How Spontaneous Electrowetting and Surface Charge affect Drop Motion

TL;DR

This paper investigates how spontaneous charge separation during water drop sliding on hydrophobic surfaces influences contact angles through electrowetting and surface charge effects, revealing compensatory mechanisms that keep the receding contact angle unchanged.

Contribution

It uncovers the interplay between spontaneous electrowetting and surface charge effects on sliding drops, a phenomenon not previously characterized.

Findings

Drop charge induces spontaneous electrowetting, reducing contact angles.

Deposited charges create a surface charge effect, also decreasing contact angles.

These effects compensate at the receding contact line, leaving the receding contact angle unchanged.

Abstract

Water drops sliding on hydrophobic surfaces spontaneously separate charges at their rear. It is unclear how this charge separation affects the contact angles of a sliding drop. We slide grounded and insulated drops on hydrophobic surfaces at low capillary numbers (\leq 10^{-4}). We find that drop charge leads to spontaneous electrowetting, which decreases the contact angles. Additionally, the deposited charges lead to a surface charge effect and decrease the contact angle. Both phenomena compensate each other at the receding contact line, resulting in an insignificant change in the receding contact angle of a sliding drop.