Drying-induced cracking and bending of nanoparticle suspension drops

The drying of sessile drops of aqueous nanoparticle suspensions with particle volume fractions >5% leads to the formation of intricate patterns. As water evaporates, a thin deposit of close-packed particles appears at the edge of the drop and grows toward the center, eventually covering the entire wetted area. Interfacial tension prevents air from penetrating inside this particle deposit, causing an outward flow of water from the bulk liquid in the center of the drop to and through the porous deposit at its edge. When the deposit reaches a certain size, radial cracks form in the deposit defining regular petals. The petals bend upwards into a blooming flower. Using poroelasticity theory, we estimate the stress caused by water flowing in the deposit and show that it can predict both the onset of crack formation for various drying conditions and particle volume fractions, and the shape of the petals as they bend. However, scaling considerations between the bending stiffness and the poroelastic torque predict that all petals will bend the same regardless of their thickness, while we observe that thinner petals bend much more.