Criteria for Crack Formation and Air Invasion in Drying Colloidal Suspensions

The drying of sessile drops of aqueous colloidal suspensions leads to the formation of a close-packed particle deposit. As water evaporates, a solidification front propagates from the edge of the drop toward the center, leaving behind a thin disk-shaped deposit. For drops with sufficiently large particle volume fractions, the deposit eventually covers the entire wetted area. In this regime, the dynamics of the deposit growth is governed by volume conservation across a large range of particle volume fractions and drying times. During drying, water flows radially through the deposit to compensate for evaporation over the solid's surface, creating a negative pore pressure in the deposit which we rationalize with a hydrodynamic model. We show that the pressure inside the deposit controls both the onset of crack formation and the onset of air invasion. Two distinct regimes of air invasion occur, which we can account for using the same model that further provides a quantitative criterion for the crossover between the two regimes.