Experimental Study of Wetting Front Instability Induced by Sudden Change of Pressure Gradient

Twenty‐two experiments were performed on stability of infiltration flows in Hele‐Shaw cells. Flows were perturbed by suddenly changing G , the pressure gradient behind the wetting front, from a definite negative to a definite positive value. Three methods were used: (i) increasing air‐pressure ahead of the front, (ii) employing a nonwetting stratum, and, (iii) suddenly stopping liquid supply and preventing air‐entry. Glycerol and silicone, with contrasting properties, were used as the infiltrating liquids. The experiments confirmed the theory of Philip (1975a): immediately G was made positive, all flows became unstable, and the observed wavelengths of fingering agreed closely with those predicted by the theory. A modification of the theory to take account of the geometry of the microscopic air‐liquid interface predicts wavelengths in soil about twice as large as does the original theory. A preliminary experiment on infiltration of water in a coarse sand confirmed the theory qualitatively, but the observed wavelength was twice that predicted theoretically. In a preliminary experiment on infiltration in a fine sand, perturbation failed to produce instability. This divergence from theory is attributed to deviations from the delta‐function model on which the theory is based. Further experimental and theoretical work is needed.