Patterns from drying
Blooming flowers in the drying of colloidal suspension drops
We use colloidal suspensions of silica nanoparticles in water to explore the mechanisms of crack formation in deposited drops occurring upon the evaporation of the solvent. Such colloidal suspensions surround us; from coffee and milk to blood and paint. Once dried, these suspensions form coatings that are prone to failure, as evidenced by the formation of craquelures in ancient paintings. Similarly, drying drops of colloidal suspensions exhibit a variety of regimes depending on the amount of suspended particles; from the well-known coffee ring effect in dilute suspensions to homogeneous coatings at larger colloid volume fractions.
Our drops dry on a glass substrate and deposit a thin layer of close-packed particles in water. The evaporation imposes a flow through this porous deposit, which generates negative pressures up to a hundred times atmospheric pressure. The resulting stress generates radial cracks which propagate by avalanche-like dynamics, leading to the formation of petals. We show that the distance between the cracks is set by the deposit thickness, which we control via the particle volume fraction. The petals bend out of plane, forming a blooming flower. Thicker petals form fewer cracks and bend less, whereas thin petals form a large number of cracks and exhibit strong bending.
Varying the initial shape of the drops by controlling the contact angle changes the shape of the deposit and the crack formation from blooming flowers on hydrophilic substrates doughnuts and eventually hollow shells at higher contact angles.
Worms and bursts in drying hydrogels
We discover very different structures in another drying material; thin films of a highly elastic hydrogel confined between two glass plates. As the water evaporates, stresses build up at the air-gel interface. These stresses get released by intermittent bubble formations, which lead either to the growth of disordered or worm-like patterns, depending on the gel modulus and the plate spacing. The bubble size is mainly set by the plate spacing.