Electrowetting Valves - Davis Miller
Currently, one of the largest issues with microfluidics is that devices are complex and require much outside equipment, such as pumps to drive flow. Ideally, these devices would be portable and usable in locations with limited resources, acting as a point-of-care-device. One alternative method to flow in a microfluidic device that is being investigated is capillary flow. This removes the need for outside assistance in fluid flow, as capillary action is intrinsic to the system. By modifying the surface of the channels to be hydrophobic or hydrophilic, this flow may be controlled. Furthermore, hydrophobic valves may be installed to stop flow at desired locations, and then by applying a voltage, these valves are "opened" due to the change of the surface to being hydrophilic. This is what's known as an electrowetting valve.
The phenomenon of electrowetting has been around for quite some time. The origins of modern electrowetting theory has its roots in the work of Gabriel Lippmann in 1875. He found that the capillary depression of mercury in contact with an electrolyte solution could be changed with an applied voltage. Additionally, it has been shown that the contact angle of water on a surface can be changed due to a charge.
In microfluidics, an electrowetting valve is a novel approach to controlling fluid flow. A basic schematic may be found in Figure 1. It takes advantage of the effects of an electric field on the wetting properties of a surface. Typically, the surface is hydrophobic, preventing flow at this barrier due to surface tension of the fluid. By applying an electric field to the surface, the chemistry can be modified, becoming hydrophilic and allowing flow. These surfaces most commonly consist of a silver electrode that has been modified with a monolayer of polytetrafluoroethylene (PTFE). PTFE is hydrophobic, but when applying an electric field, it becomes polarized and therefore hydrophilic. Capillary flow then resumes in the channel. The surface that these electrodes are printed on can vary, and is discussed in the fabrication section. It should be noted, though, that the surface changes electrode interactions in addition to fluid flow as different interactions are present on different surfaces. This in turn affects the wettability and voltages required.
The following is a video demonstrating an electrowetting valve controlling fluid flow.
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The electrowetting phenomenon is described as when the contact angle between an electrolyte and solid decreases as a result of an applied electric field (see Figure 2) This is because the surface tension is altered in the presence of this electric potential. This may be demonstrated by examining the forces acting on the droplet. Forces present from the applied electric field act to "pull" the corners of the droplet down onto the electrode, reducing contact angle.
Alternatively, as has been described by Kang et al., this phenomenon may be viewed from a thermodynamic perspective. Considering the minimum free energy requirement for thermodynamic equilibrium, the electrowetting equation becomes:
whereis the contact angle without the electric potential, is the contact angle with a potential of applied, is the electric permittivity of the dielectric layer, is the interfacial tension between the droplet and solid, and is the dielectric layer thickness. This equation is what's known as the Lippmann-Young equation. From this equation it is clear that voltage required to change contact angle depends heavily on the dielectric constant of the surface (the electric permittivity) in addition to the thickness of the layer. This typically results in high voltage requirements to sufficiently change contact angle and drive flow.
Essentially, the addition of a voltage at the interface between a liquid and dielectric acts to reduce the interfacial tension between the two, resulting in a reduced contact angle (increased wettability). This happens due to the redistribution of charges and dipoles, changing the surface energy at the interface.
Fabrication and Current Performance
Typically, microfluidic devices utilizing electrowetting valves are fabricated in a "sandwich" design with two parallel substrates so as to initialize electrical contact with the liquid. One of the most common substrates is polyethylene teraphthalate (PET) . When flow is governed by capillary forces, it is necessary for the surface to be hydrophilic. Therefore, PET is treated with UV/ozone to induce hydrophilicity, and this modification has been shown to be long-lasting.
Electrodes are commonly made of silver, and fabricated from inkjet printers using inkjettable conductive silver ink . Surface modification of the silver electrode to create a hydrophobic monolayer is typically achieved using 1H,1H,2H,2H-Perfluorodecanethiol (PFDT) or PTFE. PFDT in deoxygenated absolute ethanol solution is applied, and then washed further with ethanol.
Additionally, the feasibility of these valves in paper-fluidics has been demonstrated. Electrodes were printed on a nitrocellulose surface and the same modifications made as on the PET surface. A later-flow device was ultimately fabricated using paper-fluidics and electrowetting valves, and the detection of nucleic acids was achieved. This can be seen in Figure 3.
The feasibility of electrowetting valves to control flow in PDMS devices has also been shown. However, hydrophilic surface modification is temporary, so this material is less ideal.
When applying this phenomenon to a valve in a microfluidic device, it has been shown that very low voltages (2-4 V) have resulted in contact angle changes of up to 84 degrees.
The major challenge currently associated with electrowetting valves is the fact that these devices are one-time actuation devices, meaning that once the valve is opened, it cannot be closed again. This is a result of what's known as the breakdown voltage, meaning the amount of voltage applied that results in a permanent change to the material, in this case hydrophilicity. This voltage is usually required in order to induce a significant contact angle change to drive flow. The breakdown voltage does change with dielectric thickness, so it is theoretically possible to modify the materials so as to have a reversible wettability. However, it is not clear whether this has been or can be achieved.
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2.  Kang, Kwan Hyoung. "How electrostatic fields change contact angle in electrowetting." Langmuir 18, no. 26 (2002): 10318-10322.
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5.  By Juan Ignacio Polanco - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=29626856
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