(147d) Surface Waves on Soft Gels in a Vibrated Cylindrical Container

Soft gels are utilized in emerging applications, e.g. bioprinting, and are distinguished in that the forces of capillarity (surface tension) and elasticity are comparable in magnitude leading to elastocapillary phenomena including the observation of analogous hydrodynamic instabilities in soft materials. Here we study the formation of surface waves on agarose gel in a mechanically-vibrated cylindrical container. The wave conforms to the geometry of the container and has spatial structure described by the mode number pair (n,l), that is identified by long exposure time white light imaging. The first 50 resonance modes are observed. A laser light system is used to detect the surface wave frequency and we observe two classes of waves; i) harmonic edge waves and ii) subharmonic Faraday waves, from which we report the frequency response and instability tongue, respectively, as it depends upon the gel elasticity. The role of meniscus geometry at the container sidewall is illustrated and can lead to complex mode mixing, similar to sessile drop experiments by Steen et al. 2014, JFM. Theoretical predictions for the natural frequency of surface oscillations for a viscoelastic material depend upon the dimensionless Ohnesorge number Oh, elastocapillary number Ec, and Bond number Bo, and show good agreement with experimental observations.