Pattern Formation and Elastocapillary Instabilities in Soft Gels

Hydrogels are complex materials that are typically utilized as bioinks in the emerging 3D bioprinting technology. Gels are distinguished by an elasticity that introduces complexity into the pinch-off process. The distinguishing feature of soft gels is that capillarity (surface tension) and elasticity are comparable which can induce an abundance of new phenomena in the elastocapillary regime. Unfortunately, elastocapillary instabilities are not fully understood because classical theories of fluid mechanics and solid mechanics can not capture the crossover between capillary-dominated and elasticity-dominated dynamics. Herein, elastocapillary surface phenomena on hydrogels are experimentally characterized and new theoretical models are proposed to interpret the discrepancies between classical theories and new experimental observations. Many first observations of dynamic elastocapillary phenomena are reported including the experimental observations of i) gel drop oscillations in ultrasonic levitation and ii) Faraday waves on mechanically-vibrated gels. The mechanism of pattern formation is investigated and the role of elasticity is revealed. By relating theory to experiment, a new diagnostic technique to measure the surface tension and rheology of soft gels is developed, which can directly support many emerging 3D bioprinting technologies.