WO2022245368A1

WO2022245368A1 - Electrowetting surfaces

Info

Publication number: WO2022245368A1
Application number: PCT/US2021/033633
Authority: WO
Inventors: Michael W. Cumbie; Chien-Hua Chen; Viktor Shkolnikov
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2021-05-21
Filing date: 2021-05-21
Publication date: 2022-11-24

Abstract

The present disclosure describes electrowetting surfaces, digital microfluidic devices that include electrowetting surfaces, and methods of making electrowetting surfaces. In one example, an electrowetting surface can include a substrate, a plurality of electrodes on the substrate, and a nanoceramic coating over the plurality of electrodes. The electrodes can include a layer of conductive material deposited over the substrate. The nanoceramic coating can include ceramic nanoparticles bound together by a polymeric binder.

Description

ELECTROWETTING SURFACES BACKGROUND [0001] Microfluidics relates to the behavior, precise control and manipulation of fluids in small quantities, such as milliliters, microliters, nanoliters, or smaller volumes. Digital microfluidics, in particular, can relate to control and movement of discrete volumes of fluids. A variety of applications for microfluidics exist, with various applications involving differing controls over fluid flow, mixing, temperature, evaporation, and so on. BRIEF DESCRIPTION OF THE DRAWINGS [0002] Additional features of the disclosure will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the present technology. [0003] FIGs.1A-1B are schematic side cross-sectional views of an example electrowetting surface in accordance with the present disclosure; [0004] FIG.2 is a schematic side cross-sectional view of another example electrowetting surface in accordance with the present disclosure; [0005] FIG.3 is a schematic side cross-sectional view of yet another example electrowetting surface in accordance with the present disclosure; [0006] FIG.4 is a schematic top-down view of another example electrowetting surface in accordance with the present disclosure; [0007] FIG.5 is a schematic side cross-sectional view of an example digital microfluidic device in accordance with the present disclosure; [0008] FIG.6 is a schematic side cross-sectional view of another example digital microfluidic device in accordance with the present disclosure; [0009] FIG.7 is a schematic side cross-sectional view of yet another example digital microfluidic device in accordance with the present disclosure; [0010] FIG.8 is a schematic side cross-sectional view of still another example digital microfluidic device in accordance with the present disclosure; [0011] FIG 9 is a schematic side cross-sectional view of another example digital microfluidic device in accordance with the present; [0012] FIG.10 is a flowchart illustrating an example method of making an electrowetting surface in accordance with the present disclosure; and [0013] FIG.11 is a flowchart illustrating further detail related to an example method of making an electrowetting surface in accordance with the present disclosure. [0014] Reference will now be made to several examples that are illustrated herein, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended. DETAILED DESCRIPTION [0015] The present disclosure describes electrowetting surfaces that can be used in digital microfluidic devices. The electrodes include a layer of conductive material deposited over the substrate. A nanoceramic coating is over the plurality of electrodes. The nanoceramic coating includes ceramic nanoparticles bound together by a polymeric binder. In some examples, the ceramic nanoparticles can include silica, alumina, zirconia, yttria, ceria, Pr₆O₁₁, Nd₂O₃, Sm₂O₃, TiO2, Graphene, Eu₂O₃, Gd₂O₃, Tb₄O₇, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm2O3, Yb2O3, Lu2O3, a functionalized version thereof, or a combination thereof. In further examples, the ceramic nanoparticles can have an average particle size from about 1 nm to about 200 nm. In other examples, the polymeric binder can include polydimethylsiloxane, epoxy, fluoroalkylsilane, silicone, polysilazane, polyvinylfluoride, polyvinylidene fluoride, polytetrafluoroethylene, polychlorotrifluoroethylene, perfluoroalkoxy polymer, fluorinated ethylene- propylene, polyethylenetetrafluoroethylene, polyethylenechlorotrifluoroethylene, perfluorinated elastomer, tetrafluoroethylene-propylene, perfluoropolyether, perfluorosulfonic acid, or a combination thereof. In still other examples, the electrowetting surface can also include a dielectric coating layer between the plurality of electrodes and the nanoceramic coating. In further examples, the electrowetting surface can also include a planarization layer between the plurality of electrodes and the dielectric coating. In certain examples, the dielectric coating layer can include polyimide, ethylene tetrafluoroethylene, polytetrafluoroethylene, polyolefin, B-staged bisbenzocyclobutene, polybenzoxazole, parylene, alumina, silica, aluminum nitride, or a combination thereof, and the planarization layer can include SU-8 photoresist, parylene, polydimethylsiloxane, acrylate, or a combination thereof. [0016] The present disclosure also describes digital microfluidic devices. In one example, a digital microfluidic device includes an electrowetting surface and a transparent top cover positioned over the electrowetting surface and separated from the electrowetting surface by a gap to accommodate a liquid droplet between the transparent top cover and the electrowetting surface. The electrowetting surface includes a substrate and a nanoceramic coating over the substrate. The nanoceramic coating includes ceramic nanoparticles bound together by a polymeric binder. In certain examples, the gap can have a gap distance from about 10 ^m to about 3 mm. In other examples, the digital microfluidic device can also include a second nanoceramic coating on the transparent top cover oriented such that the second nanoceramic coating faces toward the nanoceramic coating of the electrowetting surface. In certain examples, the ceramic nanoparticles can include silica, alumina, zirconia, yttria, ceria, Pr₆O₁₁, Nd₂O₃, Sm₂O₃, TiO2, Graphene, Eu₂O₃, Gd₂O₃, Tb₄O₇, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃, a functionalized version thereof, or a combination thereof, and the polymeric binder can include polydimethylsiloxane, epoxy, fluoroalkylsilane, silicone, polysilazane, polyvinylfluoride, polyvinylidene fluoride, polytetrafluoroethylene, polychlorotrifluoroethylene, perfluoroalkoxy polymer, fluorinated ethylene-propylene, polyethylenetetrafluoroethylene, polyethylenechlorotrifluoroethylene, perfluorinated elastomer, tetrafluoroethylene- propylene, perfluoropolyether, perfluorosulfonic acid, or a combination thereof. [0017] The present disclosure also describes methods of making electrowetting surfaces. In one example, a method of making an electrowetting surface includes depositing a layer of conductive material over a substrate to form a plurality of electrodes on the substrate, and depositing a nanoceramic coating over the plurality of electrodes, wherein the nanoceramic coating includes ceramic nanoparticles bound together by a polymeric binder. In some examples, the nanoceramic coating can be deposited using a process selected from the group consisting of spray coating, dip coating, spin coating, transfer coating, roller coating, extrusion coating, wipe-on coating, screen printing, and ink-jetting. In certain examples, the nanoceramic coating can be deposited using a particular transfer coating process. The transfer coating process can include: treating a first film with a plasma treatment; dispensing a nanoceramic coating composition onto a second film; pressing the nanoceramic coating composition between the second film and the first film to form a nanoceramic coating adhered to the first film; separating the second film from the nanoceramic coating, wherein the nanoceramic coating remains adhered to the first film; pressing the nanoceramic coating and the first film over the substrate and the plurality of electrodes to deposit the nanoceramic coating over the plurality of electrodes; and removing the first film from the nanoceramic coating. In further examples, the method can also include depositing a dielectric coating layer, a planarization layer, or both, between the plurality of electrodes and the nanoceramic coating. [0018] It is noted that when discussing the electrowetting surfaces, digital microfluidic devices, and methods, these discussions can be considered applicable to other examples whether or not they are explicitly discussed in the context of that example unless expressly indicated otherwise. Thus, for example, when discussing a substrate in an electrowetting surface, such disclosure is also relevant to and directly supported in context of digital microfluidic devices, methods, and vice versa. Furthermore, for simplicity and illustrative purposes, the present disclosure is described by referring mainly to certain examples. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be readily apparent however, that the present disclosure can be practiced without limitation to some of these specific details. In other instances, certain methods, compounds, compositions, and structures have not been described in detail so as not to obscure the present disclosure. Electrowetting Surfaces [0019] The present disclosure describes certain electrowetting surfaces. In some examples, these electrowetting surfaces can be used in digital microfluidic devices. Electrowetting refers to a change in contact angle between a liquid and a solid surface when an electric field is applied between the liquid and the solid surface. In some cases, an electrowetting surface can include a relatively hydrophobic surface that is in contact with the liquid droplet. Thus, the surface can have a relatively large contact angle with the liquid droplet, such as greater than 90° in some examples. However, applying an electric field can effectively make the surface more wettable. In other words, the surface and the liquid droplet can behave as if the surface is more hydrophilic when the electric field is applied. This effect can be due to a combination of forces including surface tension and electric forces. [0020] The electrowetting effect can be used, in some examples, to cause liquid droplets to move across the electrowetting surface. For example, an electric field can be applied to an area of the surface near or adjacent to the location of a liquid droplet. The liquid can have a smaller contact angle with the surface in the area of the electric field than in the area outside the electric field. This can cause the liquid to preferentially wet the surface in the adjacent area where the electric field is applied. Thus, the liquid droplet can physically move into the area where the electric field is applied as the liquid wets the surface in this area, while leaving the more hydrophobic area of the surface outside the electric field. [0021] Such surfaces can be included in digital microfluidic devices. Digital microfluidic devices can be designed in a variety of ways. In many examples, digital microfluidic devices can be capable of moving multiple discrete droplets of liquid across their electrowetting surfaces. In some cases, the movement of many droplets can be controlled independently, which can allow the individual droplets to be directed to locations, combined with other droplets, split to form smaller droplets, and so on. Some digital microfluidic devices include an array of electrodes located under an insulating hydrophobic layer. A voltage can be applied to an individual electrode to cause a liquid droplet to move to the surface over the individual electrode. By individually controlling the voltage of the electrodes in the array, such devices can control the movement of multiple liquid droplets across the hydrophobic surface. These devices can be used for a variety of applications, such as dividing a quantity of liquid into multiple droplets having a known volume, or separating specific species from other species in a liquid, or combining droplets containing different reactants to cause chemical reactions, or other applications. In some specific examples, digital microfluidic devices can be used to perform immunoassays, such as by manipulating droplets containing antigens to interact with antibodies that are either in separately manipulated droplets or immobilized on the electrowetting surface. [0022] Digital microfluidic devices are often designed to manipulate many individual droplets by moving the droplets across an electrowetting surface in specific patterns. The effectiveness of these devices can be influenced by the contact drag of the electrowetting surface. As used herein, “contact drag” refers to a force opposing the motion of a liquid droplet when the electrowetting effect is used to move the liquid droplet across the electrowetting surface. The contact drag can be caused by friction between the surface and the liquid, adherence of the liquid to defects in the surface, reduced hydrophobicity of the surface, and other factors. A low contact drag can be useful, because less energy can be used to move droplets on the surface when the contact drag is low. When the electrowetting surface has high contact drag, then a high voltage may be used to move droplets on the surface. The cost of the digital microfluidic device can be reduced when the electrowetting surface has low contact drag and lower voltages can be used. Additionally, it can be useful to have a surface with a consistent and repeatable contact drag. If the contact drag changes over time or with extended use of the device, then the performance of the device can be reduced over time. For example, the device can fail to manipulate droplets in a desired way if the contact drag of the electrowetting surface increases. [0023] Some types of electrowetting surfaces that have previously been used in digital microfluidic devices have included a hydrophobic monolayer coating. Examples of such hydrophobic monolayer coatings include FLUOROPEL™ hydrophobic coatings, available from CYTONIX (USA); RAIN-X® coatings, available from ITW Global Brands (USA); AQUAPEL™ coatings, available from PGW Auto Glass, LLC (USA); octadecyltrichlorosilane; dodecyltrichlorosilane; and others. These coatings often have very low contact drag, which is useful in digital microfluidic devices. However, these coatings are often fragile, and can be easily degraded chemically and/or mechanically. Because of degradation over time, the contact drag can increase over time and can be unpredictable. Therefore, digital microfluidic devices that include hydrophobic monolayer coatings often have decreasing performance over time. [0024] Other digital microfluidic devices have previously been made with electrowetting surfaces having a layer of a bulk hydrophobic material, such as a bulk polymer or a bulk ceramic material. The terms “bulk polymer” and “bulk ceramic” refer to a thicker layer of a solid homogeneous material, as opposed to a monolayer coating. Some examples of bulk polymers that have been used include TEFLON™ AF 1600 and AF 2400, available from The Chemours Company (USA); CYTOP® fluoropolymer, available from AGC chemicals Company (USA); NOVEC™ 1700 available from 3M (USA); and others. Examples of bulk ceramic materials that have been used include silicon oxycarbide, cerium oxide, and others. These bulk materials can be more durable than the hydrophobic monolayer coatings mentioned above. However, these materials also often have a greater contact drag than the hydrophobic monolayer coatings. Furthermore, some bulk polymers are soluble in solvents that may be used in microfluidic processes. Therefore, digital microfluidic devices including these polymer materials can be limited in the range of solvents and reagents that can be used with the microfluidic devices. Bulk ceramic materials are also comparatively costly to deposit and incorporate into the microfluidic devices. [0025] The present disclosure describes electrowetting surfaces that can be particularly useful because the electrowetting surfaces can have low contact drag while also having high durability and therefore consistent and reproducible contact drag over time. The electrowetting surfaces can include a substrate having a nanoceramic coating over the substrate. The nanoceramic coating can include ceramic nanoparticles bound together by a polymeric binder. This coating can provide high durability and low contact drag for liquid droplets on the surface of the coating. [0026] As used herein, the term “over” can be used to describe the location of a particular layer relative to another layer. For example, a nanoceramic coating can be applied over a substrate. The nanoceramic coating can be in direct contact with the substrate in some examples, while in other examples the nanoceramic coating may not be in direct contact with the substrate. In certain examples, another layer of a different material, or multiple layers of different materials, can be located between the substrate and the nanoceramic coating. For example, a layer of conductive material can be deposited on the substrate to form an array of electrodes in some examples. The nanoceramic coating can then be applied over the substrate and the array of electrodes. Thus, the term “over” does not imply that a layer is applied directly in contact with another layer, and there may be intervening layers in some examples. [0027] In certain examples, an electrowetting surface in accordance with the present disclosure can include a substrate and a plurality of electrodes on the substrate. The electrodes can include a layer of conductive material deposited over the substrate. A nanoceramic coating can be over the plurality of electrodes. As mentioned above, the nanoceramic coating can include ceramic nanoparticles bound together by a polymeric binder. FIG.1A shows one example electrowetting surface 100. This example includes a substrate 110 with two electrodes 120, 122 on the substrate. The electrodes are formed as a layer of conductive material deposited on the substrate. A nanoceramic coating is applied over the electrodes. The nanoceramic coating includes ceramic nanoparticles 130 bound together by a polymeric binder 140. A liquid droplet 102 is shown on the surface of the nanoceramic coating. The liquid droplet is not a part of the electrowetting surface itself, but is shown to illustrate how a liquid droplet can be positioned over an electrode on the electrowetting surface. [0028] FIG.1B shows how the liquid droplet 102 can move across the electrowetting surface 100. The droplet can begin in a location over a first electrode 120, as shown in FIG.1A. In FIG.1A, both electrodes 120, 122 are turned off so the wettability of the hydrophobic surface of the nanoceramic coating is unchanged. In FIG.1B, the second electrode 122 is activated and a voltage is applied to the second electrode. This changes the contact angle between the liquid and the surface in the area over the second electrode. As shown in FIG.1B, the liquid begins to wet the area of the surface over the second electrode. This can cause the entire liquid droplet to move in the direction 104 from the original location of the droplet, over the first electrode, to a new location over the second electrode. This illustrates how a liquid droplet can be manipulated and moved across the electrowetting surface simply by applying voltage to an adjacent electrode. It is noted that many other examples of electrowetting surfaces can have a larger number of electrodes. These figures illustrate an example with two electrodes for the sake of simplicity. However, other examples can include an array of many more electrodes, such as 10 electrodes, 20 electrodes, 40 electrodes, 100 electrodes, or more. [0029] As mentioned above, the nanoceramic coating can be a composite including ceramic nanoparticles bound together by a polymeric binder. As used herein, “nanoparticles” can refer to particles that are from about 1 nm to about 1,000 nm in size. In particular examples, the nanoceramic nanoparticles used in the coating can have an average particle size from about 1 nm to about 200 nm, or from about 5 nm to about 100 nm, or from about 10 nm to about 60 nm, or from about 60 nm to about 150 nm. [0030] As used herein, “average particle size” refers to a number average of the diameter of the particles for spherical particles, or a number average of the volume equivalent sphere diameter for non-spherical particles. The volume equivalent sphere diameter is the diameter of a sphere having the same volume as the particle. Average particle size can be measured using a particle analyzer such as the MASTERSIZER™ 3000 available from Malvern Panalytical (United Kingdom). The particle analyzer can measure particle size using laser diffraction. A laser beam can pass through a sample of particles and the angular variation in intensity of light scattered by the particles can be measured. Larger particles scatter light at smaller angles, while smaller particles scatter light at larger angles. The particle analyzer can then analyze the angular scattering data to calculate the size of the particles using the Mie theory of light scattering. The particle size can be reported as a volume equivalent sphere diameter. [0031] The morphology of the nanoceramic particles can vary, depending on the particular nanoceramic material used. In some examples, the nanoceramic particles can be spherical, or non-spherical with an aspect ratio from about 1.1 to about 2, or a larger aspect ratio from about 2 to about 100. In further examples, the ceramic nanoparticles can have the form of flakes, platelets, nanorods, nanotubes, nanofibers, or another form. [0032] Some specific examples of ceramic materials that can be included in the ceramic nanoparticles are: silica, alumina, zirconia, yttria, ceria, Pr₆O₁₁, Nd₂O₃, Sm₂O₃, TiO2, Graphene, Eu₂O₃, Gd₂O₃, Tb₄O₇, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, and Lu₂O₃. In some examples, the nanoceramic coating can include nanoparticles of one of these ceramic materials, or a combination of multiple ceramic materials. In further examples, the ceramic nanoparticles can be functionalized with various functionalizing compounds. As an example, silica nanoparticles can be functionalized with a fluoroalkylsilane compound. Some specific examples include 3-(1H,1H,2H,2H-perfluorooxamidepropyl)- triethoxysilane; 3-(1H,1H,2H,2H-perfluorooctylaminepropyl)-triethoxysilane; and 3-(1H,1H,2H,2H-perfluorodecoxyamidepropyl)-triethoxysilane. [0033] The ceramic nanoparticles can be bound together with a polymeric binder. A variety of polymers can be used. Additionally, additives such as functionalizing compounds can be included with the polymeric binder. In various examples, the polymeric binder can include polydimethylsiloxane, epoxy, fluoroalkylsilane, silicone, polysilazane, polyvinylfluoride, polyvinylidene fluoride, polytetrafluoroethylene, polychlorotrifluoroethylene, perfluoroalkoxy polymer, fluorinated ethylene-propylene, polyethylenetetrafluoroethylene, polyethylenechlorotrifluoroethylene, perfluorinated elastomer, tetrafluoroethylene- propylene, perfluoropolyether, perfluorosulfonic acid, or a combination thereof. In a particular example, the nanoceramic coating can include silica particles bound together by polydimethyl siloxane. In another particular example, the silica particles can be functionalized with a fluoroalkylsilane compound such as 1H,1H,2H,2H-perfluorooctyltriethoxysilane. [0034] The amounts of the ceramic nanoparticles and the polymeric binder can be from about 0.1 wt% to about 95 wt% ceramic nanoparticles, and from about 5 wt% to about 99 wt% polymeric binder with respect to the total dry weight of the nanoceramic coating, in some examples. In other examples, the amount of ceramic nanoparticles can be from about 1 wt% to about 50 wt%, and the amount of polymeric binder can be from about 50 wt% to about 99 wt%. In still other examples, the amount of ceramic nanoparticles can be from about 1 wt% to about 20 wt%, and the amount of polymeric binder can be from about 20 wt% to about 99 wt%. In further examples, the amount of ceramic nanoparticles can be from about 1 wt% to about 15 wt%, and the amount of polymeric binder can be from about 85 wt% to about 99 wt%. [0035] In some examples, the nanoceramic coating can have a thickness from about 5 nm to about 100 ^m, or from about 10 nm to about 50 ^m, or from about 50 nm to about 10 ^m, or from about 100 nm to about 1 ^m. [0036] The nanoceramic coating can be water-repellant, meaning that the coating can have a relatively high contact angle with water when no electric field is being applied. In some examples, the nanoceramic coating can have a water contact angle from about 91° to about 175°, or from about 100° to about 160°, or from about 115° to about 150°, or from about 150° to about 175°. In some examples, the nanoceramic coating can also be somewhat oil-repellant. In certain examples, the nanoceramic coating can have an oil contact angle from about 70° to about 140°, or from about 80° to about 140°, or from about 80° to about 100°, or from about 100° to about 140°. The coating can also have a low contact drag. The low contact drag can result in a small water roll-off angle (or water sliding angle). The water roll-off angle can refer to the incline angle at which a droplet of water will roll or slide off a coated surface under the force of gravity. In some examples, the water roll-off angle of the nanoceramic coating can be from about 0.5° to about 25°, or from about 5° to about 20°, or from about 10° to about 20°. [0037] In certain examples, the nanoceramic coating can be formed by applying a liquid coating composition. The liquid coating composition can include the ceramic nanoparticles and polymeric binder. In some examples, the polymeric binder can be in an uncured liquid form in the coating composition. The polymeric binder can then be cured after applying the coating composition forming a solid coating. In other examples, the polymeric binder can be dissolved, emulsified, or dispersed in a solvent such as water and/or an organic solvent. Such coating compositions can be applied and dried to evaporate the solvent. Some specific example coating compositions that can be used include NASIOL® ZR53, NASIOL® C, NASIOL® T-WB, NASIOL® T, NASIOL® T1, NASIOL® W, NASIOL® NL272, and NASIOL® SHBC available from Artekya Technology (Turkey). [0038] Regarding the other components of the electrowetting surfaces, the substrate can be made from a variety of materials. In certain examples, the substrate can include single crystalline silicon, polycrystalline silicon, gallium arsenide, glass, silica, ceramics, indium tin oxide, a semiconducting material, a printed circuit board, a polyimide film, plastic, metal, sapphire, or a combination thereof. Some plastics that can be used include polycarbonate, cyclic olefin copolymer, acrylic, and others. Some metals that can be used include aluminum, copper, stainless steel, and others. In a particular example, the substrate can have a thickness from 500 ^m to 5 mm, or from 500 ^m to 2 mm, or from 500 ^m to 1 mm. [0039] The electrodes can be formed of a conductive material, such as metal, a conductive ceramic, or other conductive materials. Some specific examples can include copper, copper plated with gold, gold, platinum, silver, aluminum, graphene, graphitic materials, indium tin oxide, zinc tin oxide, and others. In some examples, the electrowetting surface can also include conductive traces that lead to the individual electrodes, and the conductive traces can be connectable to a power source and/or an electronic controller to allow individual electrodes to be powered. In some examples, the conductive electrodes and traces can be deposited using a suitable deposition process, such as physical vapor deposition, chemical vapor deposition, electroplating, electroless plating, conductive ink printing, photo-etching, or combinations thereof. The thickness of the electrodes can be from about 50 nm to about 100 ^m, or from about 100 nm to about 10 ^m, or from about 100 nm to about 1 ^m, in some examples. [0040] The electrowetting surfaces can also include additional material layers in some examples. In certain examples, the nanoceramic coating can be positioned so that the nanoceramic coating is in direct contact with the liquid droplets on the electrowetting surface. Therefore, in some cases additional layers can be located beneath the nanoceramic coating, such as between the nanoceramic coating and the substrate. [0041] In some examples, the electrowetting surface can include a dielectric coating layer between the electrodes and the nanoceramic coating. FIG.2 shows one such example electrowetting surface 100. This example includes a substrate 110, electrodes 120, 122, and a nanoceramic coating with ceramic nanoparticles 130 and a polymeric binder 140 as in the previous example. However, this example also includes a dielectric coating layer 150 between the nanoceramic coating and the electrodes. The dielectric coating layer can include a dielectric material, such as polyimide, ethylene tetrafluoroethylene, polytetrafluoroethylene, polyolefin, B-staged bisbenzocyclobutene, polybenzoxazole, parylene, alumina, silica, aluminum nitride, or a combination thereof. Specific examples of polyimide films that can be used include KAPTON® films from DuPont (USA) and UPILEX® films from UBE Industries (Japan). The dielectric layer can have a thickness from about 100 nm to about 1 mm or from about 100 nm to about 100 ^m, or from about 100 nm to about 10 ^m, in some examples. [0042] In further examples, the electrowetting surface can include a planarization layer. The planarization layer can help form an even, flat surface for the electrowetting surface. For example, the planarization layer can fill in spaces between electrodes and reduce the thickness variation cause by the electrodes. FIG.3 shows another example electrowetting surface 100 that includes a planarization layer 160. This particular example also includes a dielectric coating layer 150. The planarization layer is located between the electrodes 120, 122 and the dielectric coating. This example also includes a substrate 110 and a nanoceramic coating made up of ceramic nanoparticles 130 and a polymeric binder 140 as in the previous examples. In some examples, the planarization layer can include materials such as SU-8 photoresist, parylene, polydimethylsiloxane, acrylate, or a combination thereof. The planarization layer can have a thickness from about 50 nm to about 100 ^m, or from about 100 nm to about 10 ^m, or from about 100 nm to about 1 ^m, in some examples. [0043] As mentioned above, the electrowetting surfaces described herein can include an array of many electrodes in some examples. FIG.4 shows a top- down view of an example electrowetting surface 100 that includes an array of electrodes 120. The electrodes are deposited as layers of conductive material on a substrate 110. The other layers overlying the electrodes are not shown in this figure for the sake of clarity. However, in practice, the nanoceramic coating can often be transparent, so the electrodes can be visible through the nanoceramic coating. This example also includes a power source 170 and an electronic controller 180, both of which are electrically connected to the array of electrodes. These components can be connected in such a way that the electronic controller can apply a voltage to selected electrodes in the electrode array. In certain examples, the electrode array can be a commercially available electrode array such as an electrode array from an OPENDROP™ cartridge available from GaudiLabs (Switzerland). Additional layers such as the nanoceramic coating, dielectric coating, and planarization layer can be added over the electrode array to form an electrowetting surface as described herein. Digital Microfluidic Devices [0044] The present disclosure also describes digital microfluidic devices that can include electrowetting surfaces, such as the electrowetting surfaces described above. In some applications, a digital microfluidic device can include a consumable or disposable cartridge that can be used once and then discarded. Accordingly, the cartridge can be designed to have a low cost. In some cases, the electrowetting surface can be a part of the cartridge. The cartridge can be designed to be used with a system that can manipulate droplets on the electrowetting surface to perform a variety of processes depending on the particular application. In some examples, such a cartridge can include a substrate and a nanoceramic coating. In further examples, the cartridge can also include electrodes formed on the substrate, beneath the nanoceramic coating. However, some systems can include mechanisms for applying electric fields to the electrowetting surface so that the cartridge itself may not include the electrodes. [0045] In alternative examples, a digital microfluidic device can include all components used for manipulating droplets in the device itself, without using a disposable cartridge. Thus, the digital microfluidic device can include the electrodes and more expensive components such as a power supply and electronic controller, in some examples. The example digital microfluidic devices described herein can be disposable cartridges for use with an additional system, or a device that includes a disposable cartridge as a part of the device, or an integrated device that does not use disposable cartridges, in various examples. [0046] In many examples, the digital microfluidic device can include a top cover located over the electrowetting surface. A gap can be between the electrowetting surface and top cover so that a liquid droplet can be accommodated in the gap between the top cover and the electrowetting surface. In some examples, the top cover can help reduce evaporation, control droplet volume, and help with visualization of droplets. FIG.5 shows one example digital microfluidic device 200 that includes an electrowetting surface 100 and a transparent top cover 210 positioned over the electrowetting surface. The electrowetting surface includes a substrate 110 and a nanoceramic coating made up of ceramic nanoparticles 130 and a polymeric binder 140. The transparent top cover is separated from the electrowetting surface by a gap to accommodate a liquid droplet 102 between the transparent top cover and the electrowetting surface. In some examples, the gap can have a gap distance from about 10 ^m to about 3 mm, or from about 10 ^m to about 1 mm, or from about 10 ^m to about 500 ^m, or from about 500 ^m to about 1 mm. [0047] The top cover of the digital microfluidic device can be transparent to allow droplets to be viewed on the electrowetting surface. However, in some applications the droplets may not be viewed and the top cover can be opaque. The material used to make the top cover can be a rigid transparent material, such as glass, plastic, and so on. In certain examples, the top cover can include a transparent electrode layer, such as a layer of indium tin oxide or zinc tin oxide. This electrode layer can be electrically grounded or electrically charged with an opposite charge from the electrodes that are positioned beneath the electrowetting surface. Thus, the transparent electrode layer can help generate the electric fields that are used to move droplets across the electrowetting surface. [0048] FIG.6 shows another example digital microfluidic device 200. This example includes an electrowetting surface 100 that has integrated electrodes 120, 122. As in previous examples, the electrodes are on a substrate 110, and a nanoceramic coating is over the electrodes. The nanoceramic coating includes ceramic nanoparticles 130 in a polymeric binder 140. The digital microfluidic device also includes a transparent top cover 210 separated from the electrowetting surface by a gap to accommodate a liquid droplet 102. [0049] FIG.7 shows a different example digital microfluidic device 200. In this example, the electrowetting surface 100 does not include integrated electrodes. Instead, a separate second substrate 220 with electrodes 230, 232 is positionable under the substrate 110 of the electrowetting surface. The electrodes on the second substrate can thus be positioned under the electrowetting surface, and the electrodes can apply voltage to generate the electrowetting effect of the electrowetting surface. As in previous examples, the electrowetting surface includes a nanoceramic coating including ceramic nanoparticles 130 in a polymeric binder 140. Additionally, a transparent top cover 210 is positioned above the electrowetting surface with a gap to accommodate a liquid droplet 102. In certain examples, the electrowetting surface and the top cover can be components of a disposable cartridge, and the second substrate and electrodes can be components of a reusable system. The disposable cartridge can be positioned in the system so that the electrodes can be used to generate the electrowetting effect of the electrowetting surface. [0050] In certain examples, the gap between the electrowetting surface and the top cover can be filled with air. Liquid droplets that are manipulated on the electrowetting surface can be separated one from another by air. In other examples, the gap can be filled with oil. Oils that can be used to fill the gap include silicone oil, fluorocarbon oil, engineered fluids, and others. Some specific examples can include 2 centistoke silicone oil, 5 centistoke silicone oil, FLUOROINERT™ FC-40 and FC-75 available from Sigma Aldrich (USA), NOVEC™ HFE 7100, HFE 7300, and HFE 7500 available from 3M (USA). [0051] FIG.8 shows an example digital microfluidic device 200 having a gap filled with oil 240. The oil fills the gap between the top cover 210 and the electrowetting surface 100. An aqueous liquid droplet 102 can move through oil across the electrowetting surface. As in the previous examples, the electrowetting surface includes a substrate 110 and a nanoceramic coating. The nanoceramic coating includes ceramic nanoparticles 130 in a polymeric binder 140. [0052] In other examples, the top cover of the digital microfluidic device can also be coated with a nanoceramic coating. In particular, the inner surface of the top cover can be coated with the nanoceramic coating, so that the nanoceramic coating is in contact with droplets located in the gap between the top cover and the electrowetting surface. FIG.9 shows one such example digital microfluidic device 200. This example includes an electrowetting surface 100 that includes a substrate 110 and a nanoceramic coating on the substrate. The nanoceramic coating includes ceramic nanoparticles 130 and a polymeric binder 140. A top cover 210 is positioned above the electrowetting surface. The top cover also has a nanoceramic coating on the bottom surface of the top cover. A gap is between the nanoceramic coating of the top cover and the nanoceramic coating of the electrowetting surface. An aqueous liquid droplet 102 is in this gap, and the remainder of the gap is filled with oil 240. In alternative examples, a digital microfluidic device can include a nanoceramic coating on both the electrowetting surface and the top cover, and the gap can be air-filled. [0053] It is noted that many of the examples described above include aqueous liquid droplets on the electrowetting surface. The electrowetting effect can be particularly useful with aqueous liquids, especially with aqueous liquids that include electrolytes. However, non-aqueous liquids can also be manipulated on the electrowetting surface. Some examples of non-aqueous liquids that can be manipulated with the electrowetting surface include formamide, formic acid, dimethyl sulfoxide, N,N-dimethylformamide, acetonitrile, methanol, ethanol, acetone, piperidine, 1-pentanol, 1-hexanol, dichloromethane, dibromomethane, tetrahydrofuran, m-dichlorobenzene, chloroform, 4-methyl-3-heptanol, and others. Some non-aqueous fluids may move across the electrowetting surface when a more intense electric field is used, such as using a higher voltage or smaller gap distance between the top electrodes and bottom electrodes. In other examples, aqueous liquids can be moved using a less intense electric field. In certain examples, the voltage applied to the electrodes can be from about 100 V to about 400 V, or from about 200 V to about 400 V, or from about 200 V to about 300 V. Methods of Making Electrowetting Surfaces [0054] Methods of making electrowetting surfaces can include forming any of the example electrowetting surfaces described above. FIG.10 is a flowchart illustrating one example method 300 of making an electrowetting surface. The method includes: depositing a layer of conductive material over a substrate to form a plurality of electrodes on the substrate 310; and depositing a nanoceramic coating over the plurality of electrodes 320. In further examples, the method can also include depositing any other layers and coatings described above, such as a dielectric coating layer, a planarization layer, and so on. [0055] As mentioned above, in some examples the nanoceramic coating can be applied as a liquid coating composition. The coating composition can then be dried and/or cured to form the solid nanoceramic coating. In some examples, the nanoceramic coating can be cured by air drying at room temperature for a curing time from about 1 hour to about 48 hours, or from about 1 hour to about 24 hours, or from about 8 hours to about 24 hours. Depending on the type of polymeric binder used in the nanoceramic coating composition, other curing processes may also be used, such as heating to a curing temperature for a curing time, or exposing the coating to ultraviolet light if an ultraviolet-curable polymer is used. [0056] The nanoceramic coating composition can be applied using a variety of application processes. In some examples, the nanoceramic coating can be applied by spray coating, dip coating, spin coating, transfer coating, roller coating, extrusion coating, wipe-on coating, screen printing, ink-jetting, or other processes. In certain examples, the nanoceramic coating can be applied using a particular transfer coating process. The transfer coating process can involve forming a nanoceramic layer between two layers of film, and then transferring the nanoceramic layer to the substrate of the electrowetting surface (over the electrodes, dielectric layer, planarization layer, and so on if such layers are present). In one example a first film can be treated with a plasma treatment. The plasma treatment can increase the adherence of the nanoceramic layer to the first film. In certain examples, the first film can be a polyethylene film or a polyethylene terephthalate film. After the plasma treatment, a nanoceramic coating composition is dispensed onto a second film, and the first film is pressed against the second film so that a nanoceramic layer forms between the two films. In certain examples, the second film can be a MYLAR® film, available from DuPont (USA). The films and nanoceramic layer can be passed through a laminator to press the films together. The nanoceramic layer can adhere more strongly to the first film because of the plasma treatment. The second film can then be peeled off of the nanoceramic layer attached to the first film. The nanoceramic layer and the first film can then be pressed against the substrate of the electrowetting surface, or the electrodes, dielectric layer, or planarization layer if such layers are present. This can cause the nanoceramic layer to transfer to the electrowetting surface. The first film can then be removed from the nanoceramic layer. [0057] FIG.11 is another flowchart illustrating an example method 400 of making an electrowetting that includes a transfer coating process, which can build upon the method described in FIG.10, for example. To illustrate, depositing the layer of the conductive material over a substrate to form a plurality of electrodes on the substrate can include treating 410 a first film with a plasma treatment, and dispensing 420 a nanoceramic coating composition onto a second film, and pressing 440 the nanoceramic coating composition between the second film and the first film to form a nanoceramic coating adhered to the first film. In further detail, the method can include separating 450 the second film from the nanoceramic coating, pressing 460 the nanoceramic coating and the first film over the substrate and the plurality of electrodes to deposit the nanoceramic coating over the plurality of electrodes, and removing 470 the first film from the nanoceramic coating 470. Definitions [0058] It is to be understood that this disclosure is not limited to the particular processes and materials disclosed herein because such processes and materials may vary somewhat. It is also to be understood that the terminology used herein is used for the purpose of describing particular examples. The terms are not intended to be limiting because the scope of the present disclosure is intended to be limited by the appended claims and equivalents thereof. [0059] It is noted that, as used in this specification and the appended claims, the singular forms ”a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. [0060] As used herein, the term “substantial” or “substantially” when used in reference to a quantity or amount of a material, or a specific characteristic thereof, refers to an amount that is sufficient to provide an effect that the material or characteristic was intended to provide. The exact degree of deviation allowable may in some cases depend on the specific context. [0061] As used herein, the term “about” is used to provide flexibility to a numerical range endpoint by providing that a given value may be “a little above” or “a little below” the endpoint. The degree of flexibility of this term can be dictated by the particular variable and determined based on the associated description herein. [0062] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though the members of the list are individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. [0063] Concentrations, amounts, and other numerical data may be expressed or presented herein in a range format. It is to be understood that such a range format is used merely for convenience and brevity and thus should be interpreted flexibly to include the numerical values explicitly recited as the limits of the range, and also to include individual numerical values or sub-ranges encompassed within that range as if the numerical values and sub-ranges are explicitly recited. As an illustration, a numerical range of “about 1 wt% to about 5 wt%” should be interpreted to include the explicitly recited values of about 1 wt% to about 5 wt%, and also to include individual values and sub-ranges within the indicated range. Thus, included in this numerical range are individual values such as 2, 3.5, and 4 and sub-ranges such as from 1-3, from 2-4, and from 3-5, etc. This same principle applies to ranges reciting a single numerical value. Furthermore, such an interpretation should apply regardless of the breadth of the range or the characteristics being described. EXAMPLES [0064] The following illustrates an example of the present disclosure. However, it is to be understood that the following are merely illustrative of the application of the principles of the present disclosure. Numerous modifications and alternative compositions, methods, and systems may be devised without departing from the scope of the present disclosure. Example 1 – Digital Microfluidic Device [0065] A microfluidic device was assembled using a printed circuit board designed by Gaudi Labs for digital microfluidic applications. The printed circuit board provided included a printed circuit board substrate with metal electrodes arranged in a grid on the surface. The electrodes were bare, without any additional coating over the electrodes. A layer of KAPTON® polyimide film from DuPont (USA) was placed over the electrodes to act as a dielectric layer. Then, a coating of NASIOL® ZR53 nanoceramic coating composition from Artekya Technology (Turkey) was applied over the polyimide film. [0066] For comparison, a control digital microfluidic device was obtained from Gaudi Labs. The control device included the same type of printed circuit board with a dielectric layer of ethylene tetrafluoroethylene and a hydrophobic layer of a FLUOROPEL™ hydrophobic coating, available from CYTONIX (USA). [0067] The example microfluidic device and the control device were tested by placing a drop of colored water on the surface of the devices and moving the droplet across the surface using the electrowetting effect. The printed circuit boards in the devices were connected to a power source and electronic controller in an OPENDROP™ V4 digital microfluidics platform from Gaudi Labs (Switzerland). Initially, both devices were able to successfully move the water droplet across the electrowetting surface. Then, the surface of both devices was rubbed with a KIMWIPE™ wipe from Kimberly Clark (USA). After being rubbed with the wipe, the example microfluidic device with the nanoceramic coating still operated as successfully as before, with no change. However, the control device had significantly degraded performance, and the device was not able to accurately move the water droplet across the surface. This indicates that the wiping caused sufficient damage to the electrowetting surface of the control device that the operation of the digital microfluidic device was inhibited. The nanoceramic coating was sufficiently durable to allow the example device to continue consistent operation after the wiping. Example 2 – Scratch Resistance [0068] To further test the durability of the nanoceramic coatings, a layer of the NASIOL® ZR53 nanoceramic coating composition from Artekya Technology (Turkey) was applied to half of a disc made of SU8 photoresist material. A lidded plastic marker was then forcefully dragged across the SU8 disc multiple times. On the side of the disc without the nanoceramic coating, the marker formed easily visible scratches in the SU8 material. However, on the side of the disc with the nanoceramic coating, no scratches were visible at all. Thus, the nanoceramic coating is hard and scratch-resistant. Example 3 – Non-stick Properties [0069] A nanoceramic coating was applied to half of a disc of glass coated with SU8 photoresist material. The nanoceramic coating composition used in this case was NASIOL® NL272 nanoceramic coating composition from Artekya Technology (Turkey). A latex ink was dropped onto the surface of the disc to form a series of dots on the surface of the disc. The latex ink was a type highly adherent and difficult to remove from many materials. After applying the ink, the ink was cured at 90 °C for 3 minutes to fully cure the latex ink. Then, a squeegee blade (80 durometer) was used to scrape the surface of the disc. The ink dots on the half of the disc that was treated with the nanoceramic coating were wiped away cleanly with the first wipe of the squeegee. In contrast, the ink dots on the other half of the disc were still present after 10 wipes with the squeegee. This demonstrates that the nanoceramic coating has very good non-stick properties compared to bare SU8 material. [0070] While the present technology has been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the disclosure.

Claims

CLAIMS What is claimed is: 1. An electrowetting surface comprising: a substrate; a plurality of electrodes on the substrate, wherein the electrodes comprise a layer of conductive material deposited over the substrate; and a nanoceramic coating over the plurality of electrodes, wherein the nanoceramic coating comprises ceramic nanoparticles bound together by a polymeric binder.

2. The electrowetting surface of claim 1, wherein the ceramic nanoparticles comprise silica, alumina, zirconia, yttria, ceria, Pr6O11, Nd2O3, Sm₂O₃, TiO2, Graphene, Eu₂O₃, Gd₂O₃, Tb₄O₇, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃, a functionalized version thereof, or a combination thereof.

3. The electrowetting surface of claim 1, wherein the ceramic nanoparticles have an average particle size from about 1 nm to about 200 nm.

4. The electrowetting surface of claim 1, wherein the polymeric binder comprises polydimethylsiloxane, epoxy, fluoroalkylsilane, silicone, polysilazane, polyvinylfluoride, polyvinylidene fluoride, polytetrafluoroethylene, polychlorotrifluoroethylene, perfluoroalkoxy polymer, fluorinated ethylene- propylene, polyethylenetetrafluoroethylene, polyethylenechlorotrifluoroethylene, perfluorinated elastomer, tetrafluoroethylene-propylene, perfluoropolyether, perfluorosulfonic acid, or a combination thereof.

5. The electrowetting surface of claim 1, further comprising a dielectric coating layer between the plurality of electrodes and the nanoceramic coating.

6. The electrowetting surface of claim 5, further comprising a planarization layer between the plurality of electrodes and the dielectric coating.

7. The electrowetting surface of claim 6, wherein the dielectric coating layer comprises polyimide, ethylene tetrafluoroethylene, polytetrafluoroethylene, polyolefin, B-staged bisbenzocyclobutene, polybenzoxazole, parylene, alumina, silica, aluminum nitride, or a combination thereof, and wherein the planarization layer comprises SU-8 photoresist, parylene, polydimethylsiloxane, acrylate, or a combination thereof.

8. A digital microfluidic device comprising: an electrowetting surface comprising: a substrate, and a nanoceramic coating over the substrate, wherein the nanoceramic coating comprises ceramic nanoparticles bound together by a polymeric binder; and a transparent top cover positioned over the electrowetting surface separated from the electrowetting surface by a gap to accommodate a liquid droplet between the transparent top cover and the electrowetting surface.

9. The digital microfluidic device of claim 8, wherein the gap has a gap distance from about 10 ^m to about 3 mm.

10. The digital microfluidic device of claim 9, further comprising a second nanoceramic coating on the transparent top cover oriented such that the second nanoceramic coating faces toward the nanoceramic coating of the electrowetting surface.

11. The digital microfluidic device of claim 8, wherein the ceramic nanoparticles comprise silica, alumina, zirconia, yttria, ceria, Pr₆O₁₁, Nd₂O₃, Sm₂O₃, TiO2, Graphene, Eu₂O₃, Gd₂O₃, Tb₄O₇, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃, a functionalized version thereof, or a combination thereof, and wherein the polymeric binder comprises polydimethylsiloxane, epoxy, fluoroalkylsilane, silicone, polysilazane, polyvinylfluoride, polyvinylidene fluoride, polytetrafluoroethylene, polychlorotrifluoroethylene, perfluoroalkoxy polymer, fluorinated ethylene-propylene, polyethylenetetrafluoroethylene, polyethylenechlorotrifluoroethylene, perfluorinated elastomer, tetrafluoroethylene- propylene, perfluoropolyether, perfluorosulfonic acid, or a combination thereof.

12. A method of making an electrowetting surface comprising: depositing a layer of conductive material over a substrate to form a plurality of electrodes on the substrate; and depositing a nanoceramic coating over the plurality of electrodes, wherein the nanoceramic coating comprises ceramic nanoparticles bound together by a polymeric binder.

13. The method of claim 12, wherein the nanoceramic coating is deposited using a process selected from the group consisting of spray coating, dip coating, spin coating, transfer coating, roller coating, extrusion coating, wipe-on coating, screen printing, and ink-jetting.

14. The method of claim 12, wherein the nanoceramic coating is deposited using a transfer coating process, wherein the transfer coating process includes: treating a first film with a plasma treatment; dispensing a nanoceramic coating composition onto a second film; pressing the nanoceramic coating composition between the second film and the first film to form a nanoceramic coating adhered to the first film; separating the second film from the nanoceramic coating, wherein the nanoceramic coating remains adhered to the first film; pressing the nanoceramic coating and the first film over the substrate and the plurality of electrodes to deposit the nanoceramic coating over the plurality of electrodes; and removing the first film from the nanoceramic coating.

15. The method of claim 12, further comprising depositing a dielectric coating layer, a planarization layer, or both, between the plurality of electrodes and the nanoceramic coating.