A maximum speed of wetting

The wetting (or dewetting) of a solid by a liquid is an integral part of many important processes such as coating, petroleum recovery, distillation and the handling of liquid fuels in low gravity conditions. Several experiments1–4 have shown that wetting lines (where liquid, air and solid phases meet) which are straight at slow rates of movement over the solid have a sawtooth shape at sufficiently high speeds. We now offer a quantitative explanation for this phenomenon based on the postulate that, for a given system, there is a maximum rate at which wetting can proceed. The consequences of this interpretation are likely to be important, since, in many practical situations, the aim is to maximise the speed of wetting without entraining the displaced phase.