WO1990000583A1

WO1990000583A1 - Induced dipole electroviscous fluids

Info

Publication number: WO1990000583A1
Application number: PCT/US1989/002705
Authority: WO
Inventors: Ronald P. Reitz
Original assignee: Reitz Ronald P
Priority date: 1988-07-15
Filing date: 1989-06-23
Publication date: 1990-01-25
Also published as: AU3861289A

Abstract

An electroviscous fluid is composed of a dielectric fluid (22) and a multiplicity of electrically polarizable aggregate particles (21) dispersed in the dielectric fluid. A substantial portion of the aggregate particles comprising a core (23) and an electrically insulative shield (24). The core is at least partially electrically conductive and said shield partially encompasses the core. The shield is adapted to prevent substantial particle to particle transmission of electric current.

Description

INDUCED DIPOLE ELECTROVISCOUS FLUIDS

Background of the Invention

Field of the Invention

This invention relates to the field of electroviscous fluids and more particularly to electroviscous fluids that exhibit long shelf life in storage and low current drain in application-

Background Information

Electroviscous fluids refer to fluids which exhibit the property of increased viscosity when the fluid is subjected to an electric field. One phenomenon for electrically controlling the viscosity of a fluid is commonly known as the inslow effect. As used in this disclosure, the term Winslow effect refers to the phenomenon of electrically controlling the viscosity of a fluid comprising a suspension of finely divided electrically polarizable matter in a dielectric fluid by subjecting the fluid to an electric field. Within this disclosure, the finely divided electrically polarizable matter is referred to as aggregate.

Electroviscous fluids sometimes referred to as electrorheological fluids are known in the prior art. Specifically, U.S. Patent 4,687,589 teaches an electrorheological fluid comprising a liquid continuous phase having dispersed therein at least one dispersed phase and which is capable of functioning as such when at least the dispersed phase is substantially anhydrous, preferably having functional capability when the fluid is substantially anhydrous.

Attempts to use fluids of the prior art in practical electromechanical devices have met with only modest success. Serious limitations of these fluids are responsible for limiting the practical commercial applications of electroviscous fluid technology. These limitations include the inability of the aggregate to remain in suspension, the inability to operate at elevated temperatures, high electric power consumption of the device under normal temperature conditions and the dependence of the magnitude of the Winslow effect and the electric energy consumption on the temperature of the fluid. Most prior art electroviscous fluids employ aggregate materials that have a permanent electric dipole moment associated with them. When immersed in an electric field, these aggregate particles align themselves along the electric lines of flux, resulting in a fine-particle periodic or lattice structure of the aggregate particles in the dielectric fluid. This structure can be compared to the periodic structure or lattice structure of molecules that comprise solids. The electromagnetic forces associated with the molecules of the solid hold the solid together and give the solid its ability to sustain a shear stress. Similarly, electroviscous fluids, when immersed in an electric field, are able to support a shear stress because of the electric field imposed upon the fluid. The alignment of the aggregate particles is energetically favored and mechanical energy would be required to disalign them when a shear stress is applied to the energized fluid. The term energized fluid is used to define a fluid wherein the aggregates are electrically aligned in the dielectric fluid by the electric field. Energized electroviscous fluids exhibit similar physical properties to those properties found in solids.

Previously, it has been found that adding small amounts of electrolyte solution to the aggregate enhances the magnitude of the Winslow effect. Although the reasons for this were unknown, the theory was proposed that, in part, the enhancement could be due to hydrogen bonding between the aggregate particles. However, investigators have conceded that this theory did not account for the totality of the enhancement. In U.S. Patent 3,427,247 to Peck it is reported that too much is still unknown about this phenomenon to enable a definite theory to be propounded. In U.S. Patent 3,970,573 to Westhaver it is stated that in addition, the electrolytes serve in a manner not fully understood to increase the relative capacitance and leakage resistance between contacting particles while decreasing their internal resistance.

Further, U.S. Patent 3,984,339 to Takeo, et al states that the reason for the increase in the Winslow effect exhibited by the hydraulic oil composition by addition of a small amount of water soluble electrolyte is not understood since the mechanism which gives rise to the Winslow effect itself has not yet been determined. In U.S. Patent 4,502,973 to Stangroom, it is disclosed that although two liquids may separately be good electroviscous fluids, they may not be so when mixed together and certain liquids may only be effective with certain solids. Further, the chemical features were not yet determined. U.S. Patent 4,687,589 to Block discloses an electroviscous fluid wherein the aggregate, therein the aggregate is called the dispersed phase, is electrically conductive and more specifically is a semiconductor. Pedersen in U.S. Patent 4,737,886 discloses an aggregate comprising electrically conductive, elongated, fibrous, particulate material with the preferred choice of materials being graphite fibers. Using electrically conductive or semi-conductive material as an aggregate has particular disadvantages. When the aggregate particles align themselves to an electric field in which the electroviscous fluid is immersed, the electrically conductive nature of the aggregate particles allows the electric charge that is induced onto the conductor surface to at least partially discharge. Secondly, most electrically conductive or semi-conductive materials usually have a density greater than 1.2 grams per cubic centimeter (g/cc) . Thus, many commercial dielectric fluids, such as mineral oil and dimethyl silicone oil, that can be found in abundance in the market cannot be used as the dielectric fluid component of an electroviscous fluid because such fluids have a density substantially less than 1.2 g/cc. For reasons as will be explained hereafter, it is desirable for the aggregate particles and the dielectric fluid to have approximately the same density. If the density of the aggregate particles is greater than the density of the dielectric fluid, then the only effective way to keep the heavier electrically conductive or semi-conductive materials in suspension is to make the aggregate particles small enough so that their masses are on the same order of magnitude as the mass of the dielectric fluid molecules around the aggregate particles. When the electroviscous fluid is made in this manner, the Brownian motion of the dielectric fluid molecules keeps the aggregate particles in suspension as in Pedersen¹s fluid of patent 4,737,886. Unfortunately, a major disadvantage exists in using the Brownian motion of the dielectric fluid molecules to keep the aggregate particles in suspension. According to the thermodynamic or physical laws which govern Brownian motion, Brownian motion may be considered as that motion which is associated with the temperature of the dielectric fluid. As the temperature of the fluid increases, so does the Brownian motion. More precisely, the Brownian motion of the dielectric fluid is motion due to the kinetic energy of each of the dielectric fluid molecules. In the dielectric fluid, the kinetic energy is randomly distributed and hence, the dielectric fluid molecules move in all different directions. The fluid molecules collide with each other and with the aggregate particles. Because the dielectric fluid molecule and the aggregate particles have roughly the same mass, the collisions are mostly elastic collisions. Consequently, when a dielectric fluid molecule collides with an aggregate particle, the dielectric fluid molecule imparts almost all of its kinetic energy to the aggregate particle. These collisions are similar to the collision observed when one billiard ball rolls across a billiard table and collides with a second billiard ball. If the first ball has no "english", the first ball stops and the second ball begins rolling across the table. Most of the kinetic energy from the first ball is imparted to the second ball. Such collisions on the billiard table are similar to the way that dielectric fluid molecules and similarly sized aggregate particles collide.

As previously stated, the kinetic energy of the dielectric fluid molecules depends upon the temperature of the dielectric fluid. When an electric field is imposed upon the electroviscous fluid, the aggregate particles align themselves along the electric lines of flux. Such alignment gives the overall electroviscous fluid periodicity in its structure. On a fine particle dimension scale, the energized electroviscous fluid appears to have a periodic structure capable of sustaining a shear stress. Because the dielectric fluid molecules have Brownian motion, kinetic energy due to temperature, the dielectric fluid molecules continue to collide with the aggregate particles, causing them to disalign even in the presence of the electric field. When this disalignment occurs, more electric energy is required to force the aggregate particles back into alignment. As the temperature of the electroviscous fluid increases, the kinetic energy of Brownian motion of the dielectric fluid molecules also increases.

Ultimately, the amount of electric energy required to solidify an electroviscous fluid such as Pedersen's increases as the temperature of the electroviscous fluid increases. Consequently, the electric power requirements of a device using fluids such as Pedersen-s is temperature dependent.

Prior investigators have both admitted the unknown nature of the Winslow effect and at the same time postulated theories for its existence. Peck in U.S. Patent 3,427,247 attributes the effect to some proton transfer mechanism involving hydrogen bonding acids and hydrogen bonding bases resulting in an increase in the zeta potential then relating the zeta potential to the viscosity of the fluid. According to Peck, both hydrogen bonding acids and hydrogen bonding bases must be present in the composition to cause the fluid to exhibit large changes in effective viscosity with change of the applied electric field.

Another conclusion in the art of electroviscous fluids is that water is necessary in the fluid for the Winslow effect to be exhibited. In U.S. Patent 4,129,513 to Stangroom, it is advocated that it is important that the polymer to be used in the electroviscous fluid of that invention be hydrophilic since it was concluded that water was necessary for the production of the electroviscous effect.

In the inventions of Block in U.S. Patent 4,687,589 and Pedersen in U.S. Patent 4,737,886, electrically conductive or semi-conductive materials are used as the aggregate material. Both of these fluids contribute to the transmission of electrical current between the positive and negative sources of the electrical field. This can be a substantial disadvantage because of the high power requirements required by the fluid. Further, the conductive or semi-conductive aggregate can cause electrical arc-over within the fluid contributing to instability in the fluid and shock hazards.

Prior art patents are replete with discussions of other problems associated with electroviscous fluids such as susceptibility of elastomers to attack by oils and solvents as reported in U.S. Patent 4,645,614 to Goossens et al; deterioration in short term storage as reported in U.S. Patents 3,367,872 and 3,397,147 to Martinek; and, inability of the fluid to operate above temperatures in the range of 110 ° C such as are exhibited by the fluid of Goossens. The temperature restraints are particularly disadvantageous in electromechanical devices where the fluids are put in shear and generate additional heat within the device. Another disadvantage is exhibited especially by fluids that are hydrophilic. When hydrophilic fluids are left exposed to the atmosphere, water vapor may be gained in an atmosphere of high humidity and water vapor may be lost in an atmosphere of low humidity. Such characteristics are exhibited by fluids such as those of Westhaver in U.S. Patent 3,970,573.

Further, a relationship between fluid electrical conductivity and the magnitude of the Winslow effect exists for all hydrophilic electroviscous solutions such as Westhaver's fluid in U.S. Patent 3,970,573. In Goossens et al it is stated that if the fluid conductivity is too high, excessively high currents and hence excessively high electric power levels are required to activate the electroviscous fluid or sufficiently powerful electrical fields cannot be generated in the electroviscous fluid.

Hydrophilic electroviscous fluids are characterized by the property that the Winslow effect increases with increasing water to a limit but after that limit has been reached, additional water will result in a reduced Winslow effect due to the electrical power required to energize the electroviscous fluid. Thus, in hydrophilic electroviscous fluids, the Winslow effect is a function of the electrical conductivity of the fluid. Further, the electrical conductivity of hydrophilic fluids is a function of the water content ^"of the aggregate. These interrelationships represent an engineering constraint with resulting tradeoffs that limit the kinds of electroviscous fluids that can be made effective.

Summary of the Invention

The present invention, an induced dipole electroviscous fluid, comprises a dielectric fluid and a multiplicity of electrically polarizable aggregate particles dispersed in the dielectric fluid. Within the fluid, a substantial portion of the aggregate particles each further comprise a core and an electrically nonconductive shield, the core being at least partially electrically conductive and the shield partially encompassing the core and adapted to prevent particle to particle transmission of electric current. Alternately, the shield further comprises a shell for completely encapsulating the core. When an encapsulating shell is used, the core of the aggregate particle may be an electrolyte, the purpose of the shell in this instance being to prevent the electrolyte from migrating into and degrading the dielectric fluid. The performance of the electroviscous fluid is enhanced by incorporating in each aggregate particle at least one buoyant body, the purpose of the buoyant body being to equalize the effective density of the aggregate with the density of the dielectric fluid, thus enhancing the ability of the aggregate to stay in suspension for long periods of time. Buoyant bodies may be created as gas pockets in the shield or adhesively attached to the core using such as for example, glass microspheres or hollow plastic bodies. Glass microspheres having a density of 0.2 g/cc are especially useful as buoyant bodies. The core is made of any convenient conductive or semi-conductive material and when a shell is used, the core can be a liquid or composite electrolyte.

Herein, the terms electrically nonconductive, electrically insulative and electrically resistive all have the same meaning and refer to any material with electrical resistance greater than that of carbon.

In accordance with the present invention, the temperature dependence of an electroviscous fluid is decreased. This decrease can be accomplished in at least two ways. The first way is to make the mass of the aggregate particle appreciably different from the mass of dielectric particles around the aggregate particles. This means that the dielectric fluid molecules will only be able to impart a small amount of their kinetic energy to the aggregate molecules during any one collision. A second way is to make the aggregate particles much larger than dielectric fluid molecules around the aggregate particles. In the latter instance, when one dielectric fluid molecule is colliding into one side of the aggregate particle, there is usually a second dielectric fluid molecule colliding into the other side of the aggregate particle. With equal collisions, the net result is that the net kinetic energy imparted to the aggregate particle by the surrounding dielectric fluid molecules tends to be negligible because the kinetic energies of the colliding fluid molecules tend to cancel. The present invention encompasses electroviscous fluids that have aggregate particles significantly greater than the surrounding dielectric fluid molecule masses and to fluids that have aggregate particles on the same order of magnitude or smaller than the mass of the surrounding dielectric fluid molecules.

It is an object of the present invention to provide an electroviscous fluid which has extremely low electrical conductivity and which thus exhibits low current drain through the fluid when the fluid is subjected to an electric field.

Yet another object of the present invention is to provide an electroviscous fluid having a long shelf life. It is a further object of this invention to provide an electroviscous fluid that has a broad range of. functional temperatures.

Still another object of this invention is to provide an electroviscous fluid that is functional at high temperatures such as temperatures of up to 500 ° F.

Yet another object of this invention is to provide an electroviscous fluid that does not rely on hydrogen bonding acids and bases and which does not require water as one of its constituents. A yet further object of this invention is to provide a fluid wherein the characteristics of the aggregate can be tailored to match the characteristics of the dielectric fluid. A still further object of this invention is to provide a fluid wherein the suspension of aggregate is relatively independent of the viscosity of the fluid.

A yet another object of this invention is to provide an aggregate for an electroviscous fluid which can reliably be fabricated having a density of less than 1.2 grams per cubic centimeter.

The above and other objects, features and advantages of the present invention will become apparent from a consideration of the following detailed description and examples presented in connection with the accompanying drawings in which:

Brief Description of the Drawings

In the drawings, to which reference will be made in the specification, similar reference characters have been employed to designate corresponding parts throughout the several views.

Fig. 1 is a diagrammatical representation of an induced dipole electroviscous fluid of a first preferred embodiment illustrating the behavior of the fluid in the presence of electrically uncharged electrodes.

Fig. 2 is a diagrammatical representation of an induced dipole electroviscous fluid of a first preferred embodiment illustrating the behavior of the fluid in the presence of electrically charged electrodes. Fig. 3 is a diagrammatical representation of an induced dipole electroviscous fluid of a second preferred embodiment illustrating the behavior of the fluid in the presence of electrically uncharged electrodes.

Fig. 4 is a diagrammatical representation of an induced dipole electroviscous fluid of a third preferred embodiment illustrating the behavior of the fluid in the presence of electrically uncharged electrodes.

Fig. 5 is a diagrammatical representation of an induced dipole electroviscous fluid of a third preferred embodiment illustrating the behavior of the fluid in the presence of electrically charged electrodes.

Detailed Description of the Preferred Embodiments Referring now to Fig. 1, a diagrammatical representation of an induced dipole electroviscous fluid of the present invention is illustrated in an apparatus for demonstrating the electroviscous nature of the fluid. A reservoir for containing the fluid is illustrated schematically by glass beaker 12. Electrodes 13 and 14 are spaced apart in beaker 12 and are at least partially inserted in glass beaker 12. Electrodes 13 and 14 are made comprising any good electrical conductor material such as for example, copper, silver, aluminum, zinc, lead, steel, or bronze or any semiconductor material made comprising for example germanium or silicon. Electrodes 13 and 14 are connected through electrically conductive wires 15 and 16 through switch 17 to high voltage power supply 18. Induced dipole electroviscous fluid 20 is in contact with electrodes 13 and 14.

Induced dipole electroviscous fluid 20 is comprised of electrically non-conductive aggregate particles 21 substantially dispersed throughout a dielectric fluid 22. The term non-conductive when used in relation to the characteristics of the aggregate means that the separate aggregate particles are individually adapted to avoid or minimize the transmission of electrical current from one aggregate particle to another aggregate particle.

Electrically non-conductive aggregate particles 21 comprise a core 23, said core 23 being at least partly conductive, and a shield 24, said shield 24 further comprising substantially non-conductive material.

Core 23 comprises any suitable material which is at least partially conductive. Good conductors such as for example metals and metal alloys comprising aluminum, zinc, bronze, iron, stainless steel, soft steel, galvanized steel, tungsten, lead, copper-nickel and Monel are usable as core 23. Partial conductors such as compositions comprising silicon and germanium of the class of materials commonly known as semiconductors are usable as materials for core^' 23. Core 23 alternately comprises an electrolytic fluid such as salt water, sulfuric acid, hydrochloric acid, acetic acid or other electrolytic fluid, it being understood that when a fluid is used and especially when an electrolyte is employed as core 23, shield 24 must further comprise a shell around core 23 to prevent the migration of core- 23 into dielectric fluid 22.

Shield 24 comprises any material that is a good electric insulator, such as for example: polyurethane elastomers, nitrile elastomers, hardened epoxy adhesive, nylon, ceramic glaze, fired clay, cement, silica, silicone rubber, Teflon^®, glass and other good dielectric materials. Preferably, shield 24 encapsulates core 23, but as will be seen in the light of the description and examples included herein, that, except in the case where core 23 is a electrolyte or a fluid, encapsulation is not necessary to the performance of induced dipole electroviscous fluids of the present invention.

Dielectric fluid 22 comprises any dielectric fluid such as for example, dimethyl silicone oil, paraffin oil or mineral oil. The particular dielectric fluid to be used in a particular application will usually require routine selection based on the anticipated end use of the induced dipole electroviscous fluid. Referring now to Fig. 1 and 2, the functioning of a first embodiment of an induced dipole electroviscous fluid is illustrated.

With switch 17 in the open position as shown, electrodes 13 and 14 have no energizing potential applied, thus there is no electric field across electroviscous fluid 20. As indicated in Fig. 1, aggregate particles 21 are randomly oriented throughout dielectric fluid 22. When switch 17 is closed as in Fig. 2, electrodes 13 and 14 become electrically charged and an electric field is permeated through electroviscous fluid 20 inducing charges 26 and 27 on the surface of core 23. Electric charges 26 and 27 on the surface of core 23 are of opposite polarity with the charges on electrodes 13 and 14 respectively. Electric charges 26 and 27 of each aggregate particle 21 cause aggregate particles 21 to align themselves along the electric lines of flux of the permeating electric field. On a fine particle dimensional scale, alignment of aggregate particles 21 gives the electroviscous fluid a structure which is periodic and is similar to the structure of crystalline solids. Thus the effective viscosity of the fluid is greater under the conditions illustrated in Fig. 2 that the effective viscosity of such fluid under the conditions illustrated in Fig. 1. When electrodes 13 and 14 are electrically discharged so that they have no net charge on them, the phenomenon is reversed and electroviscous fluid 20 returns to the conditions illustrated in Fig. 1. Even though aggregate particles 21 may be touching, current will not be transmitted through the particles from electrode 13 to electrode 14 because shield 24 prevents current transmission between cores 23.

On the other hand, a fluid, such as the fluids of Block and Pedersen constructed with conductive aggregate exhibits particle to particle transmission of electrical current where induced charges then discharge as the aggregate particles align themselves along the electric lines of flux so as to touch each other and the charging electrodes. There, new charges continue to form only to be discharged. Such continual charging and discharging causes large electrical power consumption in order to keep the electroviscous fluid "solidified" and further causes heating of the electroviscous fluid.

When electrolyte solutions such as salt water, sulfuric acid, hydrochloric acid, acetic acid or other electrolytic fluid are employed for core 23, shield 24 must encapsulate or form a shell around each core 23 to prevent the migration of core 23 into dielectric fluid 22. B complete encapsulation of core 23, migration of the electrolyte solution comprising core 23 into dielectric fluid 22 is prevented. Naturally, migration of the conductive electrolyte into dielectric fluid 22 would cause an increase in current through electroviscous fluid 20 when fluid 20 is subjected to an electric field as between electrodes 13 and 14 shown in Fig. 2. When an electrolyte is used as core 23, charges 26 and 27 may be formed by means of ions as well as by means of electrons or holes. The dubious distinction between electron or hole electric conduction as opposed to ionic electric conduction is found in Block's definition of "electronic conductivity" in U.S. Patent 4,687,589.

It is readily apparent that in light of this disclosure, the present invention may be practiced by a wide range of combinations of cores and shields with some shields further comprising encapsulating shells and that significant advantages flow from the ability to practice the invention in a varied manner. Another advantage results from the ability to make core 23 and shield 24 from different materials. By so doing, aggregate particles 21 may be tailored to match the density of dielectric fluid 22. By matching the density of dielectric fluid 22 with the density of aggregate particles 21, aggregate particles 21 can be kept in suspension in dielectric fluid 22 regardless of the size of each of aggregate particles 21. Thus, the density of each of aggregate particles 21 can be reduced to values substantially lower than 1.2 g/cc. This is important as many inexpensive readily available dielectrics have densities below 1.2 g/cc. Density matching is accomplished by starting with electrically conductive core 23 and attaching thereto an electrically nonconductive shield 24, using material having a different density than core 23, so that the. overall density of aggregate particle 21 is made to be the same as the density of dielectric fluid 22 in which aggregate particles 21 are immersed.

Referring now to Fig. 3, in an alternate embodiment, the density matching of aggregate 21 to dielectric fluid 22 can be enhanced when aggregate 21 further comprises at least one buoyant body 28. Buoyant body 28 is formed either by entrapment of a gas such as air in shield 24 or buoyant body may be a separately manufactured article which is incorporated with shield 24 during the process of attaching shield 24 to core 23. In addition to illustrating the construction and operation of a first preferred embodiment, Fig. 1 illustrates the general test configuration which was used to determine the electroviscous nature of the electroviscous fluids described in the foregoing examples. A probe was constructed wherein electrodes 13 and 14 each comprised a planar surface of about 1.9 centimeters (cm) by 2.5 cm and were oppositely opposed at a spacing which was adjustable from about 0.32 cm to about 0.48 cm. High voltage power supply 18 was adjustable from near zero to more than 5000 volts. Power supply 18 was equipped with a voltmeter and a current milliammeter. The full scale of the milliammeter was 50 milliamperes (ma) with the lowest seale graduation being 0.5 ma. Aggregate particles were classified as being either positively buoyant, that is they rose to the top of the fluid; negatively buoyant, that is they settled to the bottom of the fluid; and, neutrally buoyant, that is they neither rose to the top or sank to the bottom of the fluid. An appropriate test criteria for the electro- viscous nature of fluids is if the fluid solidifies between the energized electrodes of the above described probe sufficient to lift a portion of the fluid from a container and if upon removal of the energizing voltage, the fluid runs freely from the probe, then the fluid is electroviscous.

Density matching of aggregate to dielectric fluid is enhanced when this first dielectric fluid and aggregate composition is suspended between a second lighter, less dense dielectric fluid and a third heavier, more dense dielectric fluid.

EXAMPLE 1 A bulk material comprising electrically conductive particles comprising aluminum and copper having particles with an individual principal dimension of approximately 10 microns was encapsulated in a plastic material commercially known as Isofoam and obtained from Read Plastics in Rockville MD. Separately cured, the plastic has a density of less than 0.2 g/cc. The encapsulation was performed by mixing the constituent parts of Isofoam with the bulk material and allowing the mixture to cure. The resulting composite material was ground and grindings or aggregate mixed in a transparent container with a Dow Corning dimethyl silicone oil having a viscosity of 50 centipoise (cp) . It was observed that some of the aggregate particles had positive buoyancy, some had negative buoyancy and others had neutral buoyancy. The naturally buoyant aggregate particles and a portion of the surrounding dielectric fluid was removed from the container using the above- described probe. The probe was placed in the mixture in the unenergized state and subsequently energized by applying a high voltage potentials ranging up to 5000 volts. With the electrodes energized, the neutrally buoyant aggregate particles aligned themselves with the lines of the electric field and the fluid within the electric field was observed to increase in viscosity to the point of "solidification". The probe with the "solidified" fluid clinging to it was lifted out of the mixture and held over a second container. When the voltage across the electrodes of the probes was discharged, the solidified fluid between the probes became liquid again and readily ran into the second container. Repeating this process resulted in transferring a small quantity of the electroviscous fluid into the second container. This procedure was repeated on several occasions. During some of these occasions, attempts were made to measure any actual current flow through the electroviscous fluid during the time that the fluid was energized. The full scale of the milliammeter on the high voltage power supply used for the tests was 50 ma with the lowest scale graduation being 0.5 ma. It was observed that there was no visible movement on the ammeter at electrode potentials high enough to substantially "solidify" the fluid. Thus, using this instrument, it could be estimated with reasonable accuracy that the current flow was not in excess of 50 icroamps.

This first dielectric fluid and aggregate mixture was made stable by suspending it between a lower density dielectric fluid comprising mineral oil and a higher density dielectric fluid comprising phenyl silicone oil.

EXAMPLE 2 Aluminum filings such as those available as residue from a machine grinding operation were mixed with one part of a two part epoxy resin set. After the filings were thoroughly mixed with the first part, epoxy hardener was added and the mixture allowed to cure into a slab of epoxy. The slab containing the hardened epoxy and the aluminum filings was then ground into fine ^"particles and the particles immersed in a mineral oil. This first dielectric fluid and aggregate particles composition was added to and suspended between a lower density dielectric fluid comprising a Mobil transmission fluid and a higher density dielectric fluid comprising a Dow Corning phenyl silicone oil. These two dielectric fluids were added to the first dielectric fluid and aggregate particles in order to make the fluid stable. Essentially, the aggregate particles had sunk to the bottom of the first dielectric fluid and could sink no further. Since the particles constituted a porous mass, there was some dielectric fluid mixed in with the aggregate^, particles at all times.

High temperature electroviscous fluids are made in accordance with the present invention.

EXAMPLE 3

About one-half cup of chromium powder with an average particle size of approximately 5 microns was placed in a container. About one cup of ceramic glaze of the type commonly used for finishing decorative ceramic articles was poured into the container and the mixture stirred. The mixture was poured onto a ceramic plate and placed in a kiln where it was fired to about 1000 ° F, melting the glaze onto the chromium particles. After 24 hours, the kiln was turned off and the glaze-chromium mixture was permitted to cool for approximately 72 hours. Afterwards, the glaze was found to have hardened into a slab in which the chromium particles were embedded. The slab was then broken up into small particles and mixed with 50 cp dimethyl silicone oil. Using the procedure outlined in Example l, the mixture was found to be electroviscous. The following is an example of an electroviscous fluid of the present invention using an electrolyte as the conductive part of the core.

EXAMPLE 4 Three grams of table salt were mixed with 30 grams of water and the resultant electrolyte was further mixed with 30 grams of starch powder of the type commonly available in a grocery store. This mixture forms an aggregate which is well known in the electroviscous art and is also well known to be conductive. This aggregate, best described as a moist powder, was added to and mixed with a first part of an epoxy resin. Subsequently, epoxy hardener was added and the mixture was allowed to cure into a hardened epoxy slab, wherein the slab had particles of the electrolytic aggregate embedded therein. The slab was then ground into small particles and mixed with 50 cp dimethyl silicone oil. Using the procedure outlined in example 1, the mixture was tested for electroviscous properties and it was found that the current drain was substantially less than an electroviscous fluid using a conventional electrolytic aggregate.

The combining of a non-conductive material with the electrolytic fluid to take an aggregate in accordance with the present invention overcomes disadvantages of prior art fluids. As noted in U.S. Patent 4,645,614, some prior art fluids using electrolytic aggregate particles are known to attach and even dissolve elastomeric materials, a material commonly found in devices employing electroviscous fluids. Obviously, this is unsuitable where the electroviscous fluid must come in contact with the elastomer. By encapsulating a core of electrolytic material in a shell which cannot be penetrated by the electrolyte, the corrosion problem can be eliminated. Thus the external shell or encapsulant will not allow the electrolyte to migrate into the dielectric liquid and hence the electrolyte will not be able to come in contact with any elastomers as may be in the device. Referring now to Fig. 3, another embodiment of the present invention is diagrammatically shown. An electrically conductive core 23 is encapsulated by a shield 24 of electrically nonconductive material embedded in said nonconductive shield is at least one buoyant body 28 such as for example a glass microsphere of the type more fully described in Example 5 below. The glass microsphere is used for the purpose of trapping a pocket of a gas in the shield of non conductive material encapsulating the conductive core. Inert gases are preferred, however, it will be appreciated that many other gaseous substances may comprise the gaseous volume without departing from the spirit and scope of the invention. Other gaseous substances include for example, air, nitrogen, any of the noble gases, oxygen, hydrogen, carbon dioxide and carbon monoxide. Alternately, buoyant body 28 is formed in shell 25 by encapsulating flexible bodies of plastic material containing a gas or by entrapping gas directly within shell 25.

EXAMPLE 5 One gram of table salt was mixed with 10 grams of water and the mixture then mixed with 100 grams of starch powder. The resultant moist powder was then mixed into several quarts of one part of a two part epoxy resin. Into the foregoing mixture, several quarts of glass microspheres of about 50 microns in diameter were mixed. These glass microspheres are hollow spheres of glass in which air is entrapped. These hollow spheres are available commercially from the 3M Company and have a density of less than 0.2 g/cc.^" Epoxy hardener was next added to the mixture and the mixture was allowed to harden into a slab. A portion of the hardened slab was then ground into small particles. A cup of these particles was added to 50 cp dimethyl silicone oil having a density of 0.98 g/cc in a glass beaker. The condition of the mixture was observed. Some of the particles were positively buoyant; some of the particles were negatively buoyant; and, some of the particles were neutrally buoyant. Thus, a portion of the fluid was found to be stable, that is, the aggregate particles did not settle or fall out of the dielectric fluid.

The stable portion of the electroviscous fluid was separated from the rest of the fluid following the general procedure outlined in Example 1. The probe was inserted in the fluid and the voltage across the electrodes was increased to about 2000 volts and it was observed that the fluid solidified between the electrodes. With the probes energized, the probe was withdrawn from the fluid and the solidified portion transferred to a second container. Once the voltage was reduced to near zero, the fluid returned to a liquid state and ran freely into the second container.

Another example of an electroviscous fluid of the present invention is illustrated in Fig. 4 and 5. Aggregate particles 21 in dielectric fluid 22 comprise an electrically conductive core 23 and a nonconductive shield 24 having encapsulated or formed therein at least one buoyant body 28. As long as there is no communica¬ tion of electrical current between cores 23, it is not required that shield 24 completely encapsulate core 23. The foregoing example will illustrate one type of shield that has been successfully employed in an electroviscous fluid.

EXAMPLE 6 A bare aluminum wire having a diameter of 4 mils (.004 in.) 0.10 millimeter (mm) was drawn through mixed but unhardened epoxy adhesive. The epoxy adhesive components were properly mixed so that when hardened, a structurally sound, electrically nonconductive encapsulant was formed around the wire. While the epoxy adhesive was still tacky, the epoxy coated wire was further drawn through a container containing many of the 50 micron hollow glass microspheres of the type described in Example 5, adhering some of the microspheres to the wire. The epoxy adhesive was then allowed to harden. The process of coating by first drawing through the uncured adhesive and then drawing through a container of microspheres was repeated. After curing, one end of the coated wire was dipped into an unhardened epoxy adhesive and the adhesive was allowed to cure. The end of the coated wire was then cut off, fabricating an aggregate particle comprising a core 23 of aluminum wire and a shield 24 of hardened epoxy adhesive in which was embedded at least one and preferably several buoyant bodies 28 in the form of glass microspheres. As shown schematically in Fig. 4, one end of the wire core was typically left unprotected by the epoxy coating. The process of first coating the wire end and then cutting off a small portion or aggregate was repeated until numerous aggregate particles comprising aluminum cores and epoxy shields with adhered microspheres were obtained. The aggregate so formed was mixed with dimethyl silicone oil. The aggregate particles were observed and some were found to be positively buoyant, some negatively buoyant and some neutrally buoyant. The resultant electroviscous fluid was collected in the same manner as in Example 1.

Referring now to Fig. 4 and Fig. 5, the functioning of an electroviscous fluid made in accordance with Example 6 is illustrated. With electrodes 13 and 14 electrically uncharged aggregate particles 21 are randomly oriented within dielectric fluid 22. When switch 17 is closed, high voltage supply 18 applies polarizing potential to electrodes 13 and 14 through wires 15 and 16. When polarizing potential is supplied by increasing the voltage of high voltage supply 18 aggregate particles 21 align themselves along the electric field as shown in Fig. 5. As is illustrated in Fig. 5, the chain of aggregate particles is not conductive from electrode to electrode because the non- conductive material covering at least one end conductive core 23 prevents the formation of an electrically conductive chain of particles thus, it is now apparent that it is not necessary to completely encapsulate-core

23 in order for shield 24 to be effective. Thus, shield

24 interdicts and prevents the overall discharge of induced electric charges that form on the surface of core 23. Consequently, it is clear that partial encapsulation of core 23 is sufficient to provided an electroviscous fluid which has a low current drain in the energized or solidified condition. Partial encapsulation also assists in controlling the density of the aggregate to be compatible with the density of the dielectric fluid. As with the other examples, applying a high voltage to electrodes 13 and 14 caused the fluid to solidify exhibiting a condition of higher viscosity and removing the high voltage causes the fluid to return to a lower effective viscosity.

EXAMPLE 7

A bulk material comprising aluminum particles was encapsulated in a plastic material commercially known as Ultra Glow and obtained from ETI, Fields Landing, CA and allowed to harden into a slab. The resulting composite material was ground and grindings or aggregate mixed with Part B of a two part adhesive identified as Duro Depend II, manufactured by Locktite Corp., Cleveland OH. An appropriate quantity of glass microspheres of the type more specifically described in Example 5 was mixed with Part A of the Duro Depend II two part adhesive. The Part A mix and the Part B mix were then mixed together to form a paste. The paste was then mixed with 50 cp dimethyl silicone oil having a density of 0.98 g/cc. It was observed that some of the aggregate particles had positive buoyancy, some had negative buoyancy, and others had neutral buoyancy. The neutrally buoyant aggregate particles and a portion of the surrounding dielectric fluid was transferred from the container using the collection procedure generally outlined in Example 1. A probe spacing ranging from 0.32 cm to 0.48 cm was used. With the electrode spacing at about 0.32 cm, it was noted that a potential of about 3000 volts between electrodes was required to solidify the fluid and with the electrodes at 0.48 cm, it was noted that a potential of about 5000 volts was required to solidify the fluid. The probe was placed in the mixture in the unenergized state and subsequently energized by applying high voltage potentials. With the electrodes energized, the neutrally buoyant aggregate particles aligned themselves with the lines of the electric field and the fluid within the electric field was observed to increase in viscosity to the point of "solidification". The probe with the "solidified" fluid clinging to it was lifted out of the mixture and held over a second container. When the voltage across the electrodes of the probes was discharged, the solidified fluid between the probes became liquid again and readily ran into the second container. Repeating this process resulted in transferring a small quantity of the electroviscous fluid into the second container. This procedure was repeated on several occasions. During some of these occasions, attempts were made to measure any actual current flow through the electroviscous fluid during the time that the fluid was energized. The full scale of the milliammeter on the high voltage power supply used for the tests was 50 ma with the lowest scale graduation being 0.5 ma. It was observed that there was no visible movement on the ammeter at electrode potentials high enough to substantially "solidify" the fluid. Thus, using this instrument, it could be estimated with reasonable accuracy that the current flow was not in excess of 50 microamps.

A more sensitive ammeter was obtained and the test was repeated on another occasion. For this test a Keithly Model 480 picoammeter manufactured by Keithly Instrument Co. Cleveland, OH was used to make an accurate measurement of current flow under conditions of fluid solidification. In this test, a probe was used having electrodes each with a surface area measuring about 0.6 cm by 2.75 cm and having a gap between the electrodes of about 0.4 cm. The picoammeter was inserted in the negative lead from the high voltage power supply. With the probe connected to the power supply through the picoammetter a voltage of about 4000 volts was applied and the meter reading was noted to be about 160 nanoampers (na) (1 x 10^"9 amperes) . The probe was inserted in the fluid and the voltage increased until the fluid solidified at about 4000 volts. When the fluid had solidified, it was noted that the current had increased to about 250 na. Assigning the initial 160 na current drain to leakage through the fluid, the power required to solidify the fluid was calculated as:

P_f = (4000 X 90 X 10 •.-9',)/(0.6 X 0.4 X 2.75) P_f = 545 microwatts per cm

EXAMPLE 8 A mixture is prepared in accordance with the procedure outlined in the above Example 3. Before the mixture is fired in a kiln, the mixture of chromium powder and ceramic glaze is aerated to introduce a gas such as air into the mixture. The mixture is then fired in the kiln as per example 3 and then broken or ground into aggregate. The aggregate particles are then mixed with a 50 cp dimethyl silicone oil having an open container temperature rating of at least 500 ° F such as for example Dow Corning Silicone Oil 200 or General Electric Silicone Oil SF1154 or General Electric Phenyl Silicone Oil SF1265. The neutrally buoyant aggregate particles and a portion of the surrounding fluid are then separated from the fluid mixture using the procedure outlined in Example 1. When it is desired to have a product comprising only the aggregate particles, the residual fluid is separated from the mixture of fluid and neutrally buoyant aggregate using a process such as centrifuging. Neutrally buoyant particles can be separated from any of the mixtures of dielectric fluids and neutrally buoyant aggregate particles described above by solidifying a small portion of the fluid and the neutrally buoyant particles in an electric field and removing the solidified mixture containing the neutrally buoyant particles; discharging the electric field to allow the solidified fluid to liquefy and then separating the neutrally buoyant particles by means such as centrifuging.

EXAMPLE 9

As stated earlier, other investigators of electroviscous fluids have stated that they do not know what causes the Winslow effect. I now state that it is due to electrically induced dipoles formed in the conductor portion of the aggregate particle when the particle is placed in an electric field. Although examples 1 through 8 show evidence of the truth of the induced dipole theory, there are other examples which demonstrate even more effectively the induced dipole nature of electroviscous fluid particles. These other examples are photoelectroviscous fluids. The induced dipoles are generated by the photoelectric effect. Stangroom would argue that water is a necessary constituent part of the Winslow effect. But, water by itself is not photosensitive. Thus, shining light on an electroviscous fluid or alternatively placing the electroviscous fluid in total darkness would have no effect on the electroviscous nature of the fluids if indeed Stangroom was correct. But he is not correct. I am correct in stating that water is not necessary and that dipoles are induced when employing the Winslow effect. As dramatic proof of this, in accordance with the induced dipole theory I have invented electroviscous fluids that have photoinduced (i.e. light i n d u c e d ) dipoles. This is accomplished by making a substantial portion of the aggregate particles each comprise a core and an electrically non-conductive shield, the core being at least partially photocontrollable with the shield partially encompassing the core, the shield adapted to prevent particle to particle transmission of electric current. The techniques for shielding the photocon¬ trollable core are generally the same as for other electroviscous fluids mentioned earlier except that the shield material must be optically translucent or transparent to light frequencies to which the photocontrollable core is sensitive. Referring now to Fig. 1, core 23 comprises any suitable material which is at least partially photocontrollable. The term photocontrollable as used herein includes both photoconductive and photogenerative materials. Photoconductive materials are those elements, alloys, compositions which exhibit the property of increased electrical conductivity in the presence of light. Alternately, core 23 comprises photogenerative material or combination. Photogenerative materials are those compositions and combinations which generate an electrical potential in the presence of light. Such photoconductive and photogenerative materials are well known in the art of electronics. Shield 24 comprises any material that is a good electric insulator as mentioned previously and that is translucent to the photon frequencies of interest. Preferably, shield 24 encap¬ sulates core 23 although complete encapsulation is not necessary to the performance of the photoinduced electroviscous fluids.

Referring now to Figs. 1 and 2, the functioning of a photogenerative aggregate particle (comprising a photogenerative core 23 and an optically translucent shield 24) is illustrated. With switch 17 in the open position, electrodes 13 and 14 have no energizing potential applied and therefore no electric field across photoelectroviscous fluid 20. As shown in Fig. 1, aggregate particles 21 are randomly oriented throughout dielectric fluid 22. When switch 17 is closed as in Fig. 2, electrodes 13 and 14 become electrically charged and an electric field is permeated through photoelectro¬ viscous fluid 20. This field attempts to induce charges on the surface of core 23. Since, in the absence of light, cores 23 are not electrogenerative, charges 26 and 27 are not formed on cores 23 and particles 21 do not align themselves as shown in Fig. 2. When a light source (not shown) illuminates fluid 20 cores 23 become electrogenerative and form charges 26 and 27, thus causing the particles 21 to align as shown in Fig. 2. Thus, the effective viscosity of the fluid is greater under the conditions illustrated in Fig. 2 than those illustrated in Fig. 1. As an example of this, a small quantity of silicon solar cells was purchased from a retail electronics supply source. Using a bench grinder, the solar cells were ground into small particles. The resulting ground solar cell material was mixed with Part B of a two part adhesive identified as Duro Depend II, manufactured by Locktite Corp., Cleveland OH. An appropriate quantity of glass microspheres of the type more specifically described in Example 5 was mixed with Part A of the Duro Depend II two part adhesive. Then the Part A mix and the Part B mix were then mixed together to form a paste. The paste was then mixed with 50 cp dimethyl silicone oil having a density of 0.98 g/cc. The paste and silicone oil were mixed thoroughly in a blender. It was observed that some of the aggregate particles had positive buoyancy, some had negative buoyancy and some had neutral buoyancy. The clear glass container containing the fluid was placed in a darkened area and an attempt was made to collect an electrically solidified portion of the fluid in a high voltage probe having a spacing ranging from 0.32 cm to 0.48 cm and opposed surface areas of about 1.8 square centimeters. With the electrode spacing at about 0.38 cm, it was noted that the fluid did not solidify even at a potential of up to about 3000 volts. A bright light in the form of a desk lamp was then used to illuminate the fluid which was in a clear glass container. Upon application of the light, it was noted that the fluid solidified and remained within the electrodes of the probe as the probe was withdrawn from the fluid, thus demonstrating that the fluid was photoelectroviscous. It was further noted that the fluid remained solidified between the electrodes even after the light source was extinguished. Thus, the fact that the dipole induction (photoinduced dipoles) can be caused by illuminating the photosensitive fluid with light shows that the theory of induced dipoles is valid and that Stangroom⁷s theory which says that water is necessary for the Winslow effect is incorrect.

It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention. It is therefore to be understood that within the scope of the appended claims. the invention may be practiced otherwise than as speci¬ fically described herein.

Claims

What Is Claimed Is:

1. An electroviscous fluid, comprising: a) a dielectric fluid; and b) a multiplicity of electrically polarizable aggregate particles dispersed in said dielectric fluid, a substantial portion of said aggregate particles each further comprising a core and an electrically insulative shield, said core being at least partially electrically conductive and said shield partially encompassing said core, said shield adapted to prevent substantial particle to particle transmission of electric current.

2. An electroviscous fluid as claimed in claim 1 wherein said shield further comprises a shell for completely encapsulating said core and said core further comprises an electrolyte.

3. An electroviscous fluid as claimed in claim 1 or 2 wherein said shield further comprises at least one buoyant body.

4. An electroviscous fluid as claimed in claim 3 wherein said at least one buoyant body is a hollow glass microsphere.

5. An electroviscous fluid as claimed in claim 3 wherein said electroviscous fluid is suspended between a first lighter, lower density dielectric fluid and a second heavier, higher density dielectric fluid.

6. An electroviscous fluid as claimed in claim 3 wherein said at least one buoyant body is a gas pocket formed in said shield.

7. An aggregate for a high temperature electroviscous fluid wherein said electroviscous fluid comprises a multiplicity of electrically polarizable aggregate particles dispersed in a dielectric fluid, a substantial portion of said aggregate particles each further comprising a core and an electrically insulative shield, said core being at least partially encompassing said core, said shield adapted to prevent substantial particle to particle transmission of electric current, said shield further comprising an electrically insulative ceramic material.

8. An aggregate for a high temperature electroviscous fluid as claimed in claim 7 wherein said shield further comprises at least one buoyant body.

9. An electroviscous fluid which comprises a liquid continuous phase and, dispersed therein, at least one dispersed phase and which is capable of functioning as such when at least the dispersed phase is substan¬ tially anhydrous and wherein said dispersed phase is a good electrical resistor.

10. A process for making an aggregate for a high temperature electroviscous fluid comprising: a) preparing a powdered core material, a substantial portion of the core material being particles of material that is at least partially conductive; b) mixing said core particles with an electrically insulative ceramic material so as to cause said core particles to adhere at least partially to the ceramic material; and c) breaking said mixture into small particles.