Contact angle hysteresis and oil film lubrication in electrowetting with two immiscible liquids

TL;DR

This paper investigates how contact angle hysteresis in electrowetting with two immiscible liquids behaves under different voltages, revealing that hysteresis increases with voltage due to oil layer thinning and surface roughness exposure.

Contribution

It demonstrates that, contrary to behavior in air, CAH in oil increases with voltage, explained by oil layer dynamics and electrostatic effects.

Findings

CAH increases with AC and DC voltage in oil

Oil layer thinning exposes surface roughness, increasing hysteresis

Electrostatic stresses influence oil film stability

Abstract

Electrowetting (EW) of water drops in ambient oil has found a wide range of applications including lab-on-a-chip devices, display screens, and variable focus lenses. The efficacy of all these applications is dependent on the contact angle hysteresis (CAH), which is generally reduced in the presence of ambient oil due to thin lubrication layers. While it is well-known that AC voltage reduces the effective contact angle hysteresis (CAH) for EW in ambient air, we demonstrate here that CAH for EW in ambient oil increases with increasing AC and DC voltage. Taking into account the disjoining pressure of the fluoropolymer-oil-water system, short range chemical interactions, viscous oil entrainment and electrostatic stresses, we find that this observation can be explained by progressive thinning of the oil layer underneath the drop with increasing voltage. This exposes the droplet to the roughness of the underlying solid and thereby increases hysteresis.