Electrification mechanisms in sliding liquid and frozen drops

Abstract The microscopic and fundamental origin of slide electrification, which happens when water drops move across insulating surfaces while accumulating and depositing electrical charges, is still debated. Charge transfer is often attributed to ion transfer at the receding contact line. However, it remains an open question whether ion transfer alone can fully account for the observed charge separation. Here we examined slide electrification of polar, self-ionizing liquids and non-polar liquids. By cooling them 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 frozen polar compounds continued to accumulate substantial charge. Non-polar liquids exhibited lower charging and nearly identical charging behaviour in both their liquid and frozen phases on different substrates. As 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 electronegativity, phase and temperature.