Abstract
The wetting of solid surfaces can be modified by altering the surface free energy balance between the solid, liquid, and vapor phases. Liquid dielectrophoresis (L-DEP) can produce wetting on normally nonwetting surfaces, without modification of the surface topography or chemistry. L-DEP is a bulk force acting on the dipoles of a dielectric liquid and is not normally considered to be a localized effect acting at the interface between the liquid and a solid or other fluid. However, if this force is induced by a nonuniform electric field across a solid-liquid interface, it can be used to enhance and control the wetting of a dielectric liquid. Recently, it was reported theoretically and experimentally that this approach can cause a droplet of oil to spread along parallel interdigitated electrodes thus forming a stripe of liquid. Here we show that by using spiral-shaped electrodes actuated with four 90° successive phase-shifted signals, a near axisymmetric spreading of droplets can be achieved. Experimental observations show that the induced wetting can achieve film formation, an effect not possible with electrowetting. We show that the spreading is reversible thus enabling a wide range of partial wetting droplet states to be achieved in a controllable manner. Furthermore, we find that the cosine of the contact angle has a quadratic dependence on applied voltage during spreading and deduce a scaling law for the dependence of the strength of the effect on the electrode size.
Highlights
The wetting of solid surfaces is of interest to a wide range of disciplines and has many applications, such as creating thin films, surfaces coating, adhesion, droplet deposition, and droplet control
The initial movement looks like a slight adjustment of the side view profile of the droplet with the contact angle falling from 102° to 90°
Our experiments show that it is possible to spread a droplet of an isotropic dielectric liquid in a near axisymmetric manner by using spiral electrodes and four-phase electric signals with successive 90° phase shifts
Summary
The wetting of solid surfaces is of interest to a wide range of disciplines and has many applications, such as creating thin films, surfaces coating, adhesion, droplet deposition, and droplet control. Achieving axisymmetric droplet shapes is essential to convert the idea of L-DEP-induced partial wetting from an interesting insight into greater practical relevance, for example in terms of liquid lenses.[14] The difficulties in doing so arise from the fact that an AC voltage on parallel IDEs30 create a nonuniform periodic field with maximum values located at the electrode edges.[35] The resulting periodic energy barriers due to the electrodes prevent the droplet spreading axisymmetrically and favor spreading along the electrodes rather than across them.
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