Electro-wetting on Dielectrics

When a conducting liquid drop is placed on an electrode having a thin dielectric coating and an electric potential is then applied between the liquid and the electrode, the drop flattens and spreads over the electrode surface. This phenomenon is often referred to as electro-wetting. Because the phenomenon is associated with the development of an electrical charge layer, an external electric field may be used to manipulate the drops causing them to move, coalesce or break apart.

Click to view the full animation.
The animation starts with the droplet steady and
holding its shape under the influence of surface
tension force alone (i.e., numerical oscillations are
minimum). Then, an electric voltage is applied
suddenly (notice the large change to the color scale),
causing the droplet to begin to flatten.

A demonstration of electro-wetting computed by FLOW-3D is shown here for a hemispherical drop of water of diameter 500 μm initially placed on a plate coated with a dielectric material of thickness 1200 A° and having a dielectric constant of 4.5. The static contact angle of the water drop on the coating is 120° and it has an electrical conductivity of 2.5e-5 S/m. A needle electrode is inserted into the top of the drop that applies a voltage drop of 20 volts across the drop. Instead of balling up on the plate because of the non-wetting contact angle, the drop spreads out to a steady configuration having an apparent contact angle of 87°. Comparisons of computational results have been made with experimental data taken from S.K. Cho, H. Moon and C.-J. Kim, J. Microelectromechanical Sys. V.12, No. 1, pp. 70-80 (2003).

3D results of a liquid drop before and after electric potential is applied.

The comparisons, shown in the plot below, indicate good agreement except at the highest voltages where experiments indicate some kind of saturation effect that limits the reduction in the apparent contact angle. No consensus exists on the cause of saturation, but it appears that it may be related to instability of the liquid surface at the contact line at high voltages. Our computations clearly indicate that the major electrical force causing the contact line to move outward is the dielectricphoresis force generated at the contact line. This force is proportional to the gradient of the square of the electric field, which is enhanced by the charge layer at the liquid-coating boundary.

Read more in our Microfluidics Tech Papers