Efficient Cavitation Generation by Non-sinusoidal Powerful Ultrasound

Abstract

In the case of high-power ultrasonic devices, the vibration waveform is generally a sinusoidal wave obtained with a resonant effect. On the other hand, in ultrasonic pumps, ultrasonic levitation and cavitation generation, sinusoidal ultrasonic vibration is not always optimal because these principles are based on the nonlinearity of the bulk modulus of water or air. In this study, we designed a transducer whose higher-order-mode resonant frequency is twice the fundamental one, and verified the waveform dependence on cavitation generation by examining a trapezoidal waveform obtained by superposing two sinusoidal waves. To a realize trapezoidal waveform, the ratio of the resonance frequencies between the fundamental mode and the higher order mode must be controlled to be precisely 1 : 2 ; however, this ratio shifts easily as a result of a temperature rise and nonlinear piezoelectric effects. In particular, for the high-power ultrasonic transducer with a high Q value, it is difficult to realize 1 : 2 the resonance frequency ratio because of the extremely narrow frequency band for the resonant effect. In this research, we developed a resonance frequency control system by separating the passive piezoelectric elements from the driving piezoelectric element. To control the resonance frequency ratio by utilizing the stiffness change depending on the electrical boundary condition of these passive piezoelectric elements. Finally, we verified that the nonsinusoidal waveform, namely, the trapezoidal waveform, is effective for cavitation generation compared with a single sinusoidal waveform.