Characteristic interfacial structure behind a rapidly moving contact line*

A solid rapidly pushed into a liquid, of viscosity η, at velocity U entrains surrounding air along with its moving surface. Using high-speed interferometry we find a robust characteristic structure of the entrained air layer: there is a thin-thick alternation of gap thickness in the transverse direction and this feature occurs both in wetting and de-wetting. In the thin regions we find that the gap thickness scales approximately as (ηU)^0.5. We present a model using the assumption that the velocity profile is robust to thickness fluctuations that gives a good quantitative estimate of this gap thickness. This is in contrast to the Landau-Levich analysis which had previously been assumed applicable to this problem.