Stress and fracture during drying of gels

Two models of drying are examined in an effort to explain why cracking during drying is more likely if the gel is large and the drying rate is high, often occurs at the critical point (i.e., at the moment when shrinkage stops), and frequently yields only 2 or 3 pieces. A microscopic model assumes that local stresses result from sequential emptying of pores of different sizes, and stress fields on the scale of the pore size cause fracture. This model fails to explain the importance of gel size and drying rate. A macroscopic model attributes stresses to a pressure gradient extending throughout the sample. The stresses increase with drying rate and sample size, but show no singularity at the critical point. By assuming that the critical flaws result from the irregular shape of the drying front, and that they are subjected to the stress predicted by the macroscopic model, all of the behavior described above can be qualitatively explained. The cracking of films is a different phenomenon that seems to be related to the amount of mechanical energy stored in the film.