Liquid slide electrification: advances and open questions

This review is about drops of a liquid with high dielectric permittivity that slide over a solid surface with high electrical resistivity. A typical situation is a water drop sliding down a tilted hydrophobic plate. It has been realized recently that such drops spontaneously acquire a charge. The opposite charge is deposited behind the drop as a surface charge. Generated electric potentials in the drops can easily reach 1 kV and more. This phenomenon has been termed slide or contact electrification. It is the soft matter analog to triboelectrification, which occurs in solid friction. Slide electrification turned out to be ubiquitous in everyday life and technical applications. It will change our common knowledge of dynamic wetting. Studying slide electrification is complex because the outcome of a wetting experiment depends on the history. For this reason, a series of drops, rather than single drops, are analyzed to gain quantitative understanding. Here, we review the fundamental understanding of slide electrification and its limits. We describe consequences, e.g. on drop motion and advancing and receding contact angles and we address open questions.