Abstract
We theoretically and experimentally study the dynamics of spherical gas bubbles in a viscoelastic material under ultrasound irradiation. First, we formulate a Rayleigh-Plesset-type nonlinear equation that describes the dynamics of spherical gas bubbles in the Voigt solid (with linear viscoelasticity). The model equation is linearized to derive the analytical solution for the case of small-amplitude oscillations. Then, we propose an experimental technique to observe ultrasound-forced dynamics of micron-sized gas bubbles in a gelatin gel. To trigger gas bubble nucleation, an infrared laser pulse is focused into gels of 3 wt % gelatin
concentration that are supersaturated with ambient air. The nucleated gas bubble grows through the incoming mass flux of air dissolved beyond Henry’s limit, allowing us to control the radius of bubbles under mechanical equilibrium (with ultrasound irradiation turned off). Low-intensity ultrasound at 28 kHz is irradiated toward the gas bubble, while the equilibrium bubble radius is tuned through the gradual mass transfer. The linearized bubble dynamics are recorded by a high-speed camera; the observed dynamics are compared to the linearized Rayleigh-Plesset solutions. Through the comparisons, we calculate viscosity and rigidity of the gel at the given frequency.
