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Modeling 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.

Lab-On-Chip Electro-wetting Applications

An electrowetting based Lab-on-chip that can manipulate discrete droplets allows designers to perform complex procedures similar to traditional lab apparatus but with much smaller volumes. These devices are required to efficiently transport, merge and split droplets. FLOW-3D can be a useful tool in the design process by allowing the user to simulate the effects of geometric parameters and voltages used to operate these devices.

The animations below demonstrate FLOW-3D's capability to simulate transport, merge and split droplets. The Lab-on-a-chip consists of two parallel plates separated by about 300μm. The bottom plate has electrodes inserted in it that are used for manipulating the droplets. The droplets are water (slightly conductive) surrounded by silicone oil. The volume of the droplet is about 800nl.

Splitting of a Droplet

Merging of Droplets


This lab-on-a-chip electrowetting simulation
demonstrates an electric field being applied
in order to split a small droplet.


This lab-on-a-chip electrowetting simulation depicts an electric field being applied in order to merge two small droplets.

Droplet Transport

 


This lab-on-a-chip electrowetting simulation depicts an electric field being applied to a small droplet to control its motion.

Simulating an Electro-Wetting Example


An electric voltage is applied suddenly
to a steady droplet, 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).

In the animation on the left, the droplet starts steady and holds 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.

 

FLOW-3D flow simulation results of electro-wetting
FLOW-3D results of a liquid drop before and after electric potential is applied.

FLOW-3D Microfluidics Validation

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.

Graph of contact angle vs. voltage curves