Resistance of droplets sliding by EWOD actuation

We report an experimental investigation of the resistive forces imparted on droplets actuated by electrowetting-on-dielectric (EWOD). While the advancing contact angle is always larger than the receding of a droplet moving on a solid surface under conventional actuation means, EWOD actuation is unique because the advancing angle is smaller than the receding. We recorded high-speed videos of sliding droplets under electrowetting to elucidate this seemingly paradoxical contact angle hysteresis. The results indicate a transition point at Caap10^-3, after which the dominant resistance force becomes strongly speed-and voltage-dependent.