Cavitation-induced shock wave behaviour in different liquids

Abstract

This paper follows our earlier work where a strong high frequency pressure peak has been observed as a consequence of the formation of shock waves due to the collapse of cavitation bubbles in water, excited by an ultrasonic source at 24 kHz. We study here the effects of liquid physical properties on the shock wave characteristics by replacing water as the medium successively with ethanol, glycerol and finally a 1:1 ethanol-water solution. The pressure frequency spectra obtained in our experiments (from more than 1.5 million cavitation collapsing events) show that the expected prominent shockwave pressure peak was barely detected for ethanol and glycerol, particularly at low input powers, but was consistently observed for the 1:1 ethanol-water solution as well as in water, with a slight shift in peak frequency for the solution. We also report two distinct features of shock waves in raising the frequency peak at MHz (inherent) and contributing to the raising of sub-harmonics (periodic). Empirically constructed acoustic pressure maps revealed significantly higher overall pressure amplitudes for the ethanol-water solution than for other liquids. Furthermore, a qualitative analysis revealed that mist-like patterns are developed in ethanol-water solution leading to higher pressures.

Keywords: Bubble cloud; Shock wave; Ultrasonic cavitation.

Fig. 1

Pressure vs frequency for all tested liquids for a specific sensor position (x = −3, y = 1 mm) and different input powers: a) 20 %, b) 60 % and c) 100 %. Each inset magnifies the plot near the prominent peak frequency. The pressure was averaged over 60 waveforms. Note the same Y-axis scales in all main and inset plots.

Fig. 2

Frequency response of acoustic emissions from kHz to MHz range for four working liquids at three input powers: a) 20 %, b) 60 % and 100 %. The top insets show the zoomed-in view of the low-frequency peaks (periodic freature), while the right insets show that of the high-frequency zone (inherent feature). Note that the sensitivity at 1 MHz was used for measuring the pressure amplitudes for f < 1 MHz. The zoomed-in view of the low-frequency peaks in log-scale is shown in Fig. A6 in the Appendix.

Fig. 3

Typical pressure vs time plots for the four working liquids at three input powers: 20% (left column), 60% (middle column) and 100% (right column). Peaks are associated with the intensity of shock waves (with their inherent frequency discussed in Fig. 1). It should be noted that plots are corresponding to cases where the distinct peak at MHz frequency was also present in accordance with Table 1 and Fig. A2.

Fig. 4

Sample snapshots from the high-speed recordings of the largest cavitation clouds of four liquids at three input powers: a) 20%, b) 60% and 100%.

Fig. 5

Contour plots of the maximum pressure (P_max) for a) ethanol, b) water, c) ethanol–water solution and d) glycerol, each for three input powers: 20% (left column), 60% (middle column) and 100% (right column). The sonotrode is axisymmetrically placed at (0,0).