3. Solvent extraction

Gel is a two-phase medium in which the liquid occupies most of the volume. The solid part, due to its low volume fraction (commonly of the order of 10%), has very low elastic properties. Typically, the Young's modulus of the solid part of a silica gel with a density of 0.2 g/cm ³ is 20 MPa. Very low stresses induce large-amplitude deformations that are easy to observe. Rupture of the solid part of the gel occurs at strains in excess of 0.5%, which is still very high for brittle materials (table 13 ). On the other hand, the solid part is usually highly reactive due to the reactive groups always present on its surface. As a result, any deformation of the solid skeleton can lead to a reaction between the solid chains if they come into contact.