Patterns from drying
Crack morphologies in drying 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.
Evaporation-driven Cellular Patterns in Confined Hydrogels
Drying-induced pattern growth is drastically altered when the material is able to undergo large-scale deformation prior to failure. We show this for hyperelastic hydrogels confined between two parallel plates, where the volume lost by evaporation is accommodated by an inward displacement of the air-hydrogel interface that induces an elastic deformation of the hydrogel. Once a critical front displacement is reached, intermittent fracture events are initiated by a geometric instability resulting in localized bursts at the interface. The bursts relax the stresses and irreversibly form air cavities that lead to mesmerizing cellular networks. We rationalize the size of the cavities by considering the elastic stress stored in the material at the onset of a burst whose spatial range can be affected by interfacial tension.
Evaporation-driven Cellular Patterns in Confined Hyperelastic Hydrogels
B. Saintyves, R. Pic, L. Mahadevan, I. Bischofberger, Phys. Rev. Lett., in press (2023)