Kinetics of Triple Line Motion during Evaporation

When a sessile drop evaporates in an unsaturated environment, it may change its geometry during mass loss in a variety of ways, depending largely on the surface state of the solid in contact. Under some circumstances, "pinning" of the wetting triple line (TL) to the solid surface may occur, leading to decrease of contact angle. Subsequent "de-pinning" leads to relatively rapid TL recession and accompanying contact angle increase, only to be followed by pinning again. Thus a "stick-slip" cycle is set up. Here we consider experimental results of ethanol drops on Teflon (R), and both apply and develop further the ideas presented some years ago in a simple theoretical study of the possible mechanisms involved in stick-slip behaviour.