Mechanics of syneresis I. Theory

Syneresis is spontaneous contraction of a gel, accompanied by expulsion of liquid from the pores. This occurs even if evaporation is prevented and the gel is immersed in liquid. In this paper, the rate of contraction is shown to be dependent on the size of the gel, because the small pores resist flow of liquid out of the gel. An analysis is presented that allows prediction of the influence of size on contraction rate for plates and cylinders of gel. An initial period of slow contraction, which is reported by some experimenters, is shown to be consistent with the gradual rise in pressure in the pore liquid as it is compressed by the viscoelastic gel network. Experimental results on syneresis in silica gels, presented in a companion paper, support the theory.