Colloidal dispersions are often coated on a substrate to allow the solvent to evaporate and leave a uniform thin film. However, cracks can form in the latter stages of drying. A negative capillary pressure develops as the air-water interface is pulled down into the interstices between particles, putting the drying film in tension. The film responds by collapsing normal to the surface but is constrained from shrinking laterally unless cracks open. Only through understanding of the mechanism can colloidal dispersions be formulated to produce crack-free films.

In this study, we use a high-pressure ultra-filtration device to measure directly the pressure responsible for cracking in uniform films of latex or silica dispersions containing particles of varying radii. The results confirm that cracking is controlled by the recovery of elastic energy with the critical pressure increasing with the modulus of the particle, decreasing with increasing film thickness, and independent of particle size. The Griffith¹s criterion for equilibrium crack propagation along with the nonlinear stress-strain relation for drying films provides a necessary, but not sufficient, condition for crack formation. As the pressure is increased beyond the critical value, additional cracks open in qualitative agreement with our elastic energy recovery model. Films that lack of defects crack at a higher pressure than predicted, while introducing flaws at the edges promotes cracking at the critical pressure. These observations suggest an important role for defects in nucleating cracks.