The dynamic response of the water retention curve in unsaturated soils during drainage to acoustic excitations

We examined the effects of acoustic excitations on the water retention curve, i.e., the relationship between capillary pressure (P^c) and water saturation (S_w) in unsaturated porous media, during drainage. The water retention curves were measured under static and dynamic conditions, where water was withdrawn from a sandbox with three different pumping rates, 12.6, 19.7, and 25.2 mL/s. Excitations with frequencies of 75, 100, 125, and 150 Hz were applied. The acoustic excitations had no effect on the static water retention curve but altered the dynamic water retention curve. The acoustic excitations lowered the dynamic (P^c), especially under the dynamic condition where the pumping rate was 25.2 mL/s and when (S_w) varied between 0.6 and 0.95. The differences between the capillary pressures measured under static and dynamic conditions decreased when acoustic excitations were applied. We link this finding to the change in contact angle induced by the acoustic excitation. The dynamic coefficients, τ, for the dynamic water retention curves that we fitted to the experimental data were smaller with than without acoustic excitations. We attribute the decrease of the dynamic coefficient to the combination of the increase in the permeability and the decline in the air-entry pressure caused by adding acoustic excitations.