First insight into mechanisms involved in cavitation enhancement by the use of bifrequency excitation

International audienceFocused ultrasound experiments were carried out on cavitation targets of controlled roughness in degassed water in order to study the cavitation threshold and the development of cavitation beyond this threshold. A monofrequency pulsed excitation was compared to a bifrequency pulsed excitation involving two neighbouring frequency components. According to experimental results, a bifrequency excitation modifies cavitation threshold and stimulates cavitation activity beyond threshold in comparison to a monofrequency waveform. For high (respectively low) monofrequency thresholds, the bifrequency excitation reduces (respectively increases) the threshold value. This shows that nonlinear effects are involved in the process. Numerical simulations of the dynamics of a single bubble in a bifrequency oscillating pressure field have been done. It appears that the cavitation threshold is modified for specific initial bubble radius, which is not sufficient to explain experimental observations. To simulate first-order nonlinear propagation effect, the difference frequency component was superimposed to the excitation field, reducing the cavitation threshold on a wider range of initial bubble radius. Considering this larger effect, the experimental observations could be related to the spectral broadening of the excitation due to higher order nonlinear combinations of primary frequencies