Role of mechanical pressure in permeation of a solvent through gels

In a system consisting of gel network and solvent, mechanical pressure and network stress can be transformed from one to the other through the frictional drag between the network and the solvent. In addition, mechanical pressure contributes to the total stress. The application of these well-known principles to concrete physical situations involves surprisingly subtle issues and, unless both roles of the pressure are properly taken into account, may lead to erroneous conclusions. To illustrate this point, we analyse the fundamental elastohydrodynamic processes which underlie several recent theoretical works and show how they depend on the geometries of set-up of gel, surrounding solvent and substrate that supports the gel sample.