Dual frequency Ultrasonic Cavitation in Various Liquids: High-Speed Imaging and Acoustic Pressure Measurements

Physics of Fluids 

Dual frequency Ultrasonic Cavitation in Various Liquids: High-Speed Imaging and Acoustic Pressure Measurements 

by Justin Morton, Mohammad Khavari, Abhinav Priyadarshi, Amanpreet Kaur, Nicole Grobert, J. Mi, Kyriakos Porfyrakis, Paul Prentice, Dmitry Eskin, & Iakovos Tzanakis 

Abstract 

Ultrasonic cavitation is used in various processes and applications, utilizing powerful shock waves and high-speed liquid jets generated by the collapsing bubbles. Typically, a single frequency source is used to produce the desired effects. However, optimization of the efficiency of ultrasound reactors is necessary to improve cavitation activity in specific applications such as for the exfoliation of two dimensional (2D) materials. This research takes the next step to investigate the effect of a dual frequency transducer system on the bubble dynamics, cavitation zone, pressure fields, acoustic spectra and induced shock waves for four liquids with a range of physical properties. Using ultra-high-speed imaging and synchronized acoustic pressure measurements, the effect of ultrasonic dual frequencies on bubble dynamics was investigated. The addition of a high frequency transducer (1174 kHz) showed that the bubble fragments and satellite bubbles induced from a low frequency transducer (24 kHz) were able to extend their lifecycle, increase spatial distribution, thus, extending the boundaries of the cavitation zone. Furthermore, this combination of ultrasonic frequencies generated higher acoustic pressures (up to 180%) and enhanced the characteristic shock wave peak, indicating more bubble collapses and the generation of additional shock waves. The dual frequency system also enlarged the cavitation cloud size under the sonotrode. These observations specifically delineated the enhancement of cavitation activity using a dual frequency system pivotal for optimization of existing cavitation-based processing technologies. 

Figure 1. a) Schematic of the chamber dimensions, transducer membrane, sonotrode and high-speed imaging setup. The laser illumination passed through the chamber walls and into the Shimadzu Hyper Vision HPV-X2 camera lens where the image was then resolved and processed on the PC unit. b) The same schematic using a Photron camera instead for high-speed imaging, and illumination provided by a powerful LED flash lamp. An FOH connected to a PCI card collected acoustic measurements, synchronized with high-speed imaging.