Wetting and dewetting phenomena are ubiquitous. Because of its
relatively high surface tension, the most common fluid - water - exhibits
partial or poor wetting properties on many surfaces. This can be
problematic when trying to coat or cover a surface with a uniform liquid
film. If the thickness or flow rate of the film is insufficient,
destabilization occurs and dry patches eventually form.
Thus, determining the shape and stability of dry patches within flowing
films has important industrial issues, as well as fundamental ones because
of the subtle hydrodynamics involved in the vicinity of a contact line.
We experimentally investigated the shape and stability of an isolated
dry patch within a fluid film flowing over an inclined plane. In the whole
range of explored parameters, dry patches exhibit a universal arch shape
characterized by a single parameter : the radius of curvature at the
apex. We provide a model based on qualitative observations, from which
the shape of the dry patch and the critical flow rate above which it
becomes unstable can be derived, in good agreement with the experiment.
Another nice example of free surface flows involving contact lines can
be observed on rainy days when drops are running over window panes. We ran
well controlled experiments in which drops are running on an inclined
plate. Depending on their velocity, drops can exhibit various shapes :
oval or rounded at low velocities, with a corner at the trailing edge
above a certain threshold, and emitting smaller droplets behind them with
a rich variety of dynamical patterns at higher velocities. These
transitions are closely related to problems of air entrainment occuring
in coating devices, when a fluid film is quickly deposited on a
substrate. I will give an interpretation for these transitions and point
out mathematical challenges arising from this study.
Converted from podgorski.abs (plain text).