Thermal expansion of gels: a novel method for measuring permeability

When a gel is heated, the thermal expansion of the pore liquid causes stretching of the solid network. If the heating rate is very high, the gel expands at the same rate as the liquid; at slower rates, some of the liquid drains out and the gel expands less. By measuring the axial deformation of a gel rod, it is possible to determine the elastic relaxation time, which involves the product of the permeability, D, and the elastic modulus of the gel. Using independently measured elastic properties, one readily obtains D. This method is ideally suited for compliant gels for which more direct methods of measurement of D are difficult. In this paper an analysis of the problem is presented showing the relationship of the dilatation of a gel to its permeability and modulus, allowing for syneresis of the gel network. Experimental data are presented for a 2-step base-catalyzed silica gel, showing that expansion can readily be observed during heating at 0.5°C/min with total temperature changes of ∼ 5°C. The permeability determined from this experiment is in good agreement with that obtained previously by a more difficult method.