Lowering Cavitation Threshold Using Bifrequency Excitation: Nonlinear Aspect And Influence Of The Difference Frequency

Abstract

The control of cavitation phenomena is a challenge in ultrasound therapy. Within the scope of investigating the influence of the excitation waveform on the cavitation activity, we have shown that using a signal combining two neighbouring frequencies (f1 and f2) instead of a pure sine wave excitation (f0), a reduction of more than 40% of the power needed to initiate inertial cavitation can be obtained. The present work focused on the influence of such parameters as dissolved gas concentration and the difference frequency Δf = f2−f1, on cavitation thresholds and cavitation activity. Experiments were carried out in a water tank, using a piezoelectric transducer focused on targets of controlled roughness. The acoustic signal diffused, either by the target or by the cavitation bubbles, was filtered using a spectral and cepstral‐like method enabling the extraction of a broadband criterion for inertial cavitation. The pulsed excitations (center frequency f0 = 550 kHz) were 1.8 ms long. For experimental conditions where low intensities were needed to trigger cavitation (high dissolved gas concentration), cavitation thresholds measured using bi‐frequency excitation were higher than when using mono‐frequency excitation. This result indicates that the mechanism responsible for the effect should be the nonlinear combination of two neighbouring frequency components f1 and f2. Concerning the influence of the difference between frequency components for bifrequency excitations, cavitation activity just beyond the threshold was much higher when using bi‐frequency signals and was increased when Δf = f2−f1 was increased.