Experimental measurements of surface stiffness on water‐saturated porous solids

The surface impedance of a fluid/fluid‐saturated 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 fluid through the surface pores. In most cases, the surface pores are inherently ‘‘open’’ and the surface impedance is negligible when the sample is fully submerged in fluid. On the other hand, due to surface tension, practically closed‐pore boundary conditions can prevail at an interface between a nonwetting fluid (e.g., air) and a porous solid saturated with a wetting fluid (e.g., water). This effect is caused by the high 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 materials by changing the hydrostatic pressure and directly measuring the average surface displacement by an acoustical sensor. Generally, the surface stiffness is proportional to the...