Hydrogels are complex materials that are typically used in many emerging 3D bioprinting technologies. One novel and distinguishing feature of soft hydrogels is that capillarity (surface tension) and elasticity are comparable which can result in new physics in this elastocapillary regime. Herein, we have experimentally characterized the elastocapillary dynamics on hydrogels and proposed new theoretical models to interpret the discrepancies between classic theories and our new experiments on soft solids. First, we describe the experimental observation of the crossover between capillary-dominated and elastic-dominated surface waves in an ultrasonically levitated gel drop. A theoretical model is proposed which enables obtaining elasticity, surface tension and viscosity of a gel drop from characteristics of its response to ultrasonic excitation. This is noteworthy as classical surface tension measurement techniques do not work for gels. Second, we explore the onset of parametrically excited surface waves (Faraday waves) on glycerol-water mixtures and agarose gels, ascertaining the effect of viscosity and elasticity on the threshold amplitude and mode selection. The instability tongues are characterized by a resonance frequency and threshold amplitude and we show that i) increasing the viscosity decreases the frequency and increases the amplitude and ii) increasing the elasticity increases both the frequency and amplitude. Our new experimental technique allows us to observe the first 50 resonant shape modes which display beautiful spatial symmetries. We hope that our study inspires many follow-on more studies of dynamic elastocapillary phenomena.

Zoom: https://psu.zoom.us/j/98891565357?pwd=Mkd5VDFBcGN6dHBlV3JVOS9yOWdBQT09