Using an interfacial tracer technique, our experiments show qualitatively different trends of total interfacial area between the wetting and non-wetting phases as a function of saturation, depending on whether the system is strongly or weakly wetted. A strongly wetted system is defined as one in which the wetting phase can spread as a thin film on the solid surface. We assess the relative contributions of fluid/fluid and fluid/solid interfaces to the total area using thermodynamic arguments. The fluid/solid contribution to area plays a crucial role in explaining the measurements.

The influence of interfacial area on relative permeability is not straightforward. Simple analysis based upon pore-level distribution of phases in a model porous medium allows quantifying the differences in the relative permeabilities for both weakly and strongly wetted systems, measured simultaneously with the interfacial area. Relative permeability correlates with fluid/solid area but not with fluid/fluid interfacial area.