Dry Hard: Crack Pattern Selection in Drying Suspension Drops

The drying of sessile droplets of polystyrene nanoparticle suspensions leads to the formation of complex fracture patterns in the dried deposit. As water evaporates from a freshly deposited drop, a thin close-packed particle deposit forms at the edge of the drop and grows inward. Water evaporation from this particle deposit causes in-plane stresses that are released by the formation of regular radial cracks that propagate as the deposit grows. Surprisingly, as drying stresses continue to increase, a second generation of cracks forms in the deposit. These new cracks are orthoradial and bridge the previously formed radial cracks. We control the deposit shape by varying the initial particle volume fraction and we measure the crack spacing of the dried deposit. We show that, as the drying stress increases in the deposit, the local radial crack spacing depends linearly on the local deposit thickness, while the orthoradial crack spacing is set by the specific shape of the deposit. We relate the orthoradial crack spacing to a recent poroelastic model that captures the dependence of the drying stress on the deposit shape.