Hydrodynamics and contact angle relaxation during unsteady spreading

We have studied the unsteady spreading of a liquid on a solid surface by ob serving the liquid-air meniscus shape and analyzing it with the hydrodynami c theory for Stokes' flow. The unsteady spreading process exhibits both qua si-steady and unsteady regimes. The unsteadiness in our moderately high vis cosity system arises from temporal relaxation of the contact angle rather t han inertial effects or momentum diffusion. The quasi-steady regime can be accurately described by a model that ascribes a steady-state flow near the contact line. "Spontaneous" spreading, i.e., spreading to final state of ze ro contact line speed, is recognized as a special case of the general unste ady spreading process with the commonly used power laws for spreading occur ring only under special conditions.