Collapse of Capillaries in the Drying of Porous Gels

Abstract

WHEN a rigid porous material, initially filled with liquid, dries out under falling vapour pressures, capillaries of particular dimensions will empty at particular vapour pressures. The remaining liquid is held under a hydrostatic tension which is transmitted to the walls but, as each capillary empties, the stresses on the walls due to this cause will fall to zero. If, however, the material is easily deformed, the tension of the liquid will cause a reduction in the size of the capillary, and evaporation will be delayed until a lower vapour pressure is reached, by which time the tension has further increased. This consideration is, so far as we are aware, a new one and is in addition to any shrinkage stresses which may result from the loss of vapour held by means other than capillary condensation. One may thus expect that, under certain conditions, capillaries may reach an unstable state where the tension in the liquid is increasing more rapidly than is the resistance of the capillary wall to further compression.