Dielectrophoresis-Driven Spreading of Immersed Liquid Droplets

In recent years electrowetting-on-dielectric (EWOD) has become an effective tool to control partial wetting. EWOD uses the liquid−solid interface as part of a capacitive structure that allows capacitive and interfacial energies to adjust by changes in wetting when the liquid−solid interface is charged due to an applied voltage. An important aspect of EWOD has been its applications in micro fluidics in chemistry and biology and in optical devices and displays in physics and engineering. Many of these rely on the use of a liquid droplet immersed in a second liquid due to the need either for neutral buoyancy to overcome gravity and shield against impact shocks or to encapsulate the droplet for other reasons, such as in microfluidic-based DNA analyses. Recently, it has been shown that nonwetting oleophobic surfaces can be forcibly wetted by nonconducting oils using nonuniform electric fields and an interface-localized form of liquid dielectrophoresis (dielectrowetting). Here we show that this effect can be used to create films of oil immersed in a second immiscible fluid of lower permittivity. We predict that the square of the thickness of the film should obey a simple law dependent on the square of the applied voltage and with strength dependent on the ratio of difference in permittivity to the liquid-fluid interfacial tension, Δε/γLF. This relationship is experimentally confirmed for 11 liquid−air and liquid−liquid combinations with Δε/γLF having a span of more than two orders of magnitude. We therefore provide fundamental understanding of dielectrowetting for liquid-in-liquid systems and also open up a new method to determine liquid−liquid interfacial tensions

Evaluation of repeated electrowetting on three different fluoropolymer top coatings

Degradation of the electrowetting effect by a repeated actuation is evaluated over an extended period (200 min) on electrowetting-on-dielectric samples for three popular fluoropolymer top coatings: Teflon, FluoroPel and Cytop. A conductive liquid droplet is tested in an air environment at electrowetting number Ew ≅ 0.34. A pulse train (6 s period and 50% duty cycle) of three different voltage types is used for the actuation: positive dc, negative dc and 1 kHz ac. For the dc actuations, electrowetting degrades gradually on Cytop but significantly faster on Teflon and FluoroPel under the tested conditions. For the ac actuation, electrowetting degrades gradually on all three materials in a similar fashion. © 2013 IOP Publishing Ltd

Two-liquid wetting properties as a surface polarity probe for hydrophobic coatings

International audienceA model for the spreading of a non-polar liquid on a surface within a polar medium is described theoretically, according to Fowkes, Good and Girifalco approximations on interfacial tension. We demonstrate both theoretically and experimentally that surface polarity measurements using the contact angle of two immiscible liquids minimize drastically the measurement error. The present method has been successfully applied to various substrates of variable polarity and overall surface energy. We also demonstrate that this method allows a direct measurement of surface sensitivity to pH

Electrowetting Force and Velocity Dependence on Fluid Surface Energy

Electrowetting on dielectric is a phenomenon in which the shape and apparent contact angle of a droplet changes when an electric field is applied across the droplet interface. If the field is asymmetric with respect to the droplet, then a net force can be applied to the droplet. In this work, we have measured the electrowetting force by confining the droplet shape beneath a glass plate and measuring the force on the plate. The force was measured as a function of voltage for a range of fluids with different surface energy. Measured forces show excellent agreement with predictions based on the Young–Lippmann equation with measured contact angles. Results also show that the electrowetting force is independent of fluid surface energy below saturation but that the peak force is proportional to the surface tension. This work shows that lowering the surface energy of the fluid can induce larger contact angle change under the same voltage, but it has no beneficial impact on the actuation force in droplet-based actuators. In contrast, velocity tests with deformable droplets show higher speeds for lower surface energy fluids, even above their saturation voltage. However, when the droplet’s shape is restrained, the highest velocity is achieved with high surface energy fluids due to the larger electrowetting actuation forces applied