Viscosity effects and confined cochlea-like geometry in laser-induced cavitation dynamics

Abstract

On the path to an optoacoustic hearing implant for stimulation of residual hearing, one possibility for tone generation in liquids is the concatenation of acoustic click events, which can be realized i. a. by the acoustic transients that accompany an optical breakdown. The application of a viscous gel is helpful in this context, as this results in an attenuation of the distortion of tone quality caused by higher harmonic components. To further understand the underlying cavitation bubble dynamics both in the viscous gel and in a confined volume that is dimensioned similarly to the human cochlea, a numerical model built in OpenFOAM was adapted and compared to additional experiments. Experimentally, the acoustic transients were generated by optical breakdown by nanosecond laser pulses with a pulse duration of 0.7 ns and a wavelength of 1064 nm. The pulses were focused on a viscous gel inside a water container. The pressure transients were measured by a needle hydrophone. The comparison of the bubble dynamics in different viscosities between the model and the experiment shows that, except for high viscosities, the experimental observations could be modeled by the simulation. We assume that the maximum size of the cavitation bubble strongly decreases with increasing viscosity, which can be used for high-frequency attenuation as reported in our previous research. In conclusion, this study aims at an application-oriented realization of the numerical cavitation bubble dynamics model to understand the experimental findings on the pathway to an optoacoustic hearing implant.