Delamination and poroelastic deformation in drying colloidal suspension drops

The drying of sessile drops of colloidal suspensions leads to the formation of fracture patterns. As water evaporates, a solidification front propagates from the edge of the drop, leaving behind a thin close-packed particle deposit. For drops of initial particle volume fraction beyond 5%, this solid deposit eventually covers the entire wetted area. Water evaporation from the deposit and adhesion to the substrate lead to the formation of regular radial cracks which define the 'petals' of a flower-like crack pattern. The deposit then progressively delaminates from the substrate and the petals bend upward. This deformation had been linked to a poroelastic bending moment created by the vertical flow of water in the drying deposit. However, by monitoring the complete shape of the petals, we show that the petal curvature remains constant except close to the moving delamination front, despite vertical flows occurring through the entire deposit. Furthermore, measuring the bending characteristics for a range of deposit thicknesses and evaporation rates, we find a scaling law distinct from that expected from the bending moment theory. These findings point towards plastic deformation caused by the mean pore pressure in the deposit as the origin of the bending.