Dielectrowetting Driven Spreading of Droplets

Abstract

The wetting of solid surfaces can be modified by altering the surface free energy balance between the solid, liquid and vapour phases. Here we show that liquid dielectrophoresis (L-DEP) induced by non-uniform electric fields can be used to enhance and control the wetting of dielectric liquids. In the limit of thick droplets, we show theoretically that the cosine of the contact angle follows a simple voltage squared relationship analogous to that found for electrowetting-on-dielectric (EWOD). Experimental observations confirm this predicted dielectrowetting behavior and show that the induced wetting is reversible. Our findings provide a non-contact electrical actuation process for meniscus and droplet control.

Voltage-induced spreading and superspreading of liquids

“…The experimental configuration uses interdigitated electrodes of linewidth 40 mm and gaps of 40 mm on glass and with a thin 0.85 mm capping layer of SU-8 photoresist with a hydrophobic treatment (see ref. 16 and inset in Fig. 1).…”

“…In both electrowetting and dielectrowetting the change of surface free energy, DF, due to a small change in area, DA, at the contact line of a liquid with a contact angle y, in the presence of a voltage, V, can be written as 7,16 ,…”

“…In the case of dielectrowetting, the ratio of interfacial liquid dielectric energy per unit area to the liquid-vapor interfacial energy per unit area predicts that a ¼ e 0 (e l À 1)/2dg LV , which depends on the permittivity difference e 0 (e l À 1) between the dielectric liquid and air and the extent of the electric potential penetration depth, d (ref. 16). …”

“…Recently, we showed that liquid dielectrophoresis using nonuniform electric fields, non-conducting liquids and without any need for direct electrical contact could be used to enhance the spreading of a droplet; so-called dielectrowetting 16 . In both electrowetting and dielectrowetting the change of surface free energy, DF, due to a small change in area, DA, at the contact line of a liquid with a contact angle y, in the presence of a voltage, V, can be written as 7,16 ,…”

Developing interface localized liquid dielectrophoresis for optical applications

“…The third regime also focuses on films, but corresponds to a near-field regime where the shape of the liquid-vapor interface reflects detailed variations in the electric field close to its source. We review our recent results showing that the droplet regime has a contact angle that obeys an equation similar to the electrowetting equation, but with the strength of the effect controlled by the relative permittivity of the liquid and the penetration depth of the electric field into the liquid 18 ; this provides an understanding of possible parameters in the design of voltage controllable lenses driven by liquid dielectrophoresis. We also describe an application of these ideas to the film regime to create a voltage programmable amplitude phase grating 19 .…”