Drop friction

Wetting phenomena have been studied quantitatively for more than 200 years, but there remain many fundamental questions that are not understood. For example, the speed of a water drop sliding down an inclined plane cannot be predicted. A drop that slides down a surface experiences a resistance. We call this resistance drop friction. It is still debated how and where energy is dissipated in a sliding drop. Particularly for the most common liquid, water, there have been considerable advances in the understanding of wetting, driven by the development of new physical, preparative and theoretical methods. Water is a special liquid, owing to its polar nature, its tendency to form hydrogen bonds, the self-ionization into OH− and H3O+, its low viscosity and its high surface tension. In recent years, water–surface interactions due to adaptation, spontaneous electrostatic charging and deformation on elastomers have been identified as important processes that increase drop friction. They may be responsible for drop friction even on seemingly smooth, homogeneous and rigid surfaces. The dynamic wetting of sliding drops, particularly of water, remains poorly understood. New experimental techniques have shown that, in addition to viscous dissipation, other energy dissipation mechanisms such as adaptation, electrostatic charging and deformation can contribute significantly and affect the motion of the drops.