WO2023239369A1

WO2023239369A1 - Anisotropic coatings

Info

Publication number: WO2023239369A1
Application number: PCT/US2022/032959
Authority: WO
Inventors: Rajesh Kelekar
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2022-06-10
Filing date: 2022-06-10
Publication date: 2023-12-14

Abstract

An anisotropic coating composition can be combined with a dielectric layer to make a coated dielectric layer. In some examples, a coated dielectric layer can include a layer of solid dielectric material and an anisotropic coating layer adhered to the layer of solid dielectric material. The anisotropic coating layer can include a polymer matrix and electrically conductive ferromagnetic particles embedded in the polymer matrix. The electrically conductive ferromagnetic particles can be aligned in a plurality of conductive paths that are spaced apart throughout the anisotropic coating layer and that extend through a thickness of the anisotropic coating layer.

Description

ANISOTROPIC COATINGS 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] FIG.1A is a cross-sectional side view of an example anisotropic coating composition in accordance with the present disclosure; [0003] FIG.1B is a cross-sectional side view of an example anisotropic coating layer in accordance with the present disclosure; [0004] FIG.1C is a top-down view of an example anisotropic coating layer in accordance with the present disclosure; [0005] FIG.2 is a side cross-sectional view of an example coated dielectric layer in accordance with the present disclosure; [0006] FIG.3 is a side cross-sectional view of an example electrowetting device in accordance with the present disclosure; [0007] FIG.4 is a side cross-sectional view of another example electrowetting device in accordance with the present disclosure; [0008] FIG.5 is a side cross-sectional view of another example electrowetting device in accordance with the present disclosure; [0009] FIG.6 is a side cross-sectional view of another example electrowetting device in accordance with the present disclosure; [0010] FIG.7 is a schematic view of example composite electrically conductive ferromagnetic particles in accordance with the present disclosure; and [0011] FIG.8 is a schematic view of example electrically conductive ferromagnetic particles dispersed with a dispersant in accordance with the present disclosure. DETAILED DESCRIPTION [0012] The present disclosure describes anisotropic coating compositions that can be used in electrowetting-on-dielectric (EWOD) digital microfluidic (DMF) devices. In some examples, the anisotropic coating compositions can be coated on a dielectric layer, which can be part of such a device. In one example, a coated dielectric layer includes a layer of a solid dielectric material and an anisotropic coating layer adhered to the layer of solid dielectric material. The anisotropic coating layer includes a polymer matrix and electrically conductive ferromagnetic particles embedded in the polymer matrix. The electrically conductive ferromagnetic particles are aligned in a plurality of conductive paths that are spaced apart throughout the anisotropic coating layer and that extend through a thickness of the anisotropic coating layer. In some examples, the polymer matrix can include an epoxy, a polyurethane, a polyacrylate, a silicone, a polyhedral oligomeric silsesquioxane, a phenolic resin, a cyanate ester resin, or a combination thereof. The electrically conductive ferromagnetic particles can include iron, nickel, cobalt, magnetite, graphite, silver, gold, an alloy thereof, or a composite thereof. The electrically conductive ferromagnetic particles can have a number average particle size from 1 μm to 50 μm. In some examples, the electrically conductive ferromagnetic particles can be included in the anisotropic coating layer in an amount from 5 wt% to 30 wt% with respect to the total weight of the anisotropic coating layer. In further examples, the anisotropic coating layer can also include a dispersant selected from the group consisting of trisilanol isooctyl polyhedral oligomeric silsesquioxane, trisilanol phenyl polyhedral oligomeric silsesquioxane, a silane coupling agent, a titanate coupling agent, a zirconate coupling agent, an aluminate coupling agent, and combinations thereof. The anisotropic coating can have a thickness from 50 μm to 2000 μm. In certain examples, the anisotropic coating layer can have a resistance-times-area less than 7.0 x 10⁵Ω·cm² in a thickness direction along which the conductive paths are aligned, and a resistivity greater than 1.0 x 10¹²Ω·cm in the plane along which the anisotropic coating layer extends. The coated dielectric layer can be part of an electrowetting device, where the electrowetting device also includes a ground plate facing a surface of the layer of solid dielectric material opposite from the anisotropic coating layer. The ground plate can be spaced apart from the layer of solid dielectric material to define a liquid droplet passageway between the ground plate and the layer of solid dielectric material. [0013] The present disclosure also describes anisotropic coating compositions. In one example, an anisotropic coating composition includes a polymer matrix and electrically conductive ferromagnetic particles embedded in the polymer matrix. The electrically conductive ferromagnetic particles include a composite of multiple materials in individual particles. In some examples, the multiple materials in the individual particles can include iron, nickel, cobalt, magnetite, graphite, silver, gold, an alloy thereof, or a combination thereof. In further examples, the individual particles can include a core of a first material and a shell of a second material, where the first material can include iron, nickel, graphite, an alloy thereof, or a combination thereof, and where the second material can include nickel, silver, gold, an alloy thereof, or a combination thereof. [0014] In another example, an anisotropic coating composition includes a polymer matrix, electrically conductive ferromagnetic particles embedded in the polymer matrix, and a dispersant dispersing the electrically conductive ferromagnetic particles in the polymer matrix. The dispersant can be selected from the group consisting of trisilanol isooctyl polyhedral oligomeric silsesquioxane, trisilanol phenyl polyhedral oligomeric silsesquioxane, a silane coupling agent, a titanate coupling agent, a zirconate coupling agent, an aluminate coupling agent, and combinations thereof. The electrically conductive ferromagnetic particles can be aligned in a plurality of conductive paths that are spaced apart throughout the polymer matrix. [0015] The anisotropic coating compositions described herein can be useful in various applications where electrical charge is transferred in one direction. Among the various applications for the anisotropic coatings, digital microfluidic devices represent one example application that is focused on in the present disclosure. In particular, the anisotropic coatings can be used in digital microfluidic devices that manipulate droplets of liquid on an electrowetting surface. 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. [0016] 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. [0017] 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 a layer of dielectric material. 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. [0018] Digital microfluidic devices are now used in the healthcare industry for testing such as nucleic acid testing for infectious diseases and neonatal testing. One useful feature of the digital microfluidic devices is that these devices are relatively easy to reprogram to perform new types of assays or other processes. In order to perform a new assay, the order of actuation of the electrodes in the electrode array can be changed. In some cases, this can be accomplished without making any changes to the underlying hardware. This can reduce the cost of developing new assays. [0019] Many assays are performed with biological fluids such as blood, saliva, or other biological material. Because these biological materials can be hazardous, testing equipment that comes in contact with biological material can be discarded after use or sterilized before a subsequent use. It is often more practical to discard a used digital microfluidic device instead of sterilizing the device. Many digital microfluidic devices are designed with a single unit that includes a dielectric surface, which directly contacts liquid droplets including biological material, and a circuit board including the electrode array for driving movement of the liquid droplets. Therefore, the entire unit is typically discarded after one use. However, manufacturing circuit boards with large arrays of independently addressable electrodes can be relatively expensive. The high cost of the circuit board can result in a high cost per unit for disposable devices. [0020] The present disclosure describes ways to make a lower-cost consumable that can include an electrowetting surface and the anisotropic coatings mentioned above. In some examples, the consumable can be separated from a circuit board that includes electrodes to control droplets on the electrowetting surface. In this arrangement, the circuit board does not come in contact with biological materials. The biological materials can be contained in the consumable. Therefore, the circuit board can be re-used and the consumable can be discarded after use. In certain examples, a consumable can include a layer of dielectric material with an anisotropic coating on one face of the layer of dielectric material. The anisotropic coating can act as a mating surface to be placed in contact with a circuit board. The anisotropic coating can be elastomeric, which can allow the anisotropic to conform to the surface of the circuit board when the consumable is placed onto the circuit board. Thus, the anisotropic coating can provide continuous physical contact between the consumable and the circuit board across the entire interface. This can be useful because the presence of air gaps – even very small air gaps – between the circuit board and the consumable can significantly increase the electric resistance between the electrodes and the dielectric surface. Such an increase in resistance can interfere with the operation of the digital microfluidic devices and may make some electrodes in the array inoperable for controlling liquid droplets on the dielectric surface. [0021] The term “anisotropic” refers to materials that have a property that is different when measured in different directions. The anisotropic coatings described herein can be anisotropic with respect to electrical resistivity. The coatings can have a low resistivity in the direction through the thickness of the coating layer (this can be referred to as the “z-axis” direction). However, the coatings can have a high resistivity in the lateral directions (i.e., the x-axis and y- axis directions). This anisotropic resistivity can be enabled by small conductive particles in the anisotropic coating. The conductive particles can be aligned in conductive paths that lead through the thickness of the coating, but the paths can be separated by resistive polymer. This allows electrical charge to flow along the conductive paths in the z-axis direction, while charge transfer is blocked in the lateral x-axis and y-axis directions. [0022] The anisotropic coatings can be made by mixing electrically conductive particles in an uncured polymer. The electrically conductive particles can also be ferromagnetic, meaning that the particles respond strongly to an applied magnetic field. While the polymer is uncured, the polymer can be a liquid or otherwise have a sufficiently low viscosity to allow the conductive particles to move through the polymer. The mixture of uncured polymer and electrically conductive ferromagnetic particles can be placed in a magnetic field. This can cause the particles to move and line up aligned with the magnetic field. The result can be a plurality of conductive paths made up of aligned conductive particles. Within an individual conductive path, the particles may be touching one another or very close to one another so that the electrical resistance through the conductive paths is low. However, the conductive paths can be separated one from another by a greater distance, where the resistive polymer occupies the space between the paths. The polymer can be cured while the particles are aligned by the magnetic field. [0023] To illustrate the formation of the anisotropic coatings, FIG.1A shows an example anisotropic coating composition 100 that includes electrically conductive ferromagnetic particles 110 in a polymer matrix 120. In this figure, the polymer matrix is in an uncured state and the electrically conductive ferromagnetic particles have not yet been aligned. FIG.1B is a side cross- sectional view of an anisotropic coating 102 after the electrically conductive ferromagnetic particles have been aligned in a magnetic field and the polymer matrix has been cured. When magnetic field is applied to the particles, the particles line up and form columns 112 of particles that are touching or close together. FIG.1C is a top-down view of the anisotropic coating. The tops of the conductive columns are visible on the top surface of the anisotropic coating layer. A bottom-up view of the anisotropic coating layer can look similar. [0024] As mentioned above, some digital microfluidic devices can include a layer of dielectric material with this type of anisotropic coating layer that is conductive in one direction. More detailed examples of coated dielectric layers and the anisotropic coating compositions are described below. It is noted that when discussing the anisotropic coating compositions, coated dielectric layers, digital microfluidic devices, and methods of making or using the same, 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 certain type of conductive particle in an anisotropic coating composition, such disclosure is also relevant to and directly supported in context of coated dielectric layers, 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. Coated Dielectric Layers [0025] The anisotropic coating compositions described herein can be used to make coated dielectric layers. In some examples, a layer of dielectric material can be prepared first and then coated with the anisotropic coating composition to form the coated dielectric layer. In other examples, an anisotropic coating layer can be prepared first and then coated with a dielectric material to form the coated dielectric layer. Thus, the term “coated dielectric layer” can include structures that include a dielectric layer and an anisotropic coating layer, regardless of which layer was formed first. This coated dielectric layer can be used as a component of a digital microfluidic device. FIG.2 shows a side cross-sectional view of an example coated dielectric layer 200. The coated dielectric layer includes a layer of a solid dielectric material 210 and an anisotropic coating layer 102 adhered to the layer of solid dielectric material. The anisotropic coating layer includes a polymer matrix 120 and electrically conductive ferromagnetic particles 110 embedded in the polymer matrix. The electrically conductive ferromagnetic particles are aligned in a plurality of conductive paths 112 that are spaced apart throughout the anisotropic coating layer. The conductive paths extend through a thickness of the anisotropic coating layer. As explained above, this allows electrical charge to be transferred through the anisotropic layer in the thickness direction (i.e., the z-axis) but not in the lateral directions (i.e., the x-axis and y- axis). It can be useful to prevent charge transfer in the lateral direction so that the charge can be contained in the particular areas where it is desired to control the movement of liquid droplets on the dielectric surface. In some examples, the coated dielectric layer can be placed onto a circuit board with the anisotropic coating in direct contact with the circuit board. Liquid droplets can be located on the surface of the layer of dielectric material opposite from the anisotropic coating. The high in-plane resistivity in the x-axis and y-axis directions can also be useful to prevent charge from being conducted from one electrode on the circuit board to another electrode on the circuit board. [0026] The resistivity of the anisotropic coating layer can be significantly lower in the thickness direction (z-axis) than in the in-plane directions (x-axis and y-axis). In some examples, the resistivity in the thickness direction can be less than the resistivity in the in-plane direction by a factor of 100 to 10,000,000, or by a factor of 100 to 100,000, or by a factor of 100 to 10,000, or by a factor of 100 to 1,000, or by a factor of 10,000 to 10,000,000, or by a factor of 100,000 to 10,000,000. In certain examples, the anisotropic coating can have a resistivity in the in-plane direction that is greater than 1.0 x 10¹² Ω·cm. For example, the in- plane resistivity can be from 1.0 x 10¹²Ω·cm to 1.0 x 10¹⁶Ω·cm, or from 1.0 x 10¹²Ω·cm to 1.0 x 10¹⁵Ω·cm, or from 1.0 x 10¹²Ω·cm to 1.0 x 10¹⁴Ω·cm, or from 1.0 x 10¹²Ω·cm to 1.0 x 10¹³Ω·cm. [0027] The ability of the anisotropic coating layer to conduct electrical charge in the thickness direction can be expressed as a resistance-times-area. The actual resistance of the coating layer is related to the area of the coating layer, with the resistance decreasing as the total area increases. Therefore, the value of the resistance of the coating layer multiplied by the area of the coating layer can be a constant. In some examples the anisotropic coating layer can have a resistance-times-area that is less than 7.0 x 10⁵Ω·cm² in the thickness direction along which the conductive paths are aligned. In further examples, the resistance-times-area can be from 1 Ω·cm² to 7.0 x 10⁵Ω·cm², or from 1.0 x 10² Ω·cm² to 7.0 x 10⁵Ω·cm², or from 1.0 x 10³Ω·cm² to 7.0 x 10⁵Ω·cm², or from 1.0 x 10⁴Ω·cm² to 7.0 x 10⁵Ω·cm², or from 1.0 x 10⁵Ω·cm² to 7.0 x 10⁵Ω·cm². The resistivity value of the anisotropic coating layer in the thickness direction can be found by dividing the resistance-times-area value by the thickness of the coating layer in centimeters. In some examples, the resistivity in the thickness direction can be from 1.0 x 10²Ω·cm to 1.0 x 10⁷Ω·cm, or from 1.0 x 10³Ω·cm to 1.0 x 10⁷ Ω·cm, or form 1.0 x 10⁴Ω·cm to 1.0 x 10⁷Ω·cm, or from 1.0 x 10⁵Ω·cm to 1.0 x 10⁷Ω·cm, or from 1.0 x 10⁶Ω·cm to 1.0 x 10⁷Ω·cm. [0028] The anisotropic coating layer can be relatively thin to provide an acceptable resistance in the thickness direction. In some examples, the anisotropic coating layer can have a thickness from 50 μm to 2000 μm. In further examples, the thickness can be from 200 μm to 2000 μm, or from 200 μm to 1500 μm, or from 300 μm to 1000 μm, or from 500 μm to 1000 μm. [0029] The layer of solid dielectric material can also be relatively thin to provide an acceptable resistance. In some examples, the layer of solid dielectric material can have a thickness from 100 μm to 3 mm. In further examples, the thickness can be from 100 μm to 2 mm, or from 100 μm to 1 mm, or from 100 μm to 500 μm, or from 500 μm to 3 mm, or from 500 μm to 2 mm, or from 500 μm to 1 mm. [0030] The resistivity in the thickness direction and in the in-plane directions can be affected by the number, size, and spacing of the conductive paths, or columns, of aligned particles in the anisotropic coating layer. These can also be related to a fraction of the surface area of the anisotropic coating that is occupied by conductive paths vs. the surface area that is resistive polymer matrix. For example, the coating can be viewed from the top or bottom, or a cross-section can be taken at a certain height along a plane parallel to the x-y plane. The area of the cross-section that is occupied by conductive pathways can be divided by the total geometric area of the coating layer to yield the fractional area of conductive paths. In some examples, the fraction of the geometric area that is occupied by conductive paths can be from 25% to 50%. [0031] In some examples, the anisotropic coating layer can be elastomeric. As explained above, an elastomeric coating can be useful as a mating surface when the coated dielectric layer is placed onto a circuit board with electrodes that contact the elastomeric coating. The elastomeric coating can provide good contact with the circuit board even if the circuit board is not perfectly flat. If the anisotropic coating is elastomeric, then the anisotropic coating can be compliant enough to “fill in” any small gaps that would otherwise be caused by imperfections in the circuit board surface. In some examples, elastomeric anisotropic coating layers can have a Young’s modulus of less than 1 GPa. For example, the Young’s modulus can be from 0.0001 GPa to 1 GPa, or from 0.001 GPa to 0.5 GPa, or from 0.001 to 0.1 GPa. [0032] In some cases, it may be useful to include a rigid anisotropic coating layer. A rigid anisotropic coating can provide more structural support than an elastomeric anisotropic coating. In certain examples, the anisotropic coating can be a double layer that includes a rigid layer and an elastomeric layer. In other examples, the coated dielectric layer can include a rigid anisotropic layer without an elastomeric anisotropic layer. One use for such a coated dielectric layer can be with a non-contact ion head, which can deposit ions on the rigid anisotropic coating layer. This type of device is described in more detail below. If the anisotropic coating is rigid, in some cases the anisotropic coating can have a Young’s modulus of 1 GPa or greater. In certain examples, the rigid anisotropic coating can have a Young’s modulus from 1 GPa to 50 GPa, or from 1 GPa to 10 GPa, or from 1 GPa to 5 GPa. [0033] The anisotropic coatings can be applied by forming a layer of an anisotropic coating composition in an uncured state on a layer of solid dielectric material. The layer of solid dielectric material and the uncured coating can then be placed in a magnetic field to align the electrically conductive ferromagnetic particles. The polymer matrix can be cured while the particles are aligned. In various examples, the anisotropic coating composition can be applied to the solid layer of dielectric material by spray coating, dip coating, spin coating, transfer coating, roller coating, extrusion coating, wipe-on coating, screen printing, ink- jetting, or other processes. [0034] In other examples, the anisotropic coating layer can be formed separate from the layer of dielectric material. A layer of the anisotropic coating composition can be placed in a magnetic field to align the particles and then the polymer matrix can be cured. The cured anisotropic coating layer can then be transferred and adhered to the layer of dielectric material. Alternately, a dielectric material can be applied in an uncured state to the anisotropic coating layer and then the dielectric material can be cured. [0035] The coated dielectric layers described above can be a part of an electrowetting device that includes additional components. In some examples, the electrowetting device can also include a ground plate facing a surface of the layer of solid dielectric material opposite from the anisotropic coating layer. The ground plate can be spaced apart from the layer of solid dielectric material. The space between the ground plate and the layer of solid dielectric material can be referred to as a liquid droplet passageway or chamber. Liquid droplets can be moved within the passageway using electrowetting. The ground plate can be electrically grounded. The difference in charge between the charge applied to the anisotropic coating and the ground plate can provide an electric field within the droplet passageway that moves droplets across the electrowetting surface. In some examples, the ground plate can include a conductive layer. The conductive layer can be made of a transparent conductive material in some cases, such as indium tin oxide or zinc tin oxide. [0036] FIG.3 shows a side cross-sectional view of an example electrowetting device 300 that includes a coated dielectric layer 200 made up of a layer of solid dielectric material 210 and an anisotropic coating layer 102. This device also includes a ground plate 310 positioned above the layer of solid dielectric material and spaced apart from the solid dielectric material. A liquid droplet passageway 320 is defined as the space between the ground plate and the solid dielectric material. A liquid droplet 330 is shown in this passageway. As in previous examples, the anisotropic coating layer includes a polymer matrix 120 with electrically conductive ferromagnetic particles 110 in the polymer matrix. The particles are aligned by a magnetic field to form columns 112 of particles, which provide conductive paths through the anisotropic coating layer. [0037] FIG.4 shows a side cross-sectional view of another example electrowetting device 300 that includes additional components. This example includes a coated dielectric layer 200 made up of a layer of solid dielectric material 210 and a rigid anisotropic coating layer 104. In this example, the anisotropic coating is made with a rigid polymer in order to add more structural strength to the device. The face of the dielectric layer opposite from the anisotropic coating is coated with a hydrophobic coating 340. The device includes a ground plate 310 positioned above the layer of solid dielectric material and spaced apart from the layer of solid dielectric material. The bottom surface of the ground plate is also coated with a hydrophobic coating. The space between the ground plate and layer of solid dielectric material is a liquid droplet passageway 320. A liquid droplet 330 is inside the passageway, in contact with both of the hydrophobic coatings. This device also includes an exterior layer 350 over the ground layer. The exterior layer can be made of glass, plastic, or another solid material. In some examples, the exterior layer and the ground plate can be transparent. This example also includes an ion head 360. The ion head can be used to charge the rigid anisotropic layer without coming in direct physical contact with the rigid anisotropic layer. The ion head can be moveable in the x-y plane so that charge can be applied to various locations on the anisotropic layer by the ion head. [0038] FIG.5 is a cross-sectional side view of another example electrowetting device 300. This example includes a coated dielectric layer 200 made up of a layer of solid dielectric material 210 and an elastomeric anisotropic coating 102. The elastomeric anisotropic coating includes a polymer matrix 120 and electrically conductive ferromagnetic particles 110 aligned in columns 112 as in the previous examples. The elastomeric anisotropic layer acts as a mating surface to contact a circuit board 370. The circuit board includes electrodes 372 that can provide electrical charge to the anisotropic layer and dielectric layer to cause the liquid droplet 330 to move over the charged electrode. The layer of solid dielectric material is coated with a hydrophobic coating 340. This example also includes a ground plate 310 that is coated on the bottom side with a hydrophobic coating. A structural layer 352 is also adhered to the top face of the ground plate. In this example, the coated dielectric, the ground plate, structural layer, and the hydrophobic coatings make up a consumable cartridge that is placed on the circuit board temporarily. After use, the cartridge can be discarded. This device also includes a vacuum chuck 380 under the circuit board. The circuit board can include vacuum channels that pass through the circuit board from the bottom to the top of the circuit board (not shown) and the vacuum chuck can pull a vacuum on the bottom of the circuit board and through the vacuum channels. When the cartridge is placed on the circuit board, the vacuum can remove air from between the elastomeric anisotropic coating layer and the top of the circuit board, which can ensure a good contact between the elastomeric anisotropic coating layer and the circuit board. [0039] FIG.6 is a side cross-sectional view of another example electrowetting device 300. This example is similar to the previous example, except that the anisotropic coating in this example is a double-layer coating, including a rigid anisotropic coating layer 104 and an elastomeric anisotropic coating layer 102. The rigid anisotropic coating layer provides structural support for the cartridge, making the cartridge more rigid. Therefore, this example does not include a structural layer as in the previous example. This example also does not include a vacuum chuck to hold the cartridge against the electrodes 372 of the circuit board 370. Instead, the cartridge can be rigid enough to be clamped in place on the circuit board. Other elements of this example are the same as in the previous example, including the coated dielectric layer 200 including a layer of solid dielectric material 210, the ground plate 310, the liquid droplet passageway 320, the liquid droplet 330 in the passageway, the hydrophobic coatings 340, the electrically conductive ferromagnetic particles 110, the conductive paths 112, and the polymer matrix 120. [0040] It is noted that the above figures may not be drawn to scale. In practice, the coated dielectric layers can have a much smaller thickness compared to their width and length. Additionally, the circuit boards shown in the figures include two electrodes for the sake of simplicity. However, in practice the circuit board is likely to include many more electrodes, such as an array having from 20 to 10,000 electrodes in some examples. It is also noted that in some examples, additional layers of materials can be added that were not shown in the figures. As an example, in some cases adhesive layers can be used between some of the material layers shown in the figures. Accordingly, the various layers are limited or constrained by having certain layers in direct contact with other layers as shown in the figures. In some examples, additional layers may be placed between layers that are shown in direct contact in the figures. [0041] Regarding the surfaces in the electrowetting devices that come in contact with the liquid droplets, it can be useful to use a hydrophobic surface. Aqueous liquid droplets can have a high contact angle on hydrophobic surfaces. However, applying a sufficient electric field across the liquid droplet passageway can reduce the contact angle of the liquid droplet. The liquid droplet can be moved by applying an electric field adjacent to the location of the liquid droplet, and this can cause the liquid droplet to move into the area of the electric field where the contact angle is lower. In some examples, a hydrophobic monolayer coating can be applied on the surfaces inside the liquid droplet passageway. 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. Other types of hydrophobic surfaces can include 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 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 include silicon oxycarbide, cerium oxide, and others. Other examples of hydrophobic surfaces include nanoceramic coatings. Nanoceramic coatings can include 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. [0042] 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. [0043] Regarding the layer of solid dielectric material, a variety of materials can be used to form this layer. In some examples, the dielectric material can include a polymer such as polydimethylsiloxane, epoxy, fluoroalkylsilane, silicone, polyolefin, polysilazane, polyvinylfluoride, polyvinylidene fluoride, polytetrafluoroethylene, polychlorotrifluoroethylene, perfluoroalkoxy polymer, fluorinated ethylene-propylene, polyethylenetetrafluoroethylene, polyethylenechlorotrifluoroethylene, perfluorinated elastomer, tetrafluoroethylene- propylene, perfluoropolyether, perfluorosulfonic acid, B-staged bisbenzocyclobutene, polybenzoxazole, parylene, or a combination thereof. Inorganic materials can also be included, such as alumina, silica, aluminum nitride, or a combination thereof. In some examples, the dielectric material can include a polyimide material such as a KAPTON® material obtainable from DuPont de Nemours, Inc. (USA) or UPILEX® films from UBE Industries (Japan). In further examples, the dielectric material can include a polyetherimide (PEI) material. In certain examples, the dielectric layer can have a thickness from 100 nm to 1 mm or from 100 nm to 100 ^m, or from 100 nm to 25 ^m, in some examples [0044] In some examples, the dielectric material can have a dielectric strength of 50 V/μm to 500 V/μm, while in some examples, the dielectric strength may be from 100 V/μm to 500 V/μm. In some examples, the dielectric strength can be from 200 V/μm to 400 V/μm. In some examples, the dielectric strength can be from 300 V/μm to 500 V/μm. [0045] The liquid droplet passageway can be a space between the layer of solid dielectric material and the ground plate above the layer of solid dielectric material. In some examples, the distance between the layer of solid dielectric material and the ground plate can be from 50 μm to 500 μm, from 100 μm to 150 μm, or from 150 μm to 250 μm. In further examples, liquid droplets in the passageway can have a droplet volume from 10 pL to 30 μL. Liquid droplets in the passageway can be surrounded by air in some examples, while in other examples the passageway can be filled with a dielectric oil and the liquid droplets can be an aqueous liquid that does not mix with the dielectric oil. In some examples, the dielectric oil can affect electrowetting forces on the aqueous liquid droplets, and/or resist evaporation of the aqueous liquid droplets, and/or facilitate sliding of the droplets and maintaining droplet integrity. 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). [0046] 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. [0047] Regarding the electrodes on the circuit board, 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 circuit board 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. In certain examples, the circuit board can be a commercially available electrode array such as an electrode array from an OPENDROP™ cartridge available from GaudiLabs (Switzerland). Anisotropic Coating Compositions [0048] Formulations of example anisotropic coating compositions will now be described in more detail. In various examples, an anisotropic coating composition can include a polymer matrix and electrically conductive ferromagnetic particles embedded in the polymer matrix. As used herein, “embedded” refers to the electrically conductive ferromagnetic particles being mixed into and surrounded by the polymer matrix. The anisotropic coating composition can be in a cured state or in an uncured state. If the composition is in an uncured state, then the electrically conductive ferromagnetic particles can be in an aligned state or an unaligned state. As explained above, the anisotropic electrical resistivity properties of the anisotropic coatings result from the conductive paths formed by aligning the particles. Thus, the term “anisotropic coating composition” does not imply that the composition already has its anisotropic electrical resistivity properties since the particles may or may not be in an aligned state. The term “anisotropic coating composition” can be understood as a composition that may be used to make an anisotropic coating by aligning the electrically conductive ferromagnetic particles in a magnetic field and curing the polymer matrix as explained above. [0049] Regarding the polymer matrix, a variety of polymers can be used. The polymer matrix can have an initial uncured state in which the polymer matrix is a liquid, paste, or has a sufficiently low viscosity to allow ferromagnetic particles to move through the polymer matrix in order to align with a magnetic field. In some examples, the viscosity of the polymer matrix in the uncured state can be less than 50,000 cP, or less than 30,000 cP, or less than 20,000 cP, or less than 10,000 cP. In certain examples, the uncured polymer matrix can include monomers that can react to form polymer chains when cured. Thus, the term “polymer matrix” can include monomers that have not formed a polymer yet when in their uncured state. In other examples, the uncured polymer matrix can include polymer chains that have already been formed, but which are dispersed or dissolved in a liquid and which can become a solid polymer upon curing. A curing process for the polymer matrix can include polymerizing monomers to form polymer chains, cross-linking polymer chains, removing solvents by evaporation, or a combination thereof. [0050] In some examples, the polymer matrix can include a heat curable polymer. Heat curable polymers can be cured by raising the temperature of the anisotropic coating composition to a curing temperature for a curing time. In some cases, the curing time can be shortened by increasing the curing temperature, or a lower curing temperature can be used for a longer curing time. The curing temperature can be from 60 °C to 300 °C, or from 60 °C to 250 °C, or from 70 °C to 200 °C, or from 80 °C to 180 °C, in some examples. The curing time can be from 30 minutes to 6 hours, or from 30 minutes to 4 hours, or from 30 minutes to 3 hours, or from 1 hour to 3 hours, in several examples. Examples of heat curing polymers can include epoxies, polyurethanes, polyacrylates, silicones, hybrid polymers, polyhedral oligomeric silsesquioxanes, phenolic resins, cyanate ester resins, and combinations thereof. [0051] The polymer matrix can be cured in a magnetic field generated by magnets in order to align the electrically conductive ferromagnetic particles. However, the magnets may be damaged by high temperatures or high temperatures may interfere with aligning the ferromagnetic particles. If a polymer matrix has a high curing temperature, it can be useful to perform pre-curing at a first temperature that is relatively low, and then remove the partially cured anisotropic coating from the magnet and cure again at a higher curing temperature. For example, the anisotropic coating can be pre-cured at a temperature from 60 °C to 100 °C, and then removed from the magnet and cured again at a temperature from 120 °C to 300 °C. [0052] In further examples, the polymer matrix can include a two-part curing polymer. This type of polymer can be made up of two or more different chemical compositions that are initially kept separate, but which cause a curing reaction when mixed together. The term “two-part curing polymer” can also encompass polymers that are made by mixing three chemical compositions, four chemical compositions, or more. The separate chemical compositions that are mixed together can include monomers, polymers in a liquid form, crosslinking agents, polymerization initiators, catalysts, or combinations thereof. Examples of two-part curing polymers can include epoxies, polyurethanes, polyacrylates, silicones, hybrid polymers, polyhedral oligomeric silsesquioxanes, phenolic resins, cyanate ester resins, and combinations thereof. [0053] If the polymer matrix includes a two-part curing polymer, then the composition may have a limited pot life after mixing the two parts together. In some examples, the electrically conductive ferromagnetic particles can be mixed into one of the parts before the two parts are combined together. In other examples, the two parts of the polymer matrix can be mixed and then the particles can be mixed in afterward. [0054] The polymer matrix can include an ultraviolet (UV) curing polymer. These polymers can be cured by the application of a particular wavelength of UV light, such as wavelengths from 200 nm to 400 nm. In further examples, the UV curing polymer can be cured with a wavelength from 250 nm to 385 nm, or from 300 to 375 nm, or from 320 to 365 nm. In some examples, the UV curing polymer can be a single-component polymer. [0055] Dual-curing polymers can include polymers that can be cured in multiple ways. For example, some polymers can be cured by UV light or by heat. Other polymers can be cured by mixing two reactive parts in a two-part curing polymer, or by heating. Some polymers may also be cured using two or more of these curing methods applied together. For example, a polymer can be cured by UV light and heat together, or by mixing a two-part system and by heating together. [0056] Non-limiting examples of polymers that can be used in the polymer matrix include UV22DC80-1, UV15DC80ND, UV15DC80LV, UV25, EP45HTAN, EP3SP5FL, EP17HT-3, and MASTERSIL® 152, from MASTERBOND® (USA); POLYTEK® 74-45, POLYTEK® 74-30, POLYTEK® 74-20, POLYTEK® 75-70, POLYTEK® 75-60, PLATSIL® 73-25, PLATSIL® 73-60, and PLATSIL® 73-20 from Polytek Development Corp. (USA); SYLGARD™ 184, SYLGARD™ 182, SYLGARD™ 186, and SYLGARD™ 170, from Dow (USA); EPON™ Resin 863 and EPON™ Resin 828 from Hexion (USA). [0057] In further examples, monomers that can be included in the polymer matrix in an uncured state can include bisphenol-A, bisphenol-F, diglycidyl ether of bisphenol-F, epichlorohydrin, methyltetrahydrophthalic anhydride, isocyanates, polyols, acrylic acid, methacrylic acid, acrylates, methacrylates, carbonates, amides, imines, lactones, unsaturated olefins, and others. [0058] The anisotropic coating compositions can also include electrically conductive ferromagnetic particles embedded in the polymer matrix. As used herein, “ferromagnetic” refers to materials that have a high susceptibility to magnetism, the strength of which depends on the strength of the applied magnetic field. Ferromagnetism is the mechanism by which iron metal is attracted to magnets. Ferromagnetic materials can include iron, nickel, cobalt, and alloys thereof. Additional ferromagnetic materials can include manganese-bismuth alloys, manganese-antimony alloys, chromium dioxide, manganese-arsenic alloys, gadolinium, terbium, dysprosium, and europium oxide. The particles used in the anisotropic coatings described herein can include a ferromagnetic material, or multiple ferromagnetic materials. In some examples, the ferromagnetic material can be electrically conductive. In other examples, the particles can include a ferromagnetic material and a conductive material. As used herein, “electrically conductive” refers to materials that conduct electrical current. These materials can have a resistivity of 1 Ω·cm or less in some examples. In further examples, the electrically conductive material can have a resistivity of 1 x 10^-8 Ω·cm to 1 Ω·cm, or from 1 x 10^-8Ω·cm to 0.1 Ω·cm, or from 1 x 10^-8Ω·cm to 0.001 Ω·cm. Some specific examples of materials that can be included in the electrically conductive ferromagnetic particles include: magnetite, silver, nickel, graphite, iron, gold, and combinations thereof. [0059] In certain examples, the electrically conductive ferromagnetic particles can include a composite of multiple materials. This means that individual particles include more than one material in the individual particles, not merely that multiple types of particles are included. In some examples, the composite of multiple materials can be in the form of a core of a first material and a shell of a second material. FIG.7 shows example electrically conductive ferromagnetic particles 110 of this type. These particles include a core 114 made of a core material and a shell 116 made of a shell material. In certain examples, the shell material can be more electrically conductive than the core material. In particular, the shell material can have a lower resistivity than the core material. In further examples, the core material can be ferromagnetic while the shell material can be non-ferromagnetic. The reverse can also be true. In other examples the shell material can be ferromagnetic while the core material can be non-ferromagnetic. In such core-shell particles, the core and shell can be formed by any suitable process, such as plating a core of one material with a shell material through electroplating, electroless plating, physical vapor deposition, chemical vapor deposition, or others. In other examples, the core-shell particles can be aggregates of a core particle with smaller particles of shell material adhered, sintered, or otherwise attached to the core particle. Composite particles can also have other arrangements of the two materials, such as having multiple cores of one material encapsulated in a shell of another material, or having multiple layers of different materials such as alternating layers of two different materials, or being formed from a collection of smaller particles of two materials mixed together. Some specific examples of composite particles that can be used in the anisotropic coating compositions described herein include silver-coated nickel powder, silver-coated iron powder, nickel-coated graphite powder, gold-coated nickel powder, gold-coated iron power, and others. [0060] The electrically conductive ferromagnetic particles can have a suitable particle size to form aligned conductive paths through the anisotropic coating layer. As such, in some examples the particle size can be less than half of the thickness of the anisotropic coating layer, or less than one tenth, or less than one twentieth. In certain examples, the particle size can be from 1/1000^th of the thickness to 1/10^th of the thickness, or from 1/1000^th of the thickness to 1/50^th of the thickness, or from 1/1000^th of the thickness to 1/100^th of the thickness. The number average particle size of the electrically conductive ferromagnetic particles can be from 1 μm to 50 μm, or from 1 μm to 25 μm, or from 1 μm to 15 μm, or from 1 μm to 10 μm, or from 5 μm to 50 μm, or from 5 μm to 25 μm. [0061] The electrically conductive ferromagnetic particles can have a spherical or nearly spherical shape in some examples, while in other examples the particles can have a high aspect ratio, such as an aspect ratio greater than 1.1. In certain examples, the particles can be shaped as flakes, platelets, rods, fibers, crystals, or other shapes. As mentioned above, the average particle size of a non-spherical particle can be the number average of the volume equivalent sphere diameter as measured using a particle analyzer such as the MASTERSIZER™ 3000 available from Malvern Panalytical (United Kingdom). In particular examples, the electrically conductive ferromagnetic particles can include nickel flakes, iron flakes, or cobalt flakes. [0062] The concentration of electrically conductive ferromagnetic particles can be sufficient to form conductive paths through the anisotropic coating layer when the particles are aligned by a magnetic field. In some examples, the concentration of particles can be sufficient to form conductive pathways that occupy from 25% to 50% of the surface area of the top, bottom, or a horizontal slice of the anisotropic layer (referring to a layer in which the conductive paths are oriented from bottom to top). The concentration of the electrically conductive ferromagnetic particles in the anisotropic coating composition can be from 5 wt% to 30 wt% with respect to the total weight of the final cured anisotropic coating layer (i.e., not including any solvents that may be in the coating composition but which are not present in the final cured anisotropic coating layer). In further examples, the electrically conductive ferromagnetic particles can be present at a concentration from 5 wt% to 20 wt%, or from 5 wt% to 15 wt%, or from 5 wt% to 10 wt%, or from 10 wt% to 20 wt%, or from 15 wt% to 20 wt%, or from 15 wt% to 30 wt% with respect to the total weight of the anisotropic coating layer. [0063] The electrically conductive ferromagnetic particles can be mixed with the polymer matrix using a suitable mixing or dispersing process. In some examples, the particles can be mixed with the polymer matrix in a liquid form using a high-shear mixer, a three-roll mill, a dual asymmetric centrifugal mixer, or a combination thereof. In specific examples, the particles can be mixed with the polymer in a high shear mixer at a speed of 6,000 rpm to 60,000 rpm, or from 10,000 rpm to 40,000 rpm, or from 20,000 rpm to 30,000 rpm. The mixing can be performed for a mixing time from 5 minutes to 30 minutes, or from 10 minutes to 30 minutes, or from 15 minutes to 30 minutes, or from 10 minutes to 20 minutes. If the polymer matrix includes a two-part cure polymer, the electrically conductive ferromagnetic particles can be mixed into one part of the two-part system before the other part of the polymer matrix is added, or the two parts of the polymer matrix can be mixed first and then the particles can be mixed in afterward. After the particles have been mixed into the polymer matrix, and the parts of a the polymer matrix have also be mixed together in the case of a two-part polymer, then the coating composition can be coated onto a dielectric layer or another surface, then placed in a magnetic field, and the polymer matrix can be cured. [0064] Dispersants can be used to help disperse the electrically conductive ferromagnetic particles in the polymer matrix. Some types of particles may be capable of being dispersed well without an additional dispersant, while other types of particles can be dispersed better when a dispersant is used. The dispersant can be a compound that is separate from the electrically conductive ferromagnetic particles, but which can associate with the particles by covalent bonding, by adsorption, or by another mechanism. FIG.8 shows another example of electrically conductive ferromagnetic particles 110 that are dispersed using a dispersant 118. The dispersant is in the form of molecules that associate with the surface of the particles. The dispersant can help to prevent the particles from coming into direct contact one with another, which can prevent clumping of the particles. The dispersant can also include organic groups that mix well with the polymer matrix, and/or polymerizable groups that can bond to the polymer matrix when the polymer matrix is cured. [0065] Non-limiting examples of dispersants for use in the anisotropic coating compositions include trisilanol isooctyl polyhedral oligomeric silsesquioxane, trisilanol phenyl polyhedral oligomeric silsesquioxane, silane coupling agents, titanate coupling agents, zirconate coupling agents, aluminate coupling agents, and combinations thereof. In some examples, the dispersant can be included at a concentration from 0.01 wt% to 5 wt%, or from 0.01 wt% to 3 wt%, or from 0.1 wt% to 3 wt%, or from 0.1 wt% to 2 wt%, or from 0.1 wt% to 1 wt%. Some types of electrically conductive ferromagnetic particles can be coated with a dispersant before the particles are mixed with the polymer matrix. For example, particles can be coated with trisilanol isooctyl polyhedral oligomeric silsesquioxane or trisilanol phenyl polyhedral oligomeric silsesquioxane prior to mixing the particles with the polymer matrix. The particles can be mixed with the dispersant and a solvent such as isopropyl alcohol and then mixed in a high shear mixer to coat the particles with the dispersant. The solvent can then be removed by decanting, evaporation, or another method. This can yield particles coated with the dispersant, which can then be mixed in the polymer matrix. In certain examples, magnetite particles can be coated with a dispersant in this way. Digital Microfluidic Devices [0066] In further examples, the anisotropic coatings described herein can be included in digital microfluidic devices. The anisotropic coating can be positioned between an electrowetting surface and a charge applicator such as an electrode array or ion head. As described above, the electrowetting surface can include a dielectric layer and in some cases additional coating materials can be added over the dielectric layer such as a hydrophobic coating. The anisotropic coating can be positioned between the charge applicator and the dielectric layer. [0067] As explained above, in some examples the anisotropic coating can be a part of a consumable that is placed on a non-consumable circuit board. The consumable and the circuit board together can be a digital microfluidic device. This type of device is shown in FIG.5, for example. In another example, a digital microfluidic device can include a consumable and an ion head, as shown in FIG. 4. In alternative examples, the anisotropic coatings can be integrated as a part of a digital microfluidic device that includes an electrode array that is permanently electrically connected to the anisotropic coating, such that the anisotropic coating is not part of a separate consumable but rather part of an integrated device that includes an electrowetting surface and the electrode array. In such examples, the entire device can be disposable or reusable depending on the application. [0068] In various examples, the anisotropic coating can be electrically connected to an electrode array. If the anisotropic coating is a part of a consumable that is attachable and detachable to the electrode array, then the electrical connection can be temporary. Alternatively, the anisotropic coating can be permanently connected to the electrode array and the device can be an integrated device as mentioned above. In some examples, the electrical connection between the anisotropic coating and the electrode array can be by direct physical contact between the anisotropic coating and the electrode array. In other examples, additional materials can be placed between the anisotropic coating and the electrode array. For example, charge-receiving electrodes can be formed on the surface of the anisotropic coating in a consumable. The charge- receiving electrodes can be formed on the surface of the anisotropic coating that is temporarily placed on a circuit board. The circuit board can include charge- supplying electrodes that physically contact the charge receiving electrodes. In some examples, the anisotropic coating can be compliant and this can help press the charge-receiving electrodes against the charge-supplying electrodes to form a good electrical contact. [0069] Digital microfluidic devices can also include a controller electrically connected or connectable to the array of electrodes or ion head that is used to apply voltage to the anisotropic coating. The controller can be configured to selectively apply charge to specific portions of the anisotropic coating layer, and these charges can create an electric field at the electrowetting surface in order to control liquid droplets as explained above. The controller can be configured to perform operations with liquid droplets, such as moving droplets, splitting droplets, merging droplets, and others. The controller can add negative or positive charge to the anisotropic layer in some examples. In further examples, the controller can neutralize charges that have already been added to the anisotropic coating. This can be accomplished by the controller by utilizing individually addressable electrodes in an electrode array, or by moving a moveable ion head to specific locations to apply electric charge to specific areas of the anisotropic coating. [0070] In some examples, the controller can be referred to as being programmed to perform the operations explained above. In certain examples, the controller can include a processor in electronic communication with a memory. The processor can execute instructions stored in the memory, and these instructions can cause the controller to perform the operations. In particular, the controller can generate and transmit electric signals to the electrode array or ion head, and any other electronic components in the digital microfluidic device, so that the digital microfluidic device manipulates liquid droplets in a desired way. In some examples, the controller can be a general purpose computing device connected to the other electronic components of the digital microfluidic device. In other examples, the controller can be a dedicated controller incorporated in a digital microfluidic device. In still further examples, a combination of a dedicated controller and a separate general purpose computing device can be used to control the digital microfluidic device. For example, a general purpose computing device can be used to provide a user interface to allow a user to input commands, while a dedicated controller integrated in the digital microfluidic device can interpret the commands and generate signals to individual electrodes in the electrode array. [0071] The memory can include random access memory (RAM), read only memory (ROM), a mass storage device, or another type of storage including a non-transitory tangible medium or non-volatile tangible medium. The memory can store machine readable instructions that can cause the controller to perform the operations described above. Definitions [0072] 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. [0073] 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. [0074] 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. [0075] 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. [0076] 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. [0077] 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 [0078] 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 – Rigid Nanosilica-filled Epoxy with 1 μm Magnetite [0079] Magnetite particles with an average particle size of 1 μm were mixed with trisilanol isooctyl polyhedral oligomeric silsesquioxane as a dispersant in isopropyl alcohol. The mixture was mixed in a high-shear mixer at 24,000 rpm for 15 minutes to coat the magnetite particles with trisilanol isooctyl polyhedral oligomeric silsesquioxane. The isopropyl alcohol was removed by decanting and evaporation. The coated magnetite particles were mixed at a concentration of 7 wt% in UV22DC80-1 nanosilica-filled epoxy resin from MASTERBOND® (USA) using a high-shear mixer. A substrate was coated with the mixture, and the substrate was placed in a magnetic field. The polymer matrix was cured by heating at 80 °C for one hour. In another example, the polymer matrix can be cured using UV light in the magnetic field, and then removed from the magnetic field and heat cured outside the magnetic field. After the substrate was removed from the magnetic field, the polymer matrix was additionally cured at 180 °C for 2 hours. Two sample anisotropic layers were formed using this process. One of the coatings was on a printed circuit board, and the other was formed in a mold and then removed as a free-standing layer. The coated circuit board was tested in an electrowetting device, and a water droplet was able to be moved by electrowetting across the circuit board. Example 2 – Elastomeric Polyurethane with 1 μm Magnetite [0080] Magnetite particles with an average particle size of 1 μm were mixed with trisilanol phenyl polyhedral oligomeric silsesquioxane as a dispersant in isopropyl alcohol. The mixture was mixed in a high-shear mixer at 24,000 rpm for 15 minutes to coat the magnetite particles with trisilanol phenyl polyhedral oligomeric silsesquioxane. The isopropyl alcohol was removed by decanting and evaporation. The coated magnetite particles were mixed at a concentration of 10 wt% in the isocyanate component of two-part POLYTEK® 74-45 elastomeric polyurethane from Polytek Development Corp. (USA). The polyol component of the polymer matrix with then added, and a dual asymmetric centrifugal mixer was used to mix the composition. A substrate was coated with the mixture, and the substrate was placed in a magnetic field. The polymer matrix was cured by heating at 80 °C for one hour. Example 3 – Elastomeric Silicon with 8 μm Silver-coated Nickel Powder [0081] Silver-coated nickel powder with an average particle size of 8 μm was mixed at a concentration of 10 wt% in SYLGARD™ 184 silicone base from Dow (USA). A curing agent was added. A dual asymmetric centrifugal mixer was used to mix the composition. Alternatively, a three-roll mill can be used to mix the composition. A circuit board was coated with the composition and placed in a magnetic field. The polymer matrix was cured by heating at 80 °C for 1 hour. The coated circuit board was tested in an electrowetting device and it was found that a water droplet could be moved by electrowetting on the surface by applying 700 V. Example 4 – Elastomeric Silicon with 1 μm Silver-coated Nickel Powder [0082] Silver-coated nickel powder with an average particle size of 1 μm was mixed at a concentration of 15 wt% in SYLGARD™ 184 silicone base from Dow (USA). A curing agent was added. A dual asymmetric centrifugal mixer was used to mix the composition. Alternatively, a three-roll mill can be used to mix the composition. A circuit board was coated with the composition and placed in a magnetic field. The polymer matrix was cured by heating at 80 °C for 1 hour. The coated circuit board was tested in an electrowetting device and it was found that a water droplet could be moved by electrowetting on the surface by applying 400 V. Example 5 – Elastomeric Silicon with 5 μm Nickel Powder [0083] Nickel powder with an average particle size of 5 μm was mixed at a concentration of 10 wt% in SYLGARD™ 184 silicone base from Dow (USA). A curing agent was added. A dual asymmetric centrifugal mixer was used to mix the composition. Alternatively, a three-roll mill can be used to mix the composition. A substrate was coated with the composition and placed in a magnetic field. The polymer matrix was cured by heating at 80 °C for 1 hour. Example 6 – Rigid Epoxy with 8 μm Nickel Powder [0084] Nickel powder with an average particle size of 8 μm was mixed at a concentration of 10 wt% in EPON™ Resin 863 epoxy from Hexion (USA). A curing agent was added. A dual asymmetric centrifugal mixer was used to mix the composition. Alternatively, a three-roll mill can be used to mix the composition. A substrate was coated with the composition and placed in a magnetic field. The polymer matrix was cured by heating at 80 °C for 1 hour. [0085] 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. A coated dielectric layer, comprising: a layer of a solid dielectric material; and an anisotropic coating layer adhered to the layer of solid dielectric material, wherein the anisotropic coating layer comprises a polymer matrix and electrically conductive ferromagnetic particles embedded in the polymer matrix, wherein the electrically conductive ferromagnetic particles are aligned in a plurality of conductive paths that are spaced apart throughout the anisotropic coating layer and that extend through a thickness of the anisotropic coating layer. 2. The coated dielectric layer of claim 1, wherein the polymer matrix comprises an epoxy, a polyurethane, a polyacrylate, a silicone, a polyhedral oligomeric silsesquioxane, a phenolic resin, a cyanate ester resin, or a combination thereof. 3. The coated dielectric layer of claim 1, wherein the electrically conductive ferromagnetic particles comprise iron, nickel, cobalt, magnetite, graphite, silver, gold, an alloy thereof, or a composite thereof. 4. The coated dielectric layer of claim 3, wherein the electrically conductive ferromagnetic particles have a number average particle size from 1 μm to 50 μm. 5. The coated dielectric layer of claim 4, wherein the electrically conductive ferromagnetic particles are included in the anisotropic coating layer in an amount from 5 wt% to 30 wt% with respect to the total weight of the anisotropic coating layer. 6. The coated dielectric layer of claim 1, wherein the anisotropic coating layer further comprises a dispersant selected from the group consisting of trisilanol isooctyl polyhedral oligomeric silsesquioxane, trisilanol phenyl polyhedral oligomeric silsesquioxane, a silane coupling agent, a titanate coupling agent, a zirconate coupling agent, an aluminate coupling agent, and combinations thereof. 7. The coated dielectric layer of claim 1, wherein the anisotropic coating has a thickness from 50 μm to 2000 μm. 8. The coated dielectric layer of claim 1, wherein the anisotropic coating layer has a resistance-times-area less than 7.0 x 10⁵Ω·cm² in a thickness direction along which the conductive paths are aligned, and a resistivity greater than 1.0 x 10¹²Ω·cm in the plane along which the anisotropic coating layer extends. 9. The coated dielectric layer of claim 1, wherein the coated dielectric layer is part of an electrowetting device, wherein the electrowetting device further comprises a ground plate facing a surface of the layer of solid dielectric material opposite from the anisotropic coating layer, wherein the ground plate is spaced apart from the layer of solid dielectric material to define a liquid droplet passageway between the ground plate and the layer of solid dielectric material. 10. An anisotropic coating composition, comprising: a polymer matrix; and electrically conductive ferromagnetic particles embedded in the polymer matrix, wherein the electrically conductive ferromagnetic particles comprise a composite of multiple materials in individual particles. 11. The anisotropic coating composition of claim 10, wherein the multiple materials in the individual particles include iron, nickel, cobalt, magnetite, graphite, silver, gold, an alloy thereof, or a combination thereof. 12. The anisotropic coating composition of claim 10, wherein the individual particles comprise a core of a first material and a shell of a second material, wherein the first material comprises iron, nickel, graphite, an alloy thereof, or a combination thereof, and wherein the second material comprises nickel, silver, gold, an alloy thereof, or a combination thereof. 13. An anisotropic coating composition, comprising: a polymer matrix; electrically conductive ferromagnetic particles embedded in the polymer matrix; and a dispersant dispersing the electrically conductive ferromagnetic particles in the polymer matrix. 14. The anisotropic coating composition of claim 13, wherein the dispersant is selected from the group consisting of trisilanol isooctyl polyhedral oligomeric silsesquioxane, trisilanol phenyl polyhedral oligomeric silsesquioxane, a silane coupling agent, a titanate coupling agent, a zirconate coupling agent, an aluminate coupling agent, and combinations thereof. 15. The anisotropic coating composition of claim 13, wherein the electrically conductive ferromagnetic particles are aligned in a plurality of conductive paths that are spaced apart throughout the polymer matrix.