Watching Droplets Dry in Wet and Dry Climates

The drying of sessile droplets of colloidal suspensions with particle volume fraction > 5% leads to the formation of fracture patterns. As water evaporates, a solidification front propagates from the edge of the droplet, leaving behind a thin close-packed deposit that eventually covers the entire wetted area. Evaporation-induced stresses generate radial cracks in the deposit that grow towards the droplet center, defining regular petals. These petals bend out of plane to form a blooming flower. Strikingly, while the distance between the cracks is independent of relative humidity, the bending behavior changes drastically when the relative humidity is increased; the curvature of the petals along their width becomes inverted, and the kinetics of the petal bending is modified. We discuss the relation between these changes in structure and kinetics and the deposit thickness.