A qualitative theory of wimples in wetting films

It has long been known that hydrodynamic pressures in a thin draining liquid film can cause inversion of the curvature of a drop surface as it approaches another surface, creating a so-called dimple. However, it was recently found that a different shape, dubbed a wimple, can be formed if a fluid drop, which is already in the field of repulsive surface forces, is abruptly pushed toward the wall. The drop shape might include a central region in which the film remains thin, surrounded by a ring of greater film thickness bounded at the outer edge by a barrier rim. Here we present a qualitative theory of the wimple formation. It is shown that this is mainly driven by the film hydrodynamics, and a qualitative criterion for the wimple/dimple transition is derived.