WETTING OF HETEROGENEOUS SURFACES - INFLUENCE OF DEFECT INTERACTIONS

We investigate the motion of a fluid interface through a thin cell, us ed as a model for porous media with heterogeneous wettability properti es. We analyze how the distribution of wettability defects modifies th e shape of the advancing interface. Solid substrates are prepared with a photolithography technique to create wettability defects. Several d istributions of defects are generated, with different surface densitie s and different position ordering. The amplitude of fluctuation of the fluid interface is measured for the different defect distributions an d with or without gravity stabilization of the interface. When the int erface is stabilized by gravity, the capillary length sets the distanc e below which defects strongly cooperate to pin the interface and trap air bubbles. Trapping of the advancing interface by the defects is de pendent on the spatial distribution of defects. The interface fluctuat ion is much larger for strongly disordered distributions and is relate d to a correlation length of the defect pattern. (C) 1997 American Ins titute of Physics.