Electrowetting and electrofluidic devices with laplace barriers and related methods

Electrowetting and electrofluidic devices and methods. The device includes a hydrophobic channel formed between first and second substrates and a polar fluid and a non-polar fluid contained in the channel. An electrode with a dielectric layer is electrically connected to a voltage source. A Laplace barrier within the hydrophobic channel defines a fluid pathway that is open to the movement of the polar fluid within the channel. The polar fluid moves to a first position when the voltage source is biased at a first voltage that is less than or equal to a threshold voltage. The polar fluid moves to a second position when the voltage source is biased with a second voltage that is greater than the first voltage.

BACKGROUND

The present invention relates to electrofluidic and electrowetting devices and methods of operating electrofluidic and electrowetting devices.

Electrowetting is a highly attractive modulation scheme for a variety of optical applications. For example, electrowetting has been used to provide optical switches for fiber optics, optical shutters or filters for cameras and guidance systems, optical pickup devices, optical waveguide materials, and video display pixels. Electrowetting has also found application in lab-on-chip devices, primarily in the form of digital droplet-driven flow.

Despite the numerous commercial applications and a large body of on-going research, nearly all conventional electrowetting-based devices require a constant application of voltage to hold a polar fluid in a particular geometry. These devices are not ‘bistable,’ that is to say, when the voltage is removed, the fluid is free to return to a spherical geometry along all non-confined fluid surfaces.

What is needed is a technology that improves on conventional devices that operate through electrowetting and electrofluidic principles.

SUMMARY

Embodiments of the invention generally relate to controlling polar fluid geometry with Laplace barriers and electrowetting or electrofluidic flow devices with Laplace barriers

In accordance with an embodiment of the invention, an electrofluidic or electrowetting device includes a polar fluid and a non-polar fluid. The fluids occupy a hydrophobic channel formed between first and second substrates. An electrode with a dielectric layer, which separates the electrode from the fluids, is electrically connected to a voltage source. This electrode and dielectric layer arrangements can cause the polar fluid to advance or move within the hydrophobic channel. A Laplace barrier within the hydrophobic channel defines a fluid pathway that is open to the movement of the polar fluid. The polar fluid moves to a first position within the hydrophobic channel by biasing the electrode with a first voltage that is less than or equal to a threshold voltage. The polar fluid then moves to a second position within the hydrophobic channel when the electrode is biased with a second voltage that is greater than the first voltage.

When the electrode is biased with the first voltage, the Laplace barrier may restrain the polar fluid at the first position. The Laplace barrier operates by reliance on Laplace pressure.

In accordance with another embodiment of the invention, a method is provided for operating a device. The method includes moving a polar fluid to a first position within the device and restraining the polar fluid at the first position by a Laplace barrier.

DETAILED DESCRIPTION

Although the invention will be described in connection with certain embodiments, the description of the embodiments is intended to cover all alternatives, modifications, and equivalent arrangements as may be included within the spirit of the present invention. In particular, those of ordinary skill in the art will recognize that the components of the various electrofluidic devices described herein could be arranged in multiple different ways.

An electromechanical force on a conductive polar fluid that is adjacent to an electrically insulated electrode underlies the physical mechanism for at least one embodiment of the present invention. This electromechanical force originates near a line of contact between the conductive polar fluid and a dielectric that insulates the electrode. The electromechanical force is proportional to electrical capacitance times the square of the bias potential, or applied voltage. The electromechanical force is generally oriented such that it is directed outward from an exposed surface of the polar fluid. When the polar fluid is confined within a cavity or channel, this electromechanical force can also be interpreted as a force per unit area or a pressure. This arrangement provides high-speed operation (on the order of milliseconds), low power capacitive operation (about 10 mJ/m2), and excellent reversibility. However, alternative embodiments of the present invention include other fluid manipulation methods that are well-known by those of ordinary skill in the art of microfluidics. These alternate methods include, but are not limited to, electrowetting without insulators, syringe-pumps, thermocapillary, photo-responsive molecules such as spiropyrans, dielectrophoresis, electrophoresis, and micro-electro-mechanical pumping. Various embodiments of Laplace barriers that are described herein will work equally well with alternate mechanisms of fluid manipulation and transport. In some embodiments, the Laplace barriers may be referred to as a partial fluid barrier or a porous fluid barrier.

A Cartesian coordinate system will be used to define specific directions and orientations. References to terms such as ‘above,’ upper,‘below,’ and ‘lower’ are for convenience of description only and represent only one possible frame of reference for describing a particular embodiment. The dimensions of the devices described herein cover a wide range of sizes from nanometers-to-meters based on the application. Terms such as visible will be used in some cases to describe a person or machine vision system or other optical source or detector that is facing towards an upper surface of the embodiments described herein. Several of the diagrams will contain a ‘side view’ and a ‘top view’, the ‘top view’ being the direction normal to a substrate surface, usually a viewable area of the substrate, and in some cases in the direction of the viewer or observer of the device. These top-view diagrams can be partial device cross-sections in order to show the arrangement of only a particular sub-set of features and should not be always be considered as the actual top-view appearance of the device features.

The devices described herein are equally useful for reflective, transmissive, and transflective displays. Therefore light can transmit through or reflect from the upper surface, the lower surface, or both surfaces of the devices. Devices can operate in dual mode transmissive/reflective at the same time, or switch between such modes of operation on demand. Backlights or other light sources used in conventional displays are also fully compatible with the devices described herein and are included within the spirit of the present invention.

Light may be provided by a source that is positioned internal to the devices such as a backlight or frontlight, by waveguide or other optics, or by the ambient surroundings, such as sunlight or conventional light fixtures. Any means of coupling a light source is applicable, including all techniques known by those skilled in the art of displays.

The term fluid is used herein to describe any material or combination of materials that is neither solid nor plasma in its physical state. A gas may also be considered as a fluid so long as the gas moves freely according to the principles of the present invention. Solid materials, such as fluid powders, can also be considered a fluid if the solid materials move freely according to the principles of the present invention. The term fluid is not confining to any particular composition, viscosity, or surface tension. Fluids may also contain any weight percent of a solid material so long as that solid material is stably dispersed in the fluid. Fluids may also contain mixtures of multiple fluids, dispersants, resins, biocides, and other additives used in commercial fluids with demanding optical, temperature, electrical, fouling, or other performance specifications.

Examples of polar fluids include water, propylene glycol, and ethylene glycol. Examples of non-polar fluids include alkanes and silicone oils. Examples of gases include argon, carbon dioxide, and nitrogen. If more than one fluid is used that contains distinct solid particles or dissolved constituents, then it is preferred that the fluid be polar if particles or constituents are to be kept separated.

Non-polar fluids often penetrate small defects or situate against non-planar geometric structures. Therefore, mixing can occur with repeated movement of the non-polar fluid over a common device area. Polar fluids, in some cases, never touch solid surfaces. For instance in the case where the non-polar fluid forms a thin film between the polar fluid and solid surface.

Solid materials described herein serve multiple purposes.

Pigments and dyes in many cases are solid particles that can be dispersed or dissolved in fluids to alter at least one optical or spectral property of the fluid.

Substrates can be glass, plastic, metal foils, paper, or a variety of other materials that support construction of the devices described herein.

Spacers can be made of solid materials that are similar to the solid materials used in constructing the substrates. In some cases, the spacers can be part of the substrate itself, such spacers being formed by etching, laser processing, microreplication, or other technique. Spacers can also be formed from optically curable epoxies or photoresists, such as MICROCHEM SU-8 or DUPONT Per-MX.

Electrodes can be constructed from a transparent solid material, such as In2O3:SnO2or PEDOT:PSS, a reflective solid material such as aluminum (Al), or colored solid material such as carbon black, so long as the electrode material provides suitable electrical conductance. Voltage sources can be direct voltage sources from a power source or a locally generated voltage or current sources, such as thin-film transistors. Numerous direct, alternating, or other types of voltage sources are known to those skilled in the art of displays or microfluidics are applicable.

Dielectrics can include any solid material which provides suitable electrical insulation and, for example, can be inorganic such as silicon nitride (SiN), organic such as Parylene C, or fluorinated such as Parylene F, mixtures thereof, layers thereof, and combinations thereof. Dielectrics thicknesses can range from 10's nm to 10's μm resulting in operating voltages between 1 V and 120 V, respectively. Solid surfaces or films may be inherently hydrophobic, or provided with an order of hydrophobicity by addition of a film or coating, by plasma treatment, by molecular mono-layer treatment, or other means. Fluoropolymers such as CYTONIX Fluoropel and ASAHI Cytop provide exemplary hydrophobicity. Additional solid materials, which are not hydrophobic to the polar fluid such as water in a gas, may still be hydrophobic if the gas is replaced with a non-polar fluid that has surface tension that is similar to the solid material.

Generally, the term hydrophobic is used herein to describe a Young's wetting angle of >90°, and the term hydrophilic is used herein to describe a Young's wetting angle of <90°. However, hydrophobic or hydrophilic functionality may extend beyond these limits in certain devices or material configurations. Superhydrophobic coatings are those exhibiting a large Young's angle for a polar liquid in a gas and are achieved by geometrically texturing a surface.

Voltage sources can be direct voltage sources from a power source, locally generated voltage, or current sources such as thin-film transistors. Numerous direct, alternating, or other types of voltage sources known to those skilled in the art of displays or microfluidics are applicable. Voltage sources may be biased by 0V, a positive DC voltage, a negative DC voltage, or AC voltage or other as appropriate.

Reflector materials may include metal films, layers with different refractive indices including multilayer dielectrics, particle filled polymers or fluids where the particles differ in refractive index from the polymer or fluid, one- or multi-dimensional photonic crystals, or other reflectors that are known by those skilled in the art of optics and displays.

Scattering mediums include polymers or fluids having particles disperse therein and where the particles differ in refractive index from the polymer or fluid, structured polymers or metals, microreplicated optics, or other scattering features that are know by those skilled in the art of optics and displays.

Black matrix and color filters are any solid or fluid material that absorbs part of or the entire spectrum of light in reflection or transmission modes.

Unless otherwise noted, the terms concave and convex refer to the geometry associated with the smallest radius of curvature along an exposed meniscus of a fluid. It is understood that other larger radii of curvatures on a meniscus can be oppositely concave or convex, but have a weaker influence on the Laplace pressure of the meniscus. These additional radii are often not shown in the figures but are readily understood in terms of their weaker influence on device design and operation.

The term channel or hydrophobic channel will be used to describe physical confinement of a fluid that is horizontally larger than it is vertical in dimension, and which in some embodiments of the present invention will provide a means to visibly display a fluid. The channel is generally defined or bounded by one or more walls, typically of a fabricated or patterned substrate.

The term reservoir can be any feature formed as part of a device, or is external to the device, including any feature that can store or hold a fluid until it is ready to be moved inside, or into, a device. Reservoirs may also be simple inlet/outlet ports or vias that may or may not be connected to additional devices, chambers, or channels.

The term duct will be used to describe a feature which provides a pathway for fluid flow and, like the reservoir, can be integrated inside of device, or in some cases could be external to the device as well.

Fluids may be dosed into devices of the present invention using one of several methods. The polar fluid can be emulsed with the non-polar fluid and then physically, chemically, or optically separated from one another after the device is completed. The polar fluid can be vacuum dosed into the reservoir, non-polar fluid added, and the device sealed. The non-polar fluid can be dosed into a reservoir, the polar fluid added, and the device sealed. The polar fluid can be electrowetted into an area by application of voltage between the polar fluid and an electrode. Numerous alternatives and combinations of dosing combinations are included within the spirit of the present invention.

The above description provides examples of materials and components for embodiments of the invention; however, the description of any particular one embodiment is intended to cover all alternative materials, components, and arrangements known by those skilled in the arts of optics, displays, microfluidics, electrowetting, electrofluidics, microfabrication, electronics, and related disciplines.

With reference now toFIGS. 1A and 1B, a conventional electrowetting device10is shown for the purpose of building an understanding of the limitations of conventional technology. The device10includes a first substrate12, a first electrode14on the first substrate12, and a second substrate16that carries a second electrode18. The first electrode14, which is patterned in a square, cross-sectional geometry, is coated with a dielectric20. The surfaces of the second electrode18and the dielectric20are hydrophobic by virtue of a hydrophobic layer22. A polar fluid24and a non-polar fluid26occupy a channel28bounded by walls29a,29bof the two substrates12,16or the fabricated layers thereon. A voltage source30is electrically coupled with the two electrodes14,18.

By virtue of the electrodes14,18, the voltage source30is electrically insulated and capacitively coupled to the polar fluid24. Alternate conducting and insulating electrode arrangements are possible and are well known by those skilled in the art of electrowetting. As a voltage is applied to the voltage source30, a contact angle for the polar fluid24will be reduced from the Young's angle (θY) to the electrowetted contact angle (θV). This reduction in the contact angle is due to an electromechanical force that is generated by the electrowetting effect of the applied voltage.

As a first approximation, the electrowetting effect can be predicted according to:
cos θV=(γod−γpd)/γpo+CV2/2γpo,
where C is capacitance per unit area of the dielectric20; γ is the interfacial surface tension between the polar fluid24(p), the non-polar fluid26(o), the dielectric20(d); and V is the DC voltage or AC RMS voltage applied by the voltage source30. The cosine of the Young's angle is predicted by (γod−γpd)/γpo. When the non-polar fluid26is an oil, a polar fluid26such as water or a glycol can exhibit a Young's angle of >160°. The θVcan range from 30° to 60°. The Young's angle is not changed during electrowetting, except for microscopic changes. Yet for simplicity in diagramming herein, the θVwill be drawn without such consideration.

With further reference toFIG. 1A, a large Young's angle and therefore a convex meniscus for the polar fluid24is apparent. This convex meniscus has a radius of curvature that leads to a Laplace pressure according to:
Δp=γpo(1/R1+1/R2),
which includes the principle radii of curvature for the meniscii of the polar fluid24(R1, R2).

With further reference to ofFIGS. 1A and 1B, the polar fluid24will have a smaller radius of curvature32that will present a dominent force over a larger radius of curvature34in determining the Laplace pressure. In the top view diagram ofFIG. 1B, the meniscus is circular (as favored from a surface-tension perspective).

InFIGS. 1C and 1D, a voltage is applied from the voltage source30, and the polar fluid24can be electromechanically forced into a geometry that is similar to the geometry of the first electrode14. However, if the voltage is removed, the polar fluid24will regain a circular geometry that was illustrated inFIGS. 1A and 1B. Therefore, although this conventional electrowetting device10can use an applied voltage to alter the shape of the polar fluid24, the device10inherently posses only one stable state, i.e., once the voltage is removed, the polar fluid24will regain the geometry ofFIGS. 1A and 1B.

Similar features and numbers will be used throughout this written description and will be diagrammed using common markings, and need not always be duplicated in description.

FIGS. 2A-2Cillustrate one example of an electrowetting or electrofluidic device40, which may be bistable, according to a first embodiment of the invention. The device40includes a first substrate42and a second substrate44. The first substrate42includes a first electrode46and a dielectric48on the first electrode46, the first electrode46being representatively shaped as a circle, as shown inFIG. 2B. The second substrate includes a second electrode50and the dielectric48thereon, the second electrode50being shaped as a star, as shown inFIG. 2C. The enclosed space, or channel52, formed between the electrodes46,50includes a wire mesh54throughout. A first voltage source56couples the first electrode46to the wire mesh54; a second voltage source58couples the second electrode50to the wire mesh54. A light source60may be arranged external to, or otherwise associated with, the device40to form a viewable design.

As will be explained in greater detail below, the device40may be operated to form two separate and distinct stable configurations. In the first configuration, shown inFIG. 2B, a voltage (+V) is supplied by the first voltage source56such that the polar fluid24covers the first electrode46and aggregates to conform to the circle shape of the first electrode46. In the second configuration, shown inFIG. 2C, a voltage (+V) is supplied by the second voltage source58such that the polar fluid24covers the second electrode50and aggregates to conform to the star shape of the second electrode50. The non-polar fluid26resides in the volume of the channel52not otherwise occupied by the polar fluid26. Because the channel52is sealed, the alternate shapes are formed by shifting the volume of polar fluid24in accordance with the principles described below. As will be taught, the wire mesh54forms a Laplace barrier that can hold the polar fluid geometry even if voltage is removed or reduced.

With reference toFIGS. 3A-3D, an electrowetting or electrofluidic device70according to one embodiment of the invention includes first and second substrates42,44, a hydrophobic layer72on a portion of the first substrate42, and first and second hydrophilic layers74a,74bon different portions of the second substrate44. The polar fluid24and non-polar fluid26are disposed in the channel52. The layers72,74a,74bare not electrically insulating, thin, nor porous. The first substrate42includes two electrodes76,78that are separated by a void or space80, which extends from the dielectric48. The polar fluid24can be considered to have a direct electrical connection to a third electrode82, which is positioned between the second substrate44and the first and second hydrophilic layers74a,74b. A hydrophobic strip84resides between the first hydrophilic layer74aand the second hydrophilic layer74bon the second substrate44and is substantially aligned with the space80between the electrodes76,78of the first substrate42. In one embodiment, the non-polar fluid26may be a gas such that the polar fluid24has a Young's angle of about 90° on the hydrophilic layers74a,74band a Young's angle of about 120° on the hydrophobic layer72.

A first voltage source86couples the first and third electrode76,82; a second voltage source88couples the second and third electrodes78,82. As shown inFIGS. 3A and 3B, in the absence of an applied voltage, the polar fluid24conforms to the geometrical shape of first hydrophilic layer74abecause wetting onto the hydrophobic layer72and the hydrophobic strip84would result in a larger Young's angle and a smaller radius of curvature90. The geometry of the polar fluid24remains stable in the absence of the applied voltage so long as the horizontal radius of curvature92is not smaller than the vertical radius of curvature90.

InFIGS. 3C and 3D, a voltage (+V) of up to a threshold voltage is applied between the second and third electrodes78,82by the second voltage source88and results in a net pressure96that moves the polar fluid24from the first electrode76toward the second electrode78. The net pressure96can be understood in terms of an electromechanical force, or by noticing that the radius of curvature94is larger than the radius of curvature90.

As shown inFIGS. 3E and 3F, after the polar fluid24is moved completely onto the second electrode78, the applied voltage (+V) from the second voltage source88can be removed. In the absence of applied voltage, the polar fluid24is held stable in the illustrated geometry, i.e., a non-circular geometry corresponding to the geometry of the second electrode78.

InFIGS. 3A-3F, the strip84of hydrophobic layer72acts as a Laplace barrier according to one embodiment of the invention. The Laplace barrier defines a fluid pathway that is open to an advancement of the polar fluid24within the channel52. The Laplace barrier regulates the polar fluid geometry, but at the same time the polar fluid24is easily advanced past, beyond, or through the Laplace barrier.

With reference toFIGS. 4-4F, a portion of another embodiment of an electrofluidic device100that includes functionalities that are not possible with conventional device structures. The electrofluidic device100includes the first and second substrates42,44, similar to the prior embodiments. A first electrode102is positioned on the first substrate42, and a second electrode104is positioned on the second substrate44. A dielectric layer106is placed over the first and second electrodes102,104, which can be covered with a hydrophobic layer108. The electrofluidic device100further includes one or more hydrophobically coated spacers110acting as the Laplace barrier. The one or more hydrophobic spacers110create two or more passageways112(i.e., a plurality of openings) on the sides of, or between, the spacers110. The passageways112collectively define the fluid pathway for the advancement of the polar fluid24within the channel52. As an example only, the spacers110can be about 10 μm in diameter, about 10 μm in height (h), and are separated by passageways112that are about 50 μm wide. For example, the device100ofFIG. 4could therefore have a dimension of about 300 μm wide (w)×about 600 μm long (l). The polar fluid24can be stabilized against the spacers110as if the spacers110were a solid wall or continuous barrier. Still under appropriate operating conditions explained in detail below, the polar fluid24can flow in the channel52and through the spacers110as though the spacers110were absent, i.e. as though the only forces acting on the polar fluid24are due to the channel configuration.

The views ofFIGS. 4A-4Fdo not show a full device structure, which will be later shown and described in connection withFIGS. 5A-5F. Rather,FIGS. 4A-4Fare used to describe the basic physics governing the Laplace barrier created by spacers110.

InFIGS. 4A and 4B, no voltage is supplied to the voltage source114connecting the first and second electrodes102,104, and the polar fluid24has a small, convex vertical radius of curvature116. As a result, the polar fluid24experiences a net pressure118that will not allow it to advance forward, or which can cause it to retract from the shown portion of the electrodfluidic device100. Said another way, the spacers110, acting as Laplace barriers, restrain the polar fluid24to a position that is proximate to, or not exceeding, the spacers110.

As shown inFIGS. 4C and 4D, the voltage source114provides a first voltage (+V) to the first and second electrodes102,104that is sufficient to invert the vertical radius of curvature116to concave, where the first voltage is less than a threshold voltage. This will cause the polar fluid24to advance toward the spacers110(indicated by “+” and “−” indicia representing the electrical charge causing the electromechanical pressure on the polar fluid24) in the channel52. However, when the polar fluid24reaches the row of spacers110, the spacers110act as the Laplace barrier by imparting a horizontal radius of curvature120on the meniscus of the polar fluid24. The Laplace pressure of the polar fluid24will therefore equalize at zero net pressure and the radii of curvature116and120being approximately equal. This is one localized example for radii of curvatures116,120and other thresholds can be realized in alternate embodiments of the present invention. Not shown, but as could be envisioned, if the meniscus of the polar fluid24were to try to move any further forward, then the horizontal radius of curvature120would be reduced, which is energetically unfavored for the device100presented inFIGS. 4A-4F.

Next, as shown inFIGS. 4E and 4F, the voltage source114can provide a second voltage (+V2) that is greater than the threshold voltage, which reduces the vertical radius of curvature116to the point that the polar fluid24will move or advance forward in the channel52with a net pressure122regardless of the horizontal radius of curvature120of the meniscus of the polar fluid24. Said another way, the when the voltage source114provides a second voltage level (+V2) that exceeds the threshold voltage, the polar fluid24is unrestrained by the spacers110and advances to a second position that is beyond the spacers110. Absent factors such as drag forces and wetting hysteresis, the theoretical voltage threshold for such forward movement of the polar fluid24is the point at which the vertical radius of curvature116is smaller than half the of the width of the passageway112between the spacers110comprising the Laplace barrier. Once the polar fluid24moves in the channel52beyond the Laplace barrier, the polar fluid24will exhibit little resistance or drag. In contrast to the advancement of a polar fluid inside a conventional, converging/diverging capillary tube or other confined channel, the polar fluid24will advance to surround, or encompass, the spacers110(i.e., Laplace barrier) at all exposed surfaces for the spacers110. The term exposed surfaces includes all available surfaces for fluidic contact, though actual contact is not necessary for the surface to be exposed. This surrounding of the exposed surfaces of the spacers110is made possible by the open passageway(s)112within the Laplace barrier. As an example, the spacers110could be about 5 μm in both height and diameter, and are separated with 50 μm of separation between them. As a result, the available fluid pathway through an area containing the spacers110is still about 90% open as compared to the total area including the spacers110.

The Laplace barrier acts in the manner of a porous barrier, a porous electrowetting barrier, and/or a partial barrier in that the Laplace barrier selectively permits the polar fluid24to advance to and past the Laplace barrier contingent upon the voltage applied to cause advancement of the polar fluid24within the channel52.

In a preferred embodiment, the spacers110(or any other variation of Laplace barrier described herein) must provide a fluid pathway that is at least 50% open. This degree of porosity or openness may support rapid advancement of the fluids within the channel52. At 50% open area, the fluid advancement within the channel52is only slowed to about 5 cm/s from the typical maximum speed of 10 cm/s. For applications such as displays, the openness of the passageway112may enhance optical contrast. With a pixel border that comprises 10% of the area, the contrast ratio is limited at best to 10:1 contrast if the spacers110are optically reflecting. Reducing the spacers110to 50% of the area boosts the optical contrast to a more acceptable maximum value of 20:1.

Referring again toFIGS. 4A-4F, once the advancing edge of the polar fluid24moves beyond the spacers110of the Laplace barrier, the polar fluid24rapidly regains a Laplace pressure that is similar to the Laplace pressure within the polar fluid24before it encountered the spacers110. As a result, the polar fluid24is capable of continuing to flow through the Laplace barrier, using the first (lower) voltage, and with an ease similar to a channel having no spacers or barriers.

With reference toFIGS. 5A-5F, another embodiment of an electrowetting or electrofluidic device130is shown. The electrofluidic device130includes first and second substrates42,44. As shown, the first substrate42includes three separate and distinct electrodes132,134,136and a splitting electrode138, which is described in greater detail below. A dielectric140covers the electrodes132,134,136,138, and a hydrophobic layer142covers the dielectric140. The second substrate44includes an upper electrode144having the hydrophobic layer142thereon. A plurality of the spacers110are included within the channel52and surrounding the perimeter of the electrodes132,134,136,138,144, and may be constructed in a manner that was described previously with reference toFIG. 4A-4F. As shown, some of the spacers145may have a different shape than those spacers110that surround the perimeter.

The illustrative embodiment ofFIG. 5Ais able to provide two or more locations for fluid positioning. Shown inFIGS. 5A and 5B, four voltage sources146,148,150,152, provide voltage between the upper electrode144and each of the first, second, third, and splitting electrodes132,134,136,138, respectively. The upper electrode144is conductive with the polar fluid24. As a result, the polar fluid24partially covers the first, second, third, and upper electrodes132,134,136,144.

As is also shown and unlike conventional devices, the polar fluid24surrounds all exposed surfaces of the spacers110when moving from one display state to another. Said another way, the spacers110are surrounded or encompassed by the polar fluid24.

With reference toFIGS. 5C and 5D, the electrofluidic device130is shown with the voltage removed from the fourth voltage source152associated with the splitting electrode138. When the voltage is removed from splitting electrode138, the vertical radius of curvature (not shown) for the meniscus of the polar fluid24in the channel52above the splitting electrode138will be small and convex, which causes the polar fluid24to dewet from the channel52above the splitting electrode138. This splits the polar fluid24into two volumes: one volume of polar fluid24aoccupying the channel52above the first electrode132and one volume24boccupying the channel52above the second electrode134, wherein the first and second voltage sources146,148are then still supplying voltage to the first and second electrodes132,134. If the voltages from the first and second voltage sources146,148are removed from the first and second electrodes132,134, then the volumes of the polar fluid24a,24bwill still reside in the position shown inFIGS. 5C and 5Dbecause of the influence of the spacers110that surround the perimeter of the electrodes132,134,136,138,144. This is because the horizontal radius of curvature154is greater than half the distance between adjacent spacers110.

Next, as shown inFIGS. 5E and 5F, the volume24b(FIG. 5C) is moved from over the second electrode134to over the third electrode136by an application of voltage from the fourth voltage source150. When the voltage is removed, the volume24bis stabilized by the spacers110.

Several other alternative embodiments are possible but are not shown. For example, the two volumes24a,24bmay be stabilized over the second and third electrodes134,136without either volume touching or mixing with each other. The prevention of liquid mixing, merging, or touching is enhanced by the larger spacers145that form a wider Laplace barrier. The particular embodiment shown inFIGS. 5A-5Fprovides superior close positioning of the two volumes24a,24b. For example, if fluids were confined in a hexagon geometry and two hexagon confined fluids were packed immediately next to each other, then the horizontal space that could not be filled with polar fluid24would comprise two out of six hexagon sub-triangles or about 30% of a hexagon area. Squares can be packed with much higher density and are more compatible with most commonly used electrode formats. Such capability of packing two square shaped liquids immediately adjacent to each other, but also allowing for the rapid advancement of the polar fluid24from square-to-square, is uniquely enabled by the various embodiments of the Laplace barriers. Laplace barriers, as specified herein, eliminate the requirement for round or hexagon-shaped containment of polar fluids.

Also, in another alternate arrangement, and starting with the case shown inFIGS. 5C and 5D, by applying a voltage to the splitting electrode138via the fourth voltage source152, the volume24bcould rejoin the volume24a. With voltage applied to second and third electrodes134,136, the single volume of the polar fluid24could then be moved over the second and third electrodes134,136and be stabilized even as these voltages are removed. Therefore, a mechanism may be provided for stabilizing the polar fluids24adjacent to each other, or in union with each other, both instances with any desired fluid geometry as determined by the Laplace barriers.

In the event that one spacer110is improperly fabricated, a second or more adjacent row(s) of spacers110can be provided to ensure proper Laplace barrier function. Such improvements may improve device function or improve manufacturing yield.

The electrode and dielectric arrangements shown inFIGS. 5A-5F, and all other embodiments described herein, are also not limited to the specific dielectric and electrode placements illustrated in the figures. For example, the splitting electrode138and the related dielectric140could be carried by the second substrate44. Any arrangement is possible so long as the electrodes and the Laplace barrier function according basic electrical and fluidic principles described herein.

With further reference toFIGS. 5A-5F, the Laplace barrier can be defined mathematically as follows. The volume of the polar fluid24is limited such that the smallest horizontal radius curvature for the polar liquid24is always greater than half the width of the passageways112(FIG. 4) between adjacent spacers110. This can also be specified as the polar fluid24having a maximum radius of curvature that is greater than a minimum radius of curvature imparted on the polar fluid24by the Laplace barrier. Generally, the width of the passageways112(FIG. 4) between adjacent spacers110should always be greater than the product of the height of the spacers110and the cosine of the minimum contact angle achieved under the applied electromechanical force.

With further reference toFIGS. 5A-5F, and all other embodiments that will be covered herein, the polar fluid24must be able to traverse the space between adjacent electrodes132,138,134,136. Several mechanisms are possible, all included within the spirit of the present invention. As known by those skilled in the art of electrowetting lab-on-chip, electrode interdigitation is effective at bridging the gap between adjacent electrodes132,138,134,136. Because electric field dissipates with distance and does not abruptly terminate in an insulating medium, the electrodes132,138,134,136can also be constructed very close to one another. In some cases, providing similar or opposite polarity voltages on the electrodes132,138,134,136may prove helpful. In other cases, the spacers110forming the Laplace barrier can be partly misaligned relative to the space between electrodes132,138,134,136, or staggered to provide an alternate means of polar fluid interdigitation across the space between the electrodes132,138,134,136. The polar fluid24can also have a volume that at least partially protrudes over the space between electrodes132,138,134,136. Numerous combinations and arrangements thereof are possible and included within the spirit of the present invention.

With further reference toFIGS. 5A-5F, the Laplace barrier is depicted as geometrically straight. Although this may be preferred for some embodiments, the Laplace barriers may also take on shapes that are more natural for the polar fluid24, or more natural during the advancement of polar fluid24due to an electromechanical force. This, for example, may allow the Laplace barriers and polar fluid24to encounter each other simultaneously at most or all possible locations.

With reference now toFIGS. 6A-6E, electrofluidic devices according to various other embodiments of the present invention are shown utilizing alternate Laplace barriers. Generally, the devices include first and second substrates42,44, with the first substrate42having first and second electrodes162,164thereon. A dielectric166covers the electrodes162,164. The second substrate44includes an upper electrode168and the dielectric166covering the upper electrode168. A first voltage source170couples the first and upper electrodes162,168; a second voltage source172couples the second and upper electrodes164,168.

Shown inFIG. 6A, the spacers110, which are constructed in a manner that is similar to those ofFIG. 5A, can be arranged in an array in the channel52. The array of spacers110can be a one-dimensional array, or a two-dimensional array such as a square or hexagonal pattern of spacers110. The only change in the method by which the device160functions is that the threshold voltage magnitude recess to move the polar fluid24(FIG. 2) past the spacers110is the same voltage magnitude required to maintain the polar fluid24moving forward and covering the electrodes162,164. The three advantages of this approach are as follows: (1) the functionality is similar to the device130ofFIG. 5A, but can be achieved without the need to precisely align the spacers110along the perimeters of the electrodes162,164; (2) the polar fluid24(FIG. 5A) could be stabilized at multiple intermediate positions between and covering the electrodes162,164; and (3) the separation between the spacers110need not be uniform. For example, though not shown, if the spacers110ofFIG. 6Aare positioned closer together over the left side of the first electrode162than the spacers110over the right side of the first electrode162, then when the polar fluid24(FIG. 4A) is moved from the second electrode164to the first electrode162the final position of the polar fluid24(FIG. 4A) could be selectable based on the magnitude of the voltage applied from the first voltage source170to the first electrode162. It would, therefore, require more voltage to move through the portion of the channel52having spacers110with less separation.

Unlike the conventional electrowetting device10ofFIG. 1A, device160ofFIG. 6Ahas spacers110that can provide such adaptive and robust influence over the movement and stabilization of the polar fluid24(FIG. 5A) with minimal displacement of the volume of the polar fluid24(FIG. 5A). For example, the spacers110could be 5 μm in both height and diameter, and separated with 50 μm separation between adjacent ones of the spacers110positioned in a square array. The net effect is equivalent to having one larger spacer with a cross-sectional area of π×2.5 μm2(about 20 μm2) inside a square unit device or pixel having a cross-sectional area of 50×50 μm (2500 μm2); or stated another way, the cross-sectional area occupied by the one spacer would be less than 1% of the total horizontal, cross-sectional area of the pixel. This is an advantage for situations when the particular fluid(s) are to be displayed visually or where the fluid(s) are to be moved with maximum speed and volume. This also advantageously makes more efficient use of space than the conventional electrowetting device10(FIG. 1A). Because the spacers110are often required to regulate the gap between the first and second substrates42,44, these spacers110can be simultaneously fabricated using a single photolithographic mask step. The spacers110can create the Laplace barrier with very little additional drag or resistance to fluid flow. Unlike hydrophilic grids or spacers used in conventional electrowetting displays, the spacers110do not substantially reduce electrowetting on one of two electrowetting substrates by creating low electrical capacitance on the substrates on which the spacer110is placed. As will be discussed forFIGS. 7A-7D, robust electrowetting on two substrates is required to introduce a fluid from a reservoir.

In another embodiment, shown inFIG. 6B, the Laplace barrier of the device180is comprised of one or more projections or posts182having a height that is less than the height of the channel52. In this case, the posts182would not act as an actual physical spacer between the first and second substrates42,44, but would still function as the Laplace barrier. Because the posts182are shorter in height than the channel, less separation between posts182while maintaining a similar Laplace barrier property may promote more rapid re-merging of the polar fluid24after traveling through the posts182. In some cases, the polar fluid24may never be split at all. Generally, to promote non-splitting of the polar fluid24as it moves past the posts182, the Laplace pressure within the polar fluid24should favor forward movement of the polar fluid24over (or under as appropriate) the posts182. While the term posts182may seem to connote a particular cylindrical shape, the posts182are not so limited. Instead, the posts182may include any physical structure or projection that extends into, but not entirely traversing, the channel52. For example, inFIG. 6C, the Laplace barrier of the device190includes a plurality of ridges192having a height that is less than the height of the channel52and/or the posts182(FIG. 6B). The ridges192differ from the posts182(FIG. 6B) in that each ridge192can extend across a majority of one dimension of the device190.

In yet another alternative embodiment of a device196shown inFIG. 6D, the wire or plastic mesh54, described previously with reference toFIG. 2Areplaces the spacers110ofFIG. 6Aas the Laplace barrier. If the mesh54is electrically conductive and coated with a very thin fluoropolymer, then the mesh54can also act as a localized electrical ground for the polar fluid24(FIG. 5A). The mesh54can be woven, fused, or other types of porous textiles or sheets, with the economical advantage that the mesh, textile, or sheet need simply be positioned between the first and second substrates42,44.

The alternative embodiment ofFIG. 6Eincludes a device200, having spacer spheres202forming the Laplace barrier, so long as the spacer spheres202exhibit an average separation that permits fluid flow as described for the present invention.

Unlike the spacers110described forFIG. 4, the mesh54and the spacer spheres202ofFIGS. 6D and 6Einfluence a radius of curvature for the polar fluid24(FIG. 5A) that transverses and encompasses any acute angle for the plane of radius of curvature. Said another way, the reduced radius of curvature of the polar fluid meniscus caused by the Laplace barrier can be in a plane that is angled with respect to the vertical plane.

The specific examples shown inFIGS. 6A-6Edo not form a limiting set. Rather, the examples shown inFIGS. 6A-6Eillustrate that multiple variations and embodiments of Laplace barriers are included within the spirit of the present invention. Additional Laplace barriers can be partially, or fully, comprised of local charges in surface energy, surface roughness, contact angle hysteresis, and/or the height of the channel between substrates. In addition, devices with Laplace barriers can also make use of partial or porous fluid barriers, such as locally missing portions of electrodes, or locally increased dielectric thickness, that locally decrease electromechanical pressure on a polar fluid.

With further reference toFIGS. 6A-6E, and other embodiments of the present invention, the second substrate44need not carry the upper electrode168, nor is there a need for a ground wire (not shown) or other electrical coupling in the channel52. Instead, co-planar electrodes are well known to those skilled in the art of electrowetting and should be considered to be included within the spirit of the present invention. One of ordinary skill in the art would readily understand how to implement co-planar electrodes in any one of the embodiments of the invention.

With reference toFIGS. 7A-7D, an electrowetting or electrofluidic device206according to another embodiment of the present invention is described. The device206includes the first and second substrates42,44. A dielectric208is formed as a block on the first substrate42defines a channel portion210while the remaining portion of the first substrate42forms a reservoir212with the second substrate44. The dielectric208includes a lower electrode214and a splitting electrode216and where the splitting electrode216is formed near the fluid pathway into the channel portion210or substantially adjacent to the reservoir212. The second substrate16includes an upper electrode218and the dielectric208covering the upper electrode218. A first voltage source220electrically couples the lower and upper electrodes214,218with the polar fluid24. A second voltage source222couples the splitting electrode216and the polar fluid24. The splitting electrode216thus provides a means to introduce the polar fluid24from the hydrophobic reservoir212and into the channel portion210.

With no voltage supplied from either of the first or second voltage sources220,222as shown inFIG. 7A, the polar fluid24will favor occupation of the hydrophobic reservoir212over a hydrophobic channel portion210because the hydrophobic reservoir212imparts a larger radius of curvature224on the polar fluid24than the smaller radius of curvature226in the channel portion210. As shown inFIG. 7A, the radii of curvatures224,226cause the polar fluid24to recoil into the hydrophobic reservoir212without applied voltage. However, according the principles of the present invention, a mechanism for stabilizing the polar fluid24in the channel portion210is needed. Furthermore, once the polar fluid24is stabilized in the channel portion210, it must be allowed to return to the hydrophobic reservoir212with appropriate electrical stimulus. Laplace barriers alone cannot achieve the functionality described above because the Laplace barrier is only functional on a polar fluid24that is advancing, i.e., not for retracting the polar fluid24. For this reason, the splitting electrode216of device206ofFIGS. 7A-7Dis constructed as to be adjacent to the hydrophobic reservoir212and at least one spacer110.

As shown inFIG. 7B, both the first and second voltage sources220,222supply a voltage to the polar fluid24and the electrodes214,216,218. As a result, a net pressure228is created that is sufficient to advance the polar fluid24into the channel portion210and through the spacers110. To stop or reduce fluid advancement, the voltage applied from the first electrode214is reduced or stopped.

To hold the polar fluid24in a given or desired position, the voltage applied by the second voltage source222is removed or reduced on the splitting electrode216. Accordingly, the polar fluid24dewets the area above splitting electrode216in a manner that is similar to that described with reference toFIG. 5C. The voltage supplied from the first voltage source220to the lower electrode214can also be removed and the polar fluid24will then be stabilized, as shown inFIG. 7C.

There are three features achieved by the device206ofFIGS. 7A-7D: (1) a bistable mechanism is achieved for moving the polar fluid24into the channel portion210from the hydrophobic reservoir212; (2) the amount of polar fluid24within the channel portion210can vary based on time or the voltage supplied from the voltage sources220,222; and (3) the Laplace barriers (shown as spacers110) stabilize the polar fluid24such that it is always adjacent to the splitting electrode216. As result of this third advantage, when a voltage is again applied by the second voltage source222to the splitting electrode216, as shown inFIG. 7D, the polar fluid24can then be retracted back into the reservoir212. A net pressure230is created that causes the retraction of the polar fluid24. At the end of the process, the voltage supplied by the second voltage source222to the splitting electrode216can be removed such that the polar fluid24returns to the positions illustrated inFIG. 7A.

Several mechanisms are possible for improving fluid communication between adjacent electrodes, all included within the spirit of the present invention. As known by those skilled in the art of electrowetting lab-on-chip, electrode interdigitation is effective at bridging the gap between adjacent electrodes. Because electric fields dissipate with distance and do not abruptly terminate in an insulating medium, electrodes can also be constructed to be very close to one another. In some cases, providing similar or opposite polarity voltages on the adjacent electrodes may prove helpful. In other cases, the Laplace barrier can be partly misaligned relative to the space between electrodes or staggered to provide an alternate means of polar fluid interdigitation across the space between the electrodes. The polar fluid can also have a volume that at least partially protrudes over the space between electrodes. Numerous combinations and arrangements thereof are possible and included within the spirit of the present invention.

The mechanism for pulling the polar fluid24into the channel portion210from the reservoir212, as shown inFIGS. 7A-7D, requires electrowetting onto both the first and second substrates42,44. Though not specifically shown, a single electrode and electrowetting surface can also be utilized to pull the polar fluid24into the channel portion210. If the spacers110included an electrode encased by a dielectric material, then the separation between adjacent pairs of the spacers could be reduced and the spacers could electrowet and ratchet the polar fluid24into the channel portion210from the hydrophobic reservoir212.

With further reference toFIGS. 7A-7D, the device206with the Laplace barrier can provide a mechanism for grayscale resets. As described previously, when the electrodes214,216,218are biased at the first voltage up to the threshold voltage, the Laplace barrier can advance the polar fluid24to the Laplace barrier, but not beyond it. Therefore, the device206may provide precise grayscale values that depend on the number of Laplace barriers included within the device206. Accordingly, a more precise mechanism for grayscale placement of the polar fluid24at two or more locations is provided. It will be appreciated that a more precise mechanism is provided for the placement of the polar fluid24between Laplace barriers, as the polar fluid24is moved to the Laplace barrier and then moved a distance away from the Laplace barrier (for example, as a position that is between two adjacent Laplace barriers). This functionality can be referred to as a grayscale reset state, which avoids grayscale accumulation error over multiple switches between two or more grayscale states (i.e., positions of the polar fluid24). Grayscale resets are usually preferred because conventionally grayscale resets had required resetting the polar fluid24to full filling of the channel portion210or the reservoir212, which would cause an observable flicker in an information display each time the grayscale state is charged.

With reference toFIG. 8A, an electrowetting or electrofluidic device234is shown according to an embodiment of the present invention that includes a plurality of electrodes236a,236b,236c,236d,236e(collectively236n) on the first substrate42an upper electrode238on the second substrate44. The first and second substrates42,44define the channel52. The polar and non-polar fluids26reside within the channel52. The channel52is in fluidic communication with the reservoir240, which may be external to the device234. Accordingly, the portions of the first and second substrates42,44adjacent the reservoir240include a hydrophilic coating242a,242b. The remainder of the first and second substrates42,44, and the electrodes236n,238formed thereon have a dielectric244thereon. The hydrophilic layers242a,242bare electrically insulating and extend to cover a portion of the upper and first electrodes238,236a. In one embodiment, the hydrophilic layers242a,242bmay be any of the previously described hydrophobic layers that was plasma or chemically treated locally to render it hydrophilic. The Laplace barrier also serves as an electrical ground with the metal wire mesh54, which as described previously, is hydrophobically coated.

Because of the hydrophilic layers242a,242b, the polar fluid24wicks to a position within the channel52while remaining capable to be electrically actuated into the device234according the principles described for the present invention. The first electrode236acould be, for instance, a splitting electrode, or any other electrode capable of acting as a splitting electrode.

In another embodiment, the device246ofFIG. 8Bis similar to the device234ofFIG. 8A, but further includes a local electrical ground for the polar fluid24, i.e., a plurality of grounding electrodes248.

Operation of the devices234,246ofFIGS. 8A-8B, while not shown, is as follows. A voltage from first and second voltage sources250,252is applied to the upper, first, and second electrodes238,236a,236bto pull the polar fluid24over the first and second electrodes236a,236b. The first electrode236aof the plurality of electrodes236nis then no longer provided with voltage from the second voltage source252and the polar fluid24splits into two volumes: the first volume is connected to the original volume of polar fluid24within the reservoir240and the second volume extends over the second electrode236b. The volume of the polar fluid24over the second electrode236bcan be moved by applying a voltage by a third voltage source254between the third electrode236cand any of the other electrodes236d,236e. A larger volume of the polar fluid24, spanning two or more electrodes, can also be created and moved as described above with reference toFIGS. 7A-7D. One advantage of using a localized electrical ground in the form of metal wire mesh54(FIG. 8A) or the grounding electrode248(FIG. 8B) is that the polar fluid24is not influenced by voltages that are applied to adjacent volumes of polar fluid. Such advantage is readily understood by those skilled in the art of passive matrix electrode addressing.

With reference toFIG. 9, an electrowetting or electrofluidic device260according to an embodiment of the present invention is described. The first substrate42is shown and includes segmented electrodes262a,262b,262c,262d(collectively262n) in the representative form of square electrode pads, to move the polar fluid24(FIG. 3A) over two or more of the segmented electrodes262n. The second substrate44(FIG. 3A) would be included to form the channel52with the first substrate42; however, the second substrate44(FIG. 3A) is not shown for simplicity in this cross-section through the channel52. Generally, the segmented electrode layout requires a plurality of electrode lines264a,264b,264c,264d(collectively264n) to electrode pads266a,266b,266c,266dat the edge of the first substrate42. Alternatively, though not shown, an electrode via may extend through the first substrate42in a manner that is similar to that used in printed circuit boards. In the representative embodiment, the electrode lines264nterminate near the first substrate edge and are respectively connected with the electrodes262n, in a one-to-one fashion. For the case where the electrodes262nare all carried on the same substrate, spacers110may be placed at each location where the electrode line264nconnects with the respective segmented electrode262n. This structure prevents the polar fluid24(FIG. 3A) from being electromechanically and prematurely pulled onto the electrode line264n. In an alternate approach, though not specifically shown, is to implement an electrode line that is horizontally narrow, such that the effective electrode line area is insufficient to pull polar fluid24(FIG. 3A) onto the electrode line.

With reference now toFIGS. 10A and 10B, an electrowetting or electrofluidic device270according to another embodiment of the present invention is shown. Again, the electrofluidic device270includes first and second substrates42,44forming the channel52there between. The first substrate42includes three lower electrodes272,274,276and a dielectric278covering the first substrate42and the electrodes272,274,276. The second substrate44includes three upper electrodes280,282,284with the dielectric278thereon. The upper electrodes280,282,284are positioned to be orthogonal to the lower electrodes272,274,276of the first substrate42so as to form a grid-like pattern of rows (the upper electrodes280,282,284of the second substrate44) and columns (the lower electrodes272,274,276of the first substrate42), shown inFIG. 10B. This arrangement of upper and lower electrodes272,274,276,280,282,284, creates a so-called passive matrix electrode operable to move the polar fluid24(FIG. 3A) between multiple locations. While one of ordinary skill in the art would readily understand the implementation of passive matrix electrodes, the device270has been illustrated as having a generic voltage source283representing a different voltage source that is electrically coupled to a separate one of the upper and lower electrodes272,274,276,280,282,284.

The electrofluidic device270moves the polar fluid24(FIG. 3A) horizontally between multiple locations to switch the polar fluid24(FIG. 3A) in a bistable pixel between two visibly different states. For example, if the polar fluid24(FIG. 3A) were to simultaneously cover more than one row or column electrode, then capacitance coupling between the covered electrodes through the polar fluid24(FIG. 3A), could complicate the drive scheme. Accordingly, the wire mesh54, acting as the Laplace barrier if metallic, can function as an electrical grounding electrode for the polar fluid24(FIG. 3A) at all locations. Then, the wire mesh54can be designed such that electrowetting on the first and second substrates42,44must be provided simultaneously to advance the polar fluid24(FIG. 3A) through the wire mesh54. As a result, the polar fluid24(FIG. 3A) will only move toward an adjacent row and/or column electrode that is provided with an appropriate voltage. This aspect of this embodiment of the present invention provides a simple means for passive matrix electrical control of polar fluid movement.

With reference toFIGS. 11A and 11B, an electrowetting or electrofluidic device296according to yet another embodiment of the present invention may be described. The electrofluidic device296is constructed with an active matrix electrode scheme, i.e., the first substrate42includes a plurality of electrodes298n(where n ranges from a to n in the illustrative embodiment) arranged in a matrix-like pattern and covered by a dielectric299. The second substrate44includes a grounding electrode300thereon and cooperates with the first substrate42to form the channel52there between for containing the polar fluid24and non-polar fluid26(FIG. 3A). The active electrode scheme can move the polar fluid24(FIG. 3A) between multiple locations that are defined by each of the electrodes298n. A thin-film transistor302nfor each electrode298nprovides local voltage control of each of the electrodes298nand thus controls electrowetting of the polar fluid24(FIG. 3A) onto that electrode298n. Alternately, the thin-film transistors302ncan provide voltage directly to the polar fluid24(FIG. 3A). In either case, as the polar fluid24(FIG. 3A) moves between the electrode298n, the electrical capacitance between the polar fluid24(FIG. 3A) and the electrode298nvaries. In active matrix drive, discharging the capacitance between the polar fluid24(FIG. 3A) and the electrode298nis easily achieved even if the capacitance is variable. However, building up the capacitance between the polar fluid24(FIG. 3A) and the electrode298nto advance the polar fluid24(FIG. 3A) over the electrode298npresents a challenge because the capacitance between the electrode298nand the polar fluid24(FIG. 3A) increases as the polar fluid24(FIG. 3A) advances above that electrode298n. During a typical write-time in active matrix drive, the polar fluid24(FIG. 3A) advances too slowly over the electrode298nto allow the capacitance to maximize. As a result, multiple row voltage write cycles for the thin-film transistor302nare required to complete polar fluid movement. Therefore, arrangements of storage capacitors, or arrangements of multiple thin-film transistors302nmay be preferred to promote rapid movement of the polar fluid24(FIG. 3A). A variety of such electrical drive schemes are well-known by those skilled in the art of active matrix displays and included within the spirit of the present invention.

With reference now toFIGS. 12A and 12B, an electrowetting or electrofluidic device310according to an embodiment of the present invention that is capable of utilizing an optically addressed virtual electrode scheme is described. The device310includes the first substrate42having a first electrode312, a dielectric314, and a photoconductor316positioned between the first electrode312and the dielectric314. The second substrate44has a second electrode318thereon, and, with the first substrate42, defines the channel52. In the absence of light from the light source60, the photoconductor316acts as a low-capacitance dielectric; in the presence of light from the light source60, the photoconductor316acts as an electrical conductor. Therefore, the polar fluid24(FIG. 3A) may be moved in the device310by providing an AC voltage between the first and second electrodes312,318, and illuminating the photoconductor316with light adjacent to a volume of polar fluid24(FIG. 3A). The light generated by the photoconductor316will cause a voltage drop across the dielectric314, but not the low-capacitance photoconductor316and, as a result, will locally increase the electrowetting effect. The polar fluid24(FIG. 3A) then moves to optically illuminated areas according to principles described herein for the present invention. Various optically address techniques are possible, such as those used for optical electrowetting or optical dielectrophoresis and are included within the spirit of the present invention. The photoconductor316can also be placed in other locations, for example, covering the second electrode318on the second substrate44in order to locally control an electrical connection with the polar fluid24(FIG. 3A). The device310can implement additional Laplace barriers as previously described forFIGS. 6A-6E.

Referring now toFIGS. 13A and 13B, an embodiment of the present invention is described where the Laplace barrier, illustrated here as a plurality of spacers110, can be used within an electrofluidic display324that is capable of alphanumeric or symbolic representations of reconfigurable information display beyond a simple ON/OFF indicator or multi-position electrowetting pixel. A channel portion326includes a matrix of pixels between the first and second substrates42,44, the latter of which is not shown due to the selection of the cross-sectional view. The non-polar fluid26and at least two polar fluids328,330are constrained between the first and second substrates42,44. The polar fluids328,330differ in at least one optical or spectral property. Each pixel is delineated by the plurality of spacers110acting as the Laplace barriers, and each pixel is capable of receiving and holding one of the fluids26,328,330in a tight, square geometry. The polar fluids328,330will be drawn as squares in each channel for simplicity, and because visually to an observer, the actual polar fluid geometry can be quite close to a square, i.e., fill more than 95% of the square geometry is possible.

The electrofluidic display324also includes a reservoir region332for holding sufficient volumes of the polar fluids328,330to fill a majority or all of the pixels. Such a reservoir region332is not required as the polar fluids328,330could be simply stored in additional pixels or another external channel (not shown); however, the reservoir region332will reduce the visible area of fluid storage and is generally preferred. The polar fluids328,330, can be introduced into a pixel that is adjacent to the reservoir region332using one or more of the techniques described above with reference toFIGS. 7A-7Dand/or8A-8B. Once one of polar fluids328,330is in the adjacent pixel, it is free from the reservoir region332and only one electrowetting surface is needed to move the polar fluid328,330to another pixel. However, two electrowetting surfaces may also be implemented.

When the non-polar fluid26fills all of the pixels, the electrofluidic display324might, for example, exhibit a blank white reflectance, or some other color, including black. One of ordinary skill in the art would readily understand that specific reflector designs, which are not discussed in detail herein, can be located at multiple possible regions in the electrofluidic display324and are well understood by those skilled in the art of displays. The electrofluidic display324may also be transparent when the non-polar fluid26is in the pixel, so long as some visual change occurs with movement of one or both of the polar fluids328,330. In addition, the spacers110may be constructed from a clear, reflective, or colored material such that they compliment, add, subtract, or otherwise modify the visual appearance of the electrofluidic display324. For example, the spacers110can be formed from SU-8 epoxy that has been dyed black or containing a pigment such as Perylene black.

With continued reference toFIG. 13A, the polar fluids328,330have been moved into the pixels to illustrate that two different polar fluids328,330can be situated in adjacent pixels without the risk of mixing or merging. In addition, a single volume of the first polar fluid328spans adjacent pixels. Therefore, multiple and alternate arrangements are possible.

With reference toFIG. 13B, the polar fluids328,330have been moved into select pixels so as to form an alphanumeric representation of ‘0.4’. As shown, the ‘.’ is displayed in a first color (use of the second polar fluid330) that is different from the ‘4’ (use of the first polar fluid328). As described previously, even if polar fluids328,330are in adjacent pixels, they do not mix by virtue of the spacers110.

In a preferred embodiment, the spacers110should comprise less than 20% of the total area projected onto the plane that is occupied by the polar and non-polar fluids328,330,26. Otherwise, the contrast ratio of the electrofluidic display324will be substantially sacrificed. Also, for alphanumeric character display, the spacers110should be optically non-obvious such that the character resembles its printed counterpart.

The polar fluids328,330move between pixels as follows. In a first instance, one of the polar fluids328,330moves easily into a single pixel that contains only the non-polar fluid26. In another instance, one of the polar fluids328,330moves into a pixel that already contains the other polar fluid330,328. In some embodiments, both polar fluids328,330may move simultaneously (one moving into the pixel as the other moves out). This latter arrangement requires more than one electrode, i.e., one associated with each pixel. While one of ordinary skill in the art would understand that each electrode would include a separate voltage source, for simplicity, only a single generic voltage source331is shown to represent an electrical connection to each electrode associated with the 18 pixels. However, this arrangement poses the risk of fluid merging, which must be mitigated by advanced pixel design. A simpler but slower alternative has one polar fluid328,330moving completely or mostly out of the pixel before the other polar fluid330,328moves in. For embodiments where the polar fluids328,330are of the same color, fluid merging might be tolerated.

In order to reconfigure the image in the electrofluidic display324inFIG. 13B, mathematical algorithms such as those developed for sliding tile puzzles may be implemented. All pixels may be filled with either of the polar fluids328,330so long as the last non-filled pixel is adjacent to the reservoir region332, which will then fill the last non-filled pixel. Most applications will likely reset the polar fluids328,330to the reservoir region332before creating the new image one row of pixels at a time. Assuming 10 cm/s speed of polar fluid motion, a 1″ high and 75 dots per inch (“DPI”), electrofluidic display324could create an image in roughly 1 second. Such speed is fully adequate for many applications, such as electronic shelf labels for retail products.

For the electrofluidic display324ofFIGS. 13A-13B, it should be noted that a space340surrounds the matrix of pixels and is enclosed by an enclosing barrier342, i.e., another Laplace barrier. Although not mandatory, the space340, or otherwise referred to as a duct, will allow the non-polar fluid26to rapidly flow back to the reservoir region332, or to an adjacent pixel, as the polar fluids328,330move from the reservoir region332to the pixels or between pixels. In fact, the entire array is ‘open-cell’ such that the pixel can be open to the non-polar fluid flow on three sides as the polar fluid328,330moves into that pixel. This feature is inherent to the design (i.e., comes without additional complexity or manufacturing cost). Although for all embodiments, it is preferred that the non-polar fluid26is an oil, the non-polar fluid26may also be a gas. In such cases, enclosing barrier342may be replaced with a series of holes that operate as a wetting barrier (due to a locally diverging capillary affect). For all embodiments shown herein, the geometry or array size of the electrofluidic display324can be modified to serve applications ranging from simple indicators on a USB flash drive, an e-book, or even billboard electronic signage.

The embodiments described herein are not limited to the specific illustrative Laplace barrier geometries. The present invention extends to using Laplace barriers that can comprise a continuous wall or perimeter on or between electrodes, having a height that does not span the entire channel. Laplace barriers may also be of varying heights that partially or completely span the channel. Some Laplace barriers may reside on the top substrate, some on the bottom substrate. Laplace barriers can take on multiple physical geometries (round, square, polygon, curved, etc. . . . ), locations, and arrangements, so long as they provide the Laplace barrier function within the spirit of the present invention.

The embodiments described herein are not limited to electrowetting control. The present invention extends to using the Laplace barriers and electrofluidic methods including electrowetting without insulators, syringe-pumps, thermocapillary, photo-responsive molecules such as spiropyrans, dielectrophoresis, electrophoresis, and micro-electro-mechanical pumping. For example, one skilled in the art of in-plane electrophoresis will recognize that the spacer posts embodiment of the Laplace barrier could also be coated with a surface charge, similar to how electrical charges are provided to pigments in electrophoretic or other types of dispersions. As a result, the Laplace barrier could be created where it would require a first voltage to move electrophoretic pigment in an insulating fluid to the Laplace barrier, but not beyond it, because of like repulsion of the pigment charge and the surface charge on the Laplace barrier. Next, a second and greater voltage could be provided to move the charged pigment beyond the Laplace barrier. As a result, a truly bistable and multi-position in-plane electrophoretic device is created.

In a first example illustrated as scanning electron microscope images inFIGS. 14A-14E, the fabrication of a device346is shown. The device346includes a glass wafer substrate having an array of spacer posts348fabricated thereon. To create the array of spacer posts348, a newly available negative-acting dry film photoresist DuPont PerMX-3020 (20 μm thick) was laminated at 80° C. and 40 psi onto the glass wafer using a Western Magnum dry film laminator. The photoresist was then UV exposed with the post-pattern, puddle developed in propylene glycol monomethyl ether acetate, and hard baked at 150° C. for 30 min. Approximately 150 nm of copper (Cu) was sputtered onto the posts348and the substrate to create the bottom electrode. For simplicity the substrate and electrode are represented by the surface352. About 1 μm of Parylene C dielectric was then conformally deposited onto the Cu using a Specialty Coating Systems PDS2010 system. The spacer posts348were then dip-coated in Cytonix Fluoropel 1601V solution and then baked at 120° C. for 20 min. to obtain a hydrophobic fluoropolymer layer of about 50 nm.

The device346was covered with a top substrate (not shown in the figures) of In2O3:SnO2and 50 nm of Fluoropel 1601V.

The device346was then dosed with tested fluids that included a 0.1 wt % NaCl aqueous polar fluid327that additionally included a self-dispersing pigment for coloration, and a tetradecane non-polar fluid350containing Dow Corning Triton X-15 (a surfactant).

In the first set of experiments, the spacer posts348were arranged in a uniform array, which included a single large area Cu electrode. Although this testing setup cannot provide programmable movement of the polar fluid327, it did confirm that closely spaced spacer posts348can cause forward propagation with one electrowetting plate and stabilization of polar fluid geometry once a voltage from voltage source (not shown) is removed. The threshold voltage for polar fluid propagation was 55V, and could be lower with use of higher capacitance dielectric coatings.

Several rows and splits of paired spacer posts348were also fabricated, tested at 55V, and time-lapse images are shown inFIGS. 14C and 14D. It would be understood that the areas not designated as polar fluid327would include the non-polar fluid329. These experiments demonstrate the preliminary basis for directional flow and channel splitting. Experiments were also performed (not shown) with differently colored aqueous solutions at opposite ends of virtual electrowetting channels and liquid mixing was confirmed. All images provided herein have been processed by the stylized find-edges and contrast-enhance functions of Adobe® Photoshop® of Adobe Systems, Inc. (San Jose, Calif.).

In the next example, shown as scanning electron microscope images inFIGS. 14F-14G, a perimeter of small diameter spacer posts348and larger diameter corner spacer posts326were implemented around a display pixel349having dimension of about 300 μm×300 μm. A plurality of pixels349comprises a device351, where each pixel349is separated by the perimeter of posts348,326. Each pixel349includes a channel positioned above an electrode, both represented by a surface350. A reservoir356and duct357are formed into each surface350.

The fluids consisted of a polar fluid327containing a red dispersed pigment and a black non-polar fluid329containing several dye mixtures. When voltage from a voltage supply (not shown) was applied to the electrodes (designated by surface350), the polar fluid327was pulled out of the reservoir356and filled the channel (designated by surface350). However, the polar fluid does not propagate beyond the pixel perimeter because of the spacer posts348.

Another example, illustrated as scanning electron microscope images inFIGS. 14H and 14I, include a glass substrate364having first, second, and third Aluminum electrodes358,360,362. The electrodes358,360,362were coated with 2.5 μm of a SU-8 epoxy dielectric and then patterned with the posts348, which were about 5 μm high and about 50 μm in pitch. The posts348were coated with 50 nm of a fluorpolymer. A top substrate, not shown in the figures, was added and the device was dosed with polar327and non-polar329fluids. The non-polar fluid329included a black dye such that when it covered one of the electrodes358,360,362, the electrode358,360,362was not visible in the photograph

InFIG. 14H, the polar fluid327was electrowetted with a first voltage to cover the first electrode358; however, the posts348between the first and second electrodes358,360confined the polar fluid327to the first electrode358.

When a second and greater voltage was applied from a voltage source (not shown) to all electrodes358,360,362, the polar fluid327advanced through the posts348such that polar fluid327covered all electrodes358,360,362. When this voltage was removed, the polar fluid327retained the geometry shown inFIG. 14I. Electrode360was also shown to act as a splitting electrode according to the principles of the present invention.

In the next example, though not shown, the spacer posts were replaced with a woven wire mesh that was sandwiched between first and second electrowetting plates. The mesh was comprised of 30 μm wire diameter mesh, which at the overlap between wire threads was therefore 60 μm in total thickness and was made hydrophobic with a fluoropolymer coating. The mesh was purchased from TWP Inc., was woven to ISO 9044, and was a 50 mesh count (per inch). It was found that the wire mesh allowed polar fluid movement by electrowetting, but also was able to regulate the polar fluid geometry in the absence of voltage. Therefore the wire mesh was also shown to act as a Laplace barrier.