Contact angle saturation in electrowetting: Injection of ions into the surrounding media

¹ Center of Soft Matter Physics and its Applications, Beihang University - Xueyuan Road 37, Beijing 100191, China
² National Composite Center, Nagoya University - Furocho, Chikusa-ku, Nagoya, 464-8603, Japan
³ School of Physics and Astronomy, Tel Aviv University - Ramat Aviv 69978, Tel Aviv, Israel

^(a) andelman@post.tau.ac.il (corresponding author)

Received: 2 October 2015
Accepted: 23 November 2015

Abstract

We use the Poisson-Boltzmann theory to predict contact angle saturation of aqueous droplets in electrowetting. Our theory predicts that injection of ions from the droplet into its surrounding medium is responsible for the deviation of the apparent contact angle from the Young-Lippmann equation for large applied voltages. The ion injection substantially decreases the Maxwell stress and increases the osmotic pressure at the interface between the two media, leading to saturation of the apparent contact angle. Moreover, we find that the contact angle does not saturate, but only has a broad minimum that increases again upon further increase of the applied voltage, in agreement with experiments.