Why do drying colloidal suspension drops bend out of plane?

The drying of sessile drops of concentrated silica nanoparticle suspensions creates fractured and bent dried deposits that resemble blooming flowers. As water evaporates from a freshly deposited drop, a solidification front propagates from the edge of the drop leaving behind a thin close-packed particle deposit that eventually covers the entire wetted area. Evaporation from the porous particle deposit and adhesion to the substrate lead to in-plane strains in the deposit that are released by the formation of regular radial cracks that define the 'petals' of the flower-like pattern. The deposit then progressively delaminates from the substrate and the petals bend upward. Using a poroelastic model combined with non-Euclidian plate simulations, we show that evaporation leads to non-uniform in-plane strains that cause the deposit to buckle out of plane. The balance between the in-plane stretching energy released by buckling and the out-of-plane bending energy cost sets the amplitude of the deformation. Our work provides a unified picture of crack formation and deformation as two ways to release strains in thin drying deposits.