Imaging stress and strain in the fracture of drying colloidal films

Abstract

Drying-induced fracture limits the applications of thin films based on colloidal materials. To better understand the mechanics of drying, we directly measure the microscopic distribution of strain and stress of a colloidal silica film during the course of drying and cracking. We observe the build-up and release of internal stresses inside the film before and after the opening of individual cracks, and extract a critical stress for fracture of approximately 1 MPa. We also find an inherent time scale for stress relaxation within the film of 25 minutes, likely due to the competition between elastic deformation of and solvent flow through the porous particle network. By correlating the stress and strain, we estimate the plane strain Young's modulus of our colloidal film of the order of 100 MPa.