Different Modes of Stress Release in Drying Drops of Colloidal Suspensions

Sessile droplets of nanoparticle suspensions dry into solid deposits subject to large drying stresses. We study two suspensions that release these stresses in distinct ways: droplets of polystyrene (PS) nanoparticle suspensions form complex crack patterns while droplets of silica (Si) nanoparticle suspensions bend into shapes resembling blooming flowers.

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 combined with adhesion of the deposit to the substrate causes tensile stresses leading to the formation of regular radial cracks. As the radial cracks form, we observe a critical difference between the two systems: the PS particle deposit remains attached to the substrate while the Si particle deposit delaminates and separates from the substrate. Stresses in the attached PS particle deposit are released by the formation of orthoradial cracks that bridge the radial cracks, creating a complex crack pattern. Conversely, no additional cracks form in the delaminated Si particle deposit. Instead, the deposit curves and bends out of plane to relieve drying stresses. We show how the combination of poroelasticity with fracture mechanics and non-Euclidian plate mechanics captures both the complex crack pattern formation and the out-of-plane deformation as distinct responses to a similar drying stress.