationary, then the position control 230 is pushed in to place the levitator in the automatic mode. This engages the automatic servo control shown in FIG. 6 so as to hold the particle fixed within the light beam. The voltage control 226 may be turned completely up so as to provide a maximum automatic servoing of the position of the particle within the levitator.

The present invention therefore provides a simple structure for positioning a microparticle within a beam of light, such as light from a laser, and maintaining that particle automatically in this position. The levitator includes a pair of parallel plate electrodes and a third electrode extending through and insulated from one of the plate electrodes. In a preferred embodiment of the invention, the pair of parallel plate electrodes provide an electric field to pull the particle down to aid the force of gravity and with the center electrode providing an electric field to pull the particle up and counterbalance the downward forces and also to pull the particle toward the center. This structure allows for a relatively strong electric field to be used to pull the particle into the central position. The maintaining of the position of the particle may be automatically controlled using an optical detector in combination with a servo system.

The invention has been described with reference to a particular embodiment but the invention is only to be limited by the appended claims.

Claims

We claim:

1. A levitator for suspending charged particles, including

a pair of parallel plate electrodes,

a third pin electrode extending through and insulated from one of the pair of parallel plate electrodes and spaced from the other electrode,

first means for supplying an electrical potential between the pair of parallel plate electrodes to produce a first electric field in a first direction,

second means for supplying an electrical potential between the pin electrode and the pair of parallel plate electrodes to produce a second electric field in a second direction opposite to the direction of the first electric field to form a composite field between the pair of plate electrodes to balance any charged particles located between the pair of plate electrodes and to produce a third radial electric field to pull any charged particles located between the pair of plate electrodes toward a position adjacent to the pin electrode.

2. The levitator of claim 1 wherein the first and second means simultaneously supply electrical potentials to the electrodes.

3. The levitator of claim 1 additionally including means for supplying individual charged particles between the pair of plate electrodes.

4. The levitator of claim 1 additionally including means detecting the position of a charged particle located between the electrodes and for providing an error signal representing the difference between the actual position of the charged particle and a desired position for the charged particle and for adjusting the potential applied to the electrodes to adjust the positions of the charged particle.

5. A levitator for suspending microparticles in a fixed location to be intercepted by a beam of light, including

an electrode structure including a pair of electrodes spaced from each other and supplied with an electrical potential between the pair of electrodes to produce a first electric field on a microparticle located between the pair of electrodes and including a third electrode extending through and insulated from one of the first pair of electrodes and supplied with an electrical potential between the third electrode and the pair of electrodes to produce a second electric field to oppose the first electric field on the microparticle located between the pair of electrodes to balance the microparticle between the first pair of electrodes and to produce a third electric field to pull the microparticle to a position adjacent to the third electrode.

6. The levitator of claim 5 wherein the pair of electrodes and the third electrode simultaneously receive electrical potentials.

7. The levitator of claim 5 additionally including means for supplying individual microparticles between the pair of electrodes.

8. The levitator of claim 5 additionally including means for detecting the position of a microparticle located between the pair of electrodes and for providing an error signal representing the difference between the actual position of the microparticle and a desired position for the microparticle and for adjusting the potential applied to the electrodes to adjust the position of the microparticle.