Drying gels. I. General theory

An assessment is given of the driving forces causing shrinkage during drying of organometallic gels. Both the elastic and viscous responses of the gel to those forces are analyzed, so that the stresses and strains resulting from drying can be calculated. The principle causes of contraction are assumed to be the large interfacial energies between the solid, liquid, and vapor phases. The extent of chemical reactions that occur during drying is assumed to be proportional to the change in interfacial area, so the corresponding change in chemical potential can be included as a contribution to interfacial energy. Three contributions to the capillary stress are identified, the largest being the redistribution pressure that drives the liquid from the wet region of the gel into the dry exterior region. This causes the contraction rate of the wet region to exceed that of the dry region. The rate of liquid flow through the pores of the gel is analyzed. The shrinkage rate is shown to be controlled by the viscosity of the solid phase of the gel. as well as the rate of transport of pore liquid.