Stress and strain during supercritical drying

Supercritical drying avoids capillary forces, but damaging stresses can be generated during solvent exchange, pressurization, and depressurization. Each of those steps has been analyzed, so the duration of the process can be minimized without risk to the sample. This work, which was initiated in collaboration with Professor Jean Phalippou’s group, is reviewed. Poromechanics is the study of stresses and strains resulting from interaction of the solid and liquid phases in a porous material. For example, if a gel is immersed in another liquid for solvent exchange, the gel network may undergo a transient contraction if the original pore liquid diffuses out faster than the replacement liquid diffuses in; poromechanical analysis quantifies the resulting stresses and indicates whether there is a risk of cracking. In the preparation of aerogels, when the pressure in the autoclave is decreased, the higher pressure inside the aerogel causes it to expand, which may result in cracking. Poromechanics reveals the rate of depressurization that can be imposed without creating damaging stresses. We also show that the sensitivity of gels to pore pressure can be exploited to measure their permeability to gases and liquids by observing the response to small loads or changes in temperature.