Thermal expansion of a viscoelastic gel

It was previously shown J. Non-Cryst. Solids, 130, 157 (1991)] that the permeability of a saturated gel can be determined from a measurement of its rate of expansion during a change in temperature. The existing analysis assumes that the solid network of the gel behaves elastically, but in many cases the gel is likely to be viscoelastic, especially when the gel exhibits syneresis. In this paper, the expansion kinetics are determined for a gel with a viscoelastic (VE) network. Sample calculations for a silica gel containing ethanol indicate that a naive application of the elastic analysis could result in errors exceeding a factor of two in the estimate of the permeability of a VE gel. However, in such cases, there are qualitative features in the experimental data that alert the experimenter to the existence of significant VE relaxation. Therefore, it is possible to avoid errors by careful examination of the data.