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

TL;DR

This paper uses Poisson-Boltzmann theory to explain contact angle saturation in electrowetting, attributing it to ion injection into the surrounding media which alters interfacial stresses and causes the apparent contact angle to plateau.

Contribution

It introduces a theoretical model linking ion injection to contact angle saturation, providing a new explanation consistent with experimental observations.

Findings

Ion injection reduces Maxwell stress at the interface.

The apparent contact angle exhibits a broad minimum with increasing voltage.

The model aligns with experimental data on contact angle behavior.

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.