Capillary spreading dynamics of electrowetted sessile droplets in air.

We report the contact line dynamics of sessile water droplets, 1.1-1.6 mm in radius, spread by electrowetting in air. Coplanar electrodes patterned on the substrate allow a true sessile condition with no wire into the droplet. The frequency response of the droplets is studied using 25 VAC ranging from 10 to 205 Hz. The effect of contact angle hysteresis is seen in form of stick-slip motion. A model developed provides a good match to the experimental result. Step response is studied with voltages in the range of 20-80 VDC. Two regimes of motion are observed. In the first regime, local flows cause the contact line speed to increase and reach a maximum while the contact angle is still changing. Global flows in the second regime cause the contact line to move with a reduced speed and attain the spherical shape pertaining to the new equilibrium contact angle. A model is used to describe the motion.