EXPERIMENTAL MEASUREMENTS OF SURFACE STIFFNESS ON WATER-SATURATED POROUS SOLIDS

The surface impedance of a fluid/fluid-saturate porous solid interface is defined as the ratio of the pressure difference between the fluids on the two sides of the interface and the volume velocity of the flui d through the surface pores. In most cases, the surface pores are inhe rently ''open'' and the surface impedance is negligible when the sampl e is fully submerged in fluid. On the other hand, due to surface tensi on, practically closed-pore boundary conditions can prevail at an inte rface between a nonwetting fluid (e.g., air) and a porous solid satura ted with a wetting fluid (e.g., water). This effect is caused by the h igh stiffness of the microscopic fluid membranes extended by capillary forces over the otherwise open surface pores. We have determined the quasistatic surface stiffness of different water-saturated porous mate rials by changing the hydrostatic pressure and directly measuring the average surface displacement by an acoustical sensor. Generally, the s urface stiffness is proportional to the surface tension of the wetting fluid and inversely proportional to the static permeability of the sp ecimen. For cylindrical pores, the measured surface stiffness is in go od agreement with theoretical predictions. For more irregular geometri es, such as consolidated spherical beads, the surface stiffness is sti ll inversely proportional to the static permeability but its value is orders of magnitudes lower than for cylindrical pores of comparable pe rmeability.