Drying of colloidal suspension drops : pattern formation and mechanical deformation

Abstract

The drying of drops of colloidal suspensions on a hydrophilic substrate leads to the formation of a close-packed solid particle deposit. The initial volume fraction of particles sets the shape and size of this deposit, from a ring at the edge of the drop to a solid film covering the initial wetted area. We show that this deposit remains saturated with water and we explain the propagation of the solidification front by considering the evaporative flux and mass conservation. As the deposit forms, tensile drying stresses generate regularly spaced radial cracks. The crack patterns define four different regimes which we relate to experimental measurements of the deposit shape. The radial cracks separate the deposit into a multitude of petals. Due to the negative water pressure inside the pores of the deposit, these petals bend upwards creating mesmerizing forms reminiscent of blooming flowers.