We investigate the influence of wetting hysteresis on the shape and stability of dry patches. Such patches are generated in a film flowing over an inclined plane in a situation of partial wetting. We performed experiments on silicon oil flowing over a glass plate coated with fluoropolymers in which direct visualizations are combined with refraction of a laser sheet to probe contact angle distribution around the patches. The shape evolution depends on the history of the flow, and is different for progressive increase or decrease of flow rate. For increasing flow rates, the shape is in agreement with a simple model based upon a balance between gravity and capillarity, and in which the contact angle is supposed to be uniform. Laser measurements indicate that its value coincides with the static advancing contact angle. For a decreasing flow rate the situation is much more complicated: the shape remains qualitatively that predicted by the model, but quantitatively, the curvature of the patch boundary is not reproducible. Laser measurements suggest that this is linked to wetting heterogeneities (wetting defects) that impose nonuniform contact angle distributions. Finally, the stability is explored in terms of two critical flow rates (expansion or advection of the patches). (C) 2006 American Institute of Physics.
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