Nano-particle based variable transmission devices

Abstract

A variable transmission medium comprises a fluid and a plurality of nanoparticles dispersed in the fluid, wherein addition of acid to the fluid causes the nanoparticles to flocculate and form aggregates of particles that scatter light. The nanoparticles may comprise at least one metal oxide, such as titanium dioxide, zinc oxide or zirconium dioxide. The fluid may have a dielectric constant less than about 10. The medium may be used in, for example, privacy glass for a conference room.

Claims

1. A method of varying the opacity of a variable transmission assembly comprising: providing a variable transmission assembly including a first light-transmissive electrode, a second light-transmissive electrode, and a variable transmission medium disposed between the first and second light-transmissive electrodes, wherein the variable transmission medium comprises zirconium dioxide nanoparticles, titanium dioxide nanoparticles, or zinc oxide nanoparticles dispersed in a fluid having a dielectric constant less than 10, wherein the zirconium dioxide nanoparticles, titanium dioxide nanoparticles, or zinc oxide nanoparticles have adsorbed thereon a stabilizing material that comprises a carboxylic acid or carboxylate group attached to a chain of at least 10 carbon atoms; providing a first electric current between the first and second light-transmissive electrodes and through the variable transmission medium, wherein the first electric current has a square or sinusoidal AC form and includes one or more rests, wherein a zero potential is applied during the rest, and wherein the first electric current produces a proton gradient in the variable transmission medium, causing the nanoparticles to flocculate and form aggregates that scatter light; and providing a second electric current, wherein the second electric current is an AC current having frequency of 60 Hz and higher, causing thermally-induced de-flocculation of the nanoparticles that increases the light-transmissivity of the variable transmission assembly.

2. The method of claim 1, wherein the variable transmission medium is contained within a plurality of microcapsules, microcells, or discrete droplets, the discrete droplets being surrounded by a continuous phase comprising a polymeric material.

3. The method of claim 1, wherein the stabilizing material comprises stearic acid.

4. The method of claim 1, wherein the fluid comprises an aliphatic hydrocarbon or a terpene.

5. The method of claim 4, wherein the terpene is limonene.

6. The method of claim 1, wherein the first or second light-transmissive electrode comprises indium tin oxide (ITO) or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT).

7. The method of claim 1, wherein the zirconium dioxide nanoparticles, titanium dioxide nanoparticles, or zinc oxide nanoparticles are between 10 and 100 nm in diameter.

8. The method of claim 1, wherein the fluid of the variable transmission medium is a petroleum distillate having molecular weight of less than about 1000 g/mole.

9. The method of claim 1, wherein the fluid of the variable transmission medium further comprises a polymer which is soluble in the fluid, wherein the polymer has molecular weight of from about 10,000 to about 1,000,000 g/mole.

10. The method of claim 1, wherein the variable transmission medium further comprises a soluble polymer having a molecular weight greater than about 1,000,000 g/mole.

11. The method of claim 1, wherein the first and second light-transmissive electrodes are not coated by a polymer.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 of the accompanying drawings is a reaction scheme illustrating schematically the chemical changes occurring in one variable transmission medium of the present invention during adsorption of a stabilizing material on to a pigment particle, and subsequent desorption of the stabilizing material upon addition of acid to the medium.

(2) FIG. 2 is a graph showing a waveform applied to a variable transmission medium of the present invention and the resultant changes in the reflectivity of the medium, as described in Example 1 below.

(3) FIG. 3 is a graph that shows the effect of acid and base titrations on the optical clarity of dispersions of nanoparticles for use in the present invention.

(4) FIG. 4 is a graph showing changes in optical clarity of nanoparticle dispersions of the present invention when addressed electrically with unblocked electrodes as compared with electrodes coated with a nonconductive polymer.

(5) FIG. 5 is a graph showing the variation of the conductivity of an electrically charged nanoparticle dispersion with concentration of the nanoparticles.

(6) FIGS. 6A and 6B are graphs illustrating the variation of haze with time in certain experiments described in Example 4 below, with FIG. 6A showing the drive from a clear to an opaque state, while FIG. 6B shows the recovery to the clear state without driving.

DETAILED DESCRIPTION

(7) As indicated above, the present invention provides a variable transmission medium comprising a fluid and a plurality of nanoparticles dispersed in the fluid. A stabilizing material is adsorbed on the nanoparticles, and this stabilizing material keeps the nanoparticles separated from one another so that the variable transmission medium is substantially non-reflective. However, upon a change in conditions in the fluid, the nanoparticles may be induced to flocculate and thereby form aggregates of particles that scatter light, causing the medium to become reflective and thus less transmissive of light. This change in conditions in the fluid may involve local introduction of acid, which may lead to desorption of the stabilizing material from the nanoparticles. Such a local introduction of acid may be achieved by passing a current through the device (i.e., by providing an electrochemically-induced chemical change in the fluid).

(8) The material used to form the nanoparticles used in the present invention may be any material which is substantially non-reflective when the nanoparticles are uniformly dispersed throughout the medium, but which causes the medium to become reflective when the nanoparticles flocculate. In general, metal oxide nanoparticles are preferred because of their high refractive index and consequent high light scattering performance. Furthermore, finely-dispersed nanoparticulate metal oxides having particle diameters in the range of 10-100 nm are readily available commercially, having been used to provide ultraviolet-absorbing, transparent coatings for applications such as thermal printing and cosmetics (sunscreens). Examples of such metal oxides include titanium dioxide, zinc oxide, zirconium dioxide and the like. When the nanoparticles are closely aggregated into clusters of diameter 100-500 nm (or larger) in a medium of relatively low refractive index, they exhibit efficient scattering of light.

(9) Preferred materials used to form the nanoparticles in the variable transmission media of the present invention include titanium dioxide and zirconium dioxide. Titanium dioxide (titania) has an especially high refractive index (about 4.23 for the rutile form) and therefore exhibits very efficient scattering of light when aggregated into clusters in a medium of lower refractive index. This material, however, has a band gap of 3.2-3.4 eV and thus can absorb light of visible wavelengths; such absorption cannot be prevented by optical filtration without leading to introduction of an undesirable tint into a variable transmission assembly of the invention. Once light has been absorbed (leading to formation of an electronically excited state of the titanium dioxide), undesirable chemical reactions, such as oxidation of water, may occur, causing degradation of the nanoparticles or of other elements comprising the assemblies of the invention. It is known in the art that such photoreactions may be alleviated by passivating the surface of titanium dioxide particles, by for example coating them with a layer of another metal oxide. However, such passivation may not be perfectly effective in eliminating undesirable photochemical side reactions. For example, the nanoparticles used in the present invention may be on the order of 10-25 nm in diameter. Applying a continuous passivating coating about 5 nm in thickness on to such a nanoparticle is synthetically difficult and can dramatically lower the average refractive index of the resulting particles. Further, such a coating may be fractured during a subsequent milling step, exposing a surface of reactive titanium dioxide.

(10) For this reason, a particularly preferred material for use in forming the nanoparticles of the present invention is zirconium dioxide (zirconia). Although the refractive index of zirconium dioxide (about 2.13) is lower than that of titanium dioxide, the band gap of zirconium dioxide is substantially larger (5-7 eV, depending upon the precise size and constitution of the nanoparticles). Zirconium dioxide therefore does not absorb any significant light in the visible spectrum and is much less prone to suffer from undesirable photochemical reactions than is titanium dioxide.

(11) Alternative materials for use in the present invention include alumina (whose refractive index is, however, only about 1.7), zinc oxide (which has a band gap of only about 3.3 eV) and silicon carbide (which, however, is colored).

(12) In the variable transmission media of the present invention, converting a fine dispersion of nanoparticles into a clustered (flocculated) configuration provides a switch of the media from a transparent to a scattering (opaque) state. The reverse transition from an opaque to a transparent state may be effected by thermal re-equilibration, and this transition can be accelerated by high-frequency AC addressing.

(13) The fluids used in the variable transmission media of the present invention will typically be of low dielectric constant (preferably less than 10 and desirably less than 3). Especially preferred solvents include aliphatic hydrocarbons such as heptane, octane, and petroleum distillates such as Isopar (Registered Trade Mark) or Isane; terpenes such as limonene; and aromatic hydrocarbons such as toluene. A particularly preferred solvent is limonene, since it combines a low dielectric constant (2.3) with a relatively high refractive index (1.47). In encapsulated media of the present invention, it is preferred that the refractive index of the dispersion of nanoparticles match as closely as possible that of the encapsulating material, and this is easier to achieve (when employing commonly-available polymeric encapsulants) when the refractive index of the solvent is close to that of the encapsulant. If the refractive index of the solvent is too low, an unacceptably high concentration of nanoparticles (whose refractive index is always higher than that of the solvent) may be required to match the refractive index of the encapsulant.

(14) As mentioned above, a stabilizing material is adsorbed on the nanoparticles when they are dispersed in the solvent, and this stabilizing material keeps the nanoparticles separated from one another so that the variable transmission medium is substantially non-reflective when the nanoparticles are in their dispersed state. As is known in the art, dispersing a nanoparticle (typically a metal oxide nanoparticle, as described above) in a solvent of low dielectric constant may be achieved by the use of a surfactant. Such a surfactant typically comprises a polar head group and a non-polar tail group that is compatible with or soluble in the solvent. In the present invention, it is preferred that the non-polar tail group be a saturated or unsaturated hydrocarbon moiety, or another group that is soluble in hydrocarbon solvents, such as for example a poly(dialkylsiloxane). The polar group may be any polar organic functionality, including ionic materials such as ammonium, sulfonate or phosphonate salts, or acidic or basic groups. Particularly preferred head groups are carboxylic acid or carboxylate groups. It is thought that such groups may facilitate the flocculation of the nanoparticles by acid-induced desorption, as described below.

(15) FIG. 1 of the accompanying drawings illustrates schematically the chemical changes that may occur in one variable transmission medium of the present invention during adsorption of a stabilizing material on to a pigment particle, and subsequent desorption of the stabilizing material upon addition of acid to the medium. Formula I in FIG. 1 illustrates the deprotonated form of a hydroxyl group which is present on most metal oxide nanoparticle pigments. In FIG. 1, the metal is illustrated as aluminum, since aluminum oxide (alumina) is commonly used as a passivating coating on metal oxides of higher refractive index, such as titanium dioxide; see, for example, U.S. Pat. No. 6,822,782, especially Example 31. The metal oxide nanoparticle is treated with a long chain fatty acid (oleic acid is illustrated) to produce, as shown in Formula II in FIG. 1, a hydroxyl group and a fatty acid anion associated with the metal oxide nanoparticle. (Although FIG. 1 shows a hydroxyl group on the alumina and a fatty acid anion, in practice the anion is likely to be hydrogen bonded to the hydroxyl group.) In the hydrocarbon solvent (or other solvent of low dielectric constant) commonly used in variable transmission media, the presence of the long chain fatty acid anion associated with the metal oxide nanoparticle changes the surface characteristics of the nanoparticle and sterically stabilizes the nanoparticle; thus, a thin layer (typically 10-50 m) of such a dispersion of nanoparticles will not appreciably scatter light. On the other hand, introduction into the dispersion of an acid stronger than oleic acid will lead to protonation of the oleic acid, and consequent release of the oleic acid from the nanoparticle surface, as illustrated in Formula III in FIG. 1. Desorption of the oleic acid removes the steric stabilization of the nanoparticles, which proceed to form aggregates which scatter light.

(16) Preferred stabilizing materials include long-chain fatty acids such as oleic acid, or oligomers of such materials such as poly(hydroxystearic acid). Many variants of these materials will be apparent to those skilled in the art, however in general the non-polar tail group will be soluble in a solvent of low dielectric constant (for example, an aliphatic carbon chain of at least 10 atoms or a poly(dialkylsiloxane)), and the polar head group will be an acid or the salt of an acid having an aqueous pKa in the range of 2-6, such as a carboxylic acid.

(17) Formation of the nanoparticle/acid complex is readily effected by, for example, milling a nanoparticulate metal oxide pigment in a solvent of low dielectric constant in the presence of the stabilizing group to form a dispersion of very small particles.

(18) The strong acid required to protonate the fatty acid anion may be introduced electrochemically; it is known that electrolysis of water creates a proton gradient in an electrochemical cell. Alternatively, the redox couple required to generate acid could comprise organic compounds that are reversibly oxidized and reduced, examples including mixtures of quinones and hydroquinones as described in detail in as described in U.S. Patent Application Publication No. 2015/0015932. The use of organic electrode materials such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT) may also lead to electrochemical reactions useful in the present invention.

(19) During electrochemical generation of acid, the nanoparticles will be flocculate in the vicinity of the anode. The flocculated material may then move from its adjacent the anode, either being carried by electrochemical currents that may be spatially organized, as described in Y. Han and D. Grier, Colloidal convection in thin horizontal cell III: Interfacial and transient patterns on electrodes, J. Chem. Phys. 2012 Jul. 7; 137(1):014504. doi: 10.1063/1.4730752, or by intrinsic electrophoretic motion. Reversal of the light scattering may be achieved thermally or by driving the cell with alternating current fields at a frequency sufficiently high that electrochemistry does not take place; experimentally this has been found to be at 60 Hz or higher. At such frequencies phenomena such as induced-charge electro-osmosis may occur, leading to enhanced mixing and thus re-equilibration of the nanoparticle dispersion. FIG. 2 shows the change in reflectivity that can be achieved by electrically addressing a variable transmission medium of the invention. Example 1 below describes in more detail how this result was obtained.

(20) As shown in FIG. 3 and described in detail in Example 2 below, dispersions of nanoparticles suitable for use in the present invention are substantially non-scattering in the resting state but may be induced to flocculate, thereby forming a state that scatters light, by the introduction of an acid. Such an acid may be, for example, a strong carboxylic acid such as trifluoroacetic acid. Similar, but weaker flocculation may be induced by adding a weaker carboxylic acid containing fewer than 10 carbon atoms. In many cases, it has been found that such flocculation may be reversed by the addition of a suitable base, but it is not a requirement of the present invention that such a reversal be evident.

(21) As is shown in FIG. 4 and described in detail in Example 3 below, in assemblies of the present invention it is necessary that a current be passed through the variable transmission medium in order for electrically-induced flocculation of nanoparticles to occur. When a dispersion of nanoparticles of the present invention is placed between two spaced-apart electrodes, flocculation of the nanoparticles occurs when an electric current is passed but can be suppressed by coating the electrodes with a thin coating of a polymer that blocks electrochemical reactions but does not greatly reduce the electric field between the electrodes.

(22) Electrochemically-induced nanoparticle flocculation is likely to be more efficient that attempting nanoparticle flocculation by electrophoresis alone. It has been found that dispersions of alumina-passivated nanoparticulate titanium dioxide in an Isopar G/oleic acid mixture have a very weakly positive zeta potential (about 5 mV). This is consistent with only a very small proportion of the particles bearing a charge. As shown in FIG. 5, the conductivity of such dispersions varies linearly with concentration, with a slope consistent with only about 1% of the nanoparticles being charged. In contrast, the present invention is not dependent upon any charge existing on individual nanoparticles, or the mobilities of such nanoparticles, at least as regards driving the nanoparticles from their dispersed, transparent state to their flocculated, light-scattering state. This is confirmed by the observation, described above, that when one attempts to drive nanoparticles from their dispersed, transparent state to their flocculated, light-scattering state using blocked electrodes (i.e., electrodes provided with a coating which does not allow charge injection into the fluid), no change in the optical state of the nanoparticle dispersion occurs.

(23) As described above, it is thought (although this is not intended to limit the scope of the present invention in any way) that an electrochemical reaction, such as electrolysis of water, may produce a proton gradient in the dispersion of nanoparticles that may lead to flocculation of the particles in the vicinity of the anode (where the proton concentration is highest). As will be appreciated by those skilled in the art, it is not necessary that the rate of flocculation and deflocculation of the nanoparticles be the same. Thus, it may be preferred to drive an assembly of the present invention using a waveform in which pulses of alternating electrical polarity are applied in a time-sequential manner. For example, an addressing waveform may comprise a square-wave or sinusoidal AC form that may include rests (times when zero potential is applied). If the rate of flocculation (in the vicinity of the anode) is higher than the rate of deflocculation (in the vicinity of the cathode) the device will still exhibit increasing optical opacity, even though such a waveform may be DC-balanced in impulse. As is known in the art, DC-balancing in impulse may be used to limit damage to the device by electrochemical degradation of critical components, including the electrodes themselves.

(24) Just as the rate of electrically-induced flocculation and deflocculation of the nanoparticles may not be identical, the rate of electrically-induced flocculation and thermally-induced deflocculation by (diffusional) equilibration may likewise not be identical. Furthermore, the rates of electrically-induced flocculation and of thermally-induced deflocculation may respond differently to changes in the rheology of the fluid in which the nanoparticles are dispersed. For example, the fluid may be made more viscous by introduction of large amounts of materials having relatively low molecular weights or small amounts of materials having relatively high molecular weights. It has been found experimentally, as described below with reference to Example 4 and FIGS. 6A and 6B, that making the fluid more viscous by incorporation of low molecular weight components (i.e., by changing the average molecular weight of a petroleum distillate by adding components having a molecular weight of less than about 1000) leads both to slower electrically-induced flocculation and to slower thermally-induced deflocculation. However, making the fluid more viscous by introduction of a relatively small amount of a soluble polymer of high molecular weight (in the case of Example 4, a polymer of molecular weight greater than about 1,000,000) does not appreciably change the rate of flocculation, but greatly decreases the rate of thermally-induced deflocculation.

(25) It will thus be apparent that independent control of the rate of flocculation (i.e., the rate of development of optical opacity in response to an applied voltage) and deflocculation (i.e., the rate of return to optical clarity in the absence of electrical driving) is possible in the present invention. As noted above, acceleration of the rate of deflocculation may also be achieved by electrical driving.

(26) The ability to control independently the rates of flocculation and deflocculation is important in practical embodiments of the present invention. For example, it may be desired to provide a variable transmission window powered by a photovoltaic element. Such a window could be self-contained and not require connection to mains electricity (although, of course an energy storage element such as a battery or capacitor may be provided to store photovoltaically-generated charge). For maximum power efficiency, it may be preferred to drive the window from a clear state to an opaque state electrically, but to allow it to return to a clear state thermally. Maintenance of the window in the opaque state could be provided by top-up electrical driving. In this embodiment, there will clearly be a need to control the rate of thermal clearing of the window relative to the rate of electrical opening as can be provided, as described above, by suitably controlling the rheology of the solvent in which the nanoparticles of the present invention are dispersed. For example, too slow a rate of thermal re-opening may be unacceptable, but too fast a rate of thermal re-opening may necessitate too great a load on the power supply for the top-up driving required when the window is desired to remain closed.

(27) The nanoparticle dispersion used in the present invention may or may not be encapsulated. The nanoparticles are so small that typically gravitational settling is minimal. It has been found empirically that, when a 36.5% solids dispersion of alumina-passivated, nanoparticulate titanium dioxide in a mixture of oleic acid and Isopar G with a sample height of 10 cm and an internal tube diameter of 5 mm was centrifuged at 30G for 65 hours, no significant sedimentation occurred. If there is no sedimentation of a 10 cm sample at 30G, there would be no sedimentation for a 3 meter tall window at 1G. An unencapsulated medium of the present invention may be prepared by providing two sheets of glass (or other transparent material) each provided with a light-transmissive electrode on one surface, positioning the two sheets spaced apart from one another with their electrodes disposed on their adjacent surfaces, and pouring the nanoparticle dispersion into the gap between the two sheets; spacers or open cell structures may be provided between the sheets if desired to maintain proper spacing. However, in some cases, it may be desirable to encapsulate the nanoparticle dispersion in the form of capsules, microcells or polymer-dispersed materials. For example, a microcell encapsulated nanoparticle dispersion could provide useful mechanical support when disposed between two pieces of glass when the variable transmission medium is to be used in a high vibration environment.

(28) The following Examples are now given, though by way of illustration only, to show details of preferred materials, conditions and techniques used in the present invention.

EXAMPLE 1

(29) This Example illustrates the development of light scattering in a variable transmission medium of the invention by electrical addressing.

(30) A pigment dispersion was prepared by adding 87.5 g of alumina passivated nanoparticulate titanium dioxide (grade MS-7, available from Kobo Corporation, South Plainfield, N.J.), 13.12 g oleic acid (150 mg oleic acid per gram of pigment), and 175 g of Isopar G or limonene to a 500 mL glass jar. The resultant mixture was shaken to disperse the pigment, then sonicated for 60 minutes in a bath sonicator, prior to high speed horizontal milling on a Netzsch MiniCer mill using milling conditions of 3500 RPM, pump speed 150 RPM, with 200 m Zirconia medium (Zirmil from Saint Gobain) for 60 minutes. The 200 m medium was then replaced with a similar 100 m medium and the dispersion was milled for additional 60 minutes under the same milling conditions as before.

(31) The dispersion (with limonene solvent) was placed between two glass plates each having an indium tin oxide (ITO) coating and the resultant cell placed on a black background. The cell was then driven with 250 millisecond pulses at 30 V, separated by rests of 250 milliseconds as shown in the lower part of FIG. 2. The upper part of the same Figure shows the reflectivity of the cell (measured as L* on the usual CIE L*a*b* system). From FIG. 2 it will be seen that as the drive pulses were applied (from t=5 to t=10.25 seconds), the reflectivity of the cell rose from 29 L* (the transparent state, viewed against the black background) to 47 L* (the scattering state, viewed against the black background). From t=10.25 to t=18 seconds, the cell was driven with an AC voltage of 40V at 55 Hz and returned to its original transparent state.

(32) In separate experiments, a transparent state haze of 3.5 percent and an opaque state of 87 percent haze were achieved by driving the same dispersion in a similar cell and in a similar way, and measuring the optical clarity using a transmission spectrometer, calculating haze as defined in ASTM standard D1003.

EXAMPLE 2

(33) This Example illustrates the effect of treating a variable transmission medium of the invention with acid and base.

(34) A sample of untreated yttria-stabilized zirconium oxide nanoparticles (ZrO.sub.2-3Y, 8522QI 25, nm) from Skyspring Nano (Houston, Tex.) with 2500 ppm chloride content (measured by combustion ion chromatography) was dispersed following the same procedure as in Example 1. This dispersion was diluted to 2 weight percent pigment by addition of limonene and a 19.5 g sample was added to a 2 cm diameter vial. The dispersion was then titrated with freshly prepared 2 weight percent trifluoroacetic acid (TFA, purchased from Sigma Aldrich) in limonene. Turbidity was measured using a Hach 2100AN turbidimeter and reported in nephelometric turbidity units (NTU). After adding acid a 10 weight percent solution of trioctylamine (purchased from Sigma Aldrich) in limonene was then titrated into the dispersion.

(35) A sample of yttria-stabilized zirconium oxide nanoparticles containing 1400 ppm chloride was prepared by washing the aforementioned ZrO.sub.2-3Y with three 1 deionized water washes. The pigment was then dried overnight under vacuum at 85 C. This pigment was then milled, diluted and titrated under identical conditions as the sample containing 2500 ppm chloride.

(36) As can be seen from FIG. 3 (open circles), after about 0.2 mmole of trifluoroacetic acid (TFA) had been added to the unwashed zirconium oxide dispersion, further addition of acid led to rapid development of turbidity. Addition of an approximately stoichiometric amount of a base, trioctylamine (TOA), reversed the effect (closed circles). The same effects were seen with the washed zirconium oxide (open and closed squares) but in this case a much larger amount of acid was needed to initiate turbidity. This result is consistent with hydrogen chloride (a possible byproduct from the production of the nanoparticles, and a strong acid) having been removed by the washing process. Accordingly, with some variable transmission media of the present invention, it may be desirable to titrate the media with acid prior to use, and adjust with acid or base to tune sensitivity; addition of acid would lead to higher sensitivity; addition of base to lower sensitivity.

EXAMPLE 3

(37) This Example illustrates that the effect of blocking current in an assembly of the invention, by coating the electrodes with a nonconductive material, is to inhibit the development of opacity, thus demonstrating that the change in light transmission is not caused by electrophoresis of the nanoparticles alone.

(38) A dispersion was prepared by combining 174.5 g of yttria-stabilized zirconium oxide nanoparticles (ZrO.sub.2-3Y), 100.6 g of high purity limonene (from Florida Chemical Company) and 84.5 g of a 24.8 weight percent solution of DSP-OL300 (polyhydroxystearic acid, PHSA, from Innospec Performance Chemicals, Ellesmere Port, United Kingdom), in limonene in a 1 plastic bottle with 1687 g of 0.6 mm Zirmil and 250 g of 5.0 mm YTZ yttria-stabilized Zirconia media and milling for 1 week at room temperature on a roll mill at approximately 200 RPM. The media were removed via filtration and the refractive index was matched to the desired target of 1.54 by addition of limonene.

(39) Liquid test cells were built by sandwiching this dispersion between two sheets of indium tin oxide (ITO) glass with a 1 mil plastic shim. Electro-optical testing was performed on a transmission bench applying a waveform consisting of 10 cycles of 750 milliseconds 30 V, 250 milliseconds 0 V, 750 milliseconds 30 V, 250 milliseconds 0 V. Coated electrodes were prepared by spin coating a solution of 5 weight percent poly(methyl methacrylate) (PMMA, Mw approximately 350 kg/mole, purchased from Sigma Aldrich, Milwaukee, Wis.), in toluene on to ITO glass slides at 3000 RPM for 30 seconds. The thickness of the deposited layer was measured using a Zygo interferometer to be approximately 100 nm.

(40) As can be seen in FIG. 4, when the polymer coating was applied to the electrodes, no change in haze was seen when addressing the sample. Under the same conditions, using the uncoated electrodes, the haze rose from about 10% to about 62%. These results show that current flow is necessary for flocculation to occur (leading to light scattering and consequent haze).

EXAMPLE 4

(41) This Example illustrates that independent control of the rates of electrically-induced flocculation and deflocculation (opacification and relaxation) may be achieved by the addition of soluble polymers to the fluid of media of the present invention

(42) Nanoparticulate titanium dioxide (grade MS-7, available from Kobo Corporation, South Plainfield, N.J.) was dispersed via milling (on a Netzsch mill) in the presence of dispersant (oleic acid). The stock dispersion was 20 weight percent nanotitania and 6 weight percent oleic acid dispersed in Isopar E.

(43) This dispersion was diluted with a) Isopar E; b) Isopar V (a more viscous hydrocarbon grade); or c) a solution of a polyisobutylene (PIB) polymer (Oppanol B200, available from BASF Corporation) in Isopar E.

(44) Each dilution was a factor of two by weight, i.e., accomplished by mixing equal weights of stock dispersion and diluent, as described in the Table below. In this Table, amounts are expressed in terms of total sample mass. For example, for sample IE/PIB, the PIB amount of 0.6% total mass corresponds to 0.69% of fluid phase mass (ignoring nanotitania and oleic acid). Upon preparation the samples were left to equilibrate at least overnight.

(45) TABLE-US-00001 TABLE Sample name Nanoparticies Oleic acid Isopar E Isopar V PIB IE/IE 10% 3% 87% 0 0 IE/IV 10% 3% 50% 37% 0 IE/PIB 10% 3% 86.4%.sup. 0 0.6%

(46) To test the electro-optical response samples were loaded into a cell comprising two ITO-coated glass slides, ITO sides facing each other, spaced apart with a ring shim spacer (1.5 inches internal diameter, 0.002 inches thick (38 mm internal diameter, 0.05 mm thick). The slides were held together by paper clips, clipped across the shim spacer to prevent slides from bowing in under stress. The test cell was loaded with enough sample dispersion to completely fill the inner ring of the shim; any overfill was dabbed away with a tissue.

(47) Optical response was tested on a custom transmission measurement bench. The waveform used was 30V, 0.25 Hz, square wave, no offset. Each test started with 20 seconds without driving, after which the waveform was applied for 30 seconds, followed by 1 hour without driving. The optical response was recorded during this whole time, at a sampling rate of 5 s.sup.1.

(48) The haze reported here is in arbitrary units (AU). However, as all the samples were tested in the same way (same mounting, same spacers, under a single and same detector calibration) relative sample-to-sample comparisons can be made. This is particularly true when comparing timescales alone.

(49) The electro-optical response of the three samples described is shown in FIGS. 6A and 6B. FIG. 6A shows the drive from a clear to an opaque state, while FIG. 6B shows the recovery to the clear state without driving. As can be seen in FIG. 6A, the all-Isopar E sample and the sample containing PIB dissolved in Isopar E developed opacity at the same rate. The sample in which the Isopar E had been diluted with the more viscous Isopar V, on the other hand, developed opacity at a slower rate.

(50) The rates of recovery from the opaque to the clear state are shown in FIG. 6B. Here the all-Isopar E sample recovered very quickly to the clear state, while the other two samples recovered at about the same, slower rate.

(51) It will be clear, therefore, that in a given fluid (here, Isopar E) the rate of recovery from the opaque state may be controlled independently of the rate of driving to the opaque state by addition of a polymer that is soluble in the fluid. In this particular experiment, the molecular weight of the polymer was on the order of 1,000,000. Polymers or oligomers having molecular weights as low as about 10,000 may also be used.

(52) From the foregoing, it will be seen that the present invention can provide a variable transmission medium having a very low haze in its transparent state with a highly opaque scattering state, and which is thus very attractive for commercial applications such as variable transmission windows. No encapsulation of the variable transmission medium is necessary, although encapsulation can be used if desired. The power requirements of the variable transmission medium of the invention are acceptable.

(53) It will be apparent to those skilled in the art that numerous changes and modifications can be made in the specific embodiments of the invention described above without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be interpreted in an illustrative and not in a limitative sense.