Wetting on heterogeneous surfaces: Experiments in an imperfect Hele-Shaw cell

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Abstract
We use a model system, a thin Hele-Shaw cell, to study the influence of surface wettability defects on the behavior of a liquid front advancing on a substrate. Defects are made by deposition of ink droplets on a glass surface. We record the deformation of the interface of a silicone oil displacing air in the cell, at very small capillary numbers. There is a trapping transition determined by the defect strength. The contact line cannot move over defects that are too large and an air bubble remains trapped on the defect. Smaller defects do hold the contact line for a while, but they are eventually wetted by the advancing liquid. From the shape of the contact line anchored on a single defect we determine the energy associated with the line deformation. From the experimental energy-displacement curves we find that the force exerted by the defect on the contact line increases linearly with the deformation. We also determine the amplitude of fluctuation of the interface when it moves over multiple defects. When the average defect strength is larger than the critical value, air bubbles with a size up to the capillary length are trapped by the defects.