Experimental Study of Wetting Front Instability in Porous Media

The stability of infiltration flow in Hele-Shaw cells was examined. Planar wetting fronts were perturbed by increasing G, the pressure gradient behind the wetting front, from negative to positive values. For experiments in which a step-change in G was imposed, the onset of instability and the wavelength of fingering agreed closely with those predicted by the theory of Philip. In experiments where G was gradually increased through zero, Philip's analysis adequately described the onset of instability and gave useful estimates of the wavelengths of fingers. Preliminary experiments using sands showed divergences from the theory. These were attributed to deficiencies in the delta-function model which forms the basis of the theory.