Wetting of heterogeneous surfaces: Influence of defect interactions

We investigate the motion of a fluid interface through a thin cell, used as a model for porous media with heterogeneous wettability properties. We analyze how the distribution of wettability defects modifies the shape of the advancing interface. Solid substrates are prepared with a photolithography technique to create wettability defects. Several distributions of defects are generated, with different surface densities and different position ordering. The amplitude of fluctuation of the fluid interface is measured for the different defect distributions and with or without gravity stabilization of the interface. When the interface is stabilized by gravity, the capillary length sets the distance below which defects strongly cooperate to pin the interface and trap air bubbles. Trapping of the advancing interface by the defects is dependent on the spatial distribution of defects. The interface fluctuation is much larger for strongly disordered distributions and is related to a correlation length of the defect pattern. (C) 1997 American Institute of Physics.