Measuring permeability of rigid materials by a beam-bending method: II, porous glass

It has been shown in a companion paper that the permeability of a porous bo dy can be found from a three-point bending experiment. When the sample is b ent, a pressure gradient is created in the liquid within the pores; as the liquid nons in response to the gradient, the force that must be applied to the rad to sustain a fixed deflection decreases with time. Bg fitting the f orce decay curve to the predicted shape, the permeability is obtained, alon g with the elastic modulus, In this paper, that theory is tested using poro us Vycor(R) glass saturated with various solvents, including several normal alcohols, water, and glycerol. The shape of the measured decay is in excel lent agreement with the prediction. Consistent with observations of previou s workers, we find that the permeability is influenced by the size of the s oh-ent molecule; by assuming that the pore surfaces are covered with a mono layer of immobile solvent, the observed variation can be explained, The adv antage of the beam-bending method is that the results are obtained in a few minutes; moreover, there is no problem with leaks or need for high pressur es, as there is in conventional measurements of low permeabilities.