Drying gels VII. Diffusion during drying

An analysis is provided for the drying behavior of a gel whose pores contain a solution of liquids differing in volatility; both viscoelastic and rigid elastic gels are considered. The more volatile liquid (V) evaporates faster, creating a concentration gradient of V in the other liquid, so the flux of V to the surface of the gel results from both flow and diffusion. The diffusive flux removes V from the interior, and can thereby reduce the pressure gradient that causes differential strain and stress. Sample calculations indicate that diffusion can drastically reduce the drying stress in a plate of gel. This is suggested to be one of the functions of "drying control chemical additives" (DCCAs), which tend to be nonvolatile liquids, such as formamide and glycerol, that would provide a medium through which alcohol and water could diffuse. Unfortunately, for every compliant gels, it will not be possible to maintain diffusion control throughout the drying process, so the practical advantage of this mechanism will be less than the potential benefit that the calculations suggest.