Mechanisms in Slide Electrification of Liquid and Frozen Drops on Hydrophobic Surfaces

TL;DR

This study investigates the fundamental mechanisms behind slide electrification of liquids on hydrophobic surfaces, revealing that both ion transfer and electron transfer contribute depending on conditions.

Contribution

It demonstrates that slide electrification involves at least two mechanisms, with the dominant pathway shifting between ion and electron transfer based on phase and temperature.

Findings

Polar liquids accumulate significant charge even when frozen.

Non-polar liquids exhibit lower charging, similar in liquid and solid phases.

Charge transfer mechanisms depend on electronegativity, phase, and temperature.

Abstract

The microscopic and fundamental origin of slide electrification, where droplets of water move across insulating surfaces accumulating and depositing electrical charges, is still debated. Charge transfer is often attributed to ion transfer at the receding contact line. However, it is still unclear whether ion transfer alone can fully account for the observed charge separation. We examined slide electrification of two polar, self-ionizing liquids (water, formamide) and two non-polar liquids (diiodomethane, bromonaphthalene). By cooling below the melting temperature, we were able to compare this process to tribocharging of the respective frozen components. Despite reduced ion mobility at sub-freezing temperatures, the ice of the polar liquids continues to accumulate significant charge. Non-polar liquids exhibit lower charging (<25% of polar liquids) and nearly identical charging behaviour in both their liquid and frozen phases on five different substrates. Since non-polar liquids contain few free ions, these observations indicate an alternative charging mechanism, which could be electron transfer. Our findings suggest that slide electrification operates through at least two mechanisms, with the dominant charge transfer pathway shifting between ions and electron transfer depending on the electronegativity, phase, and temperature.