What it takes to break a liquid: analysis of the cavitation threshold in various media

Abstract

Cavitation has historically been related to parameters measured at equilibrium, such as vapor pressure and surface tension. However, nucleation might occur when the liquid is metastable, especially for fast phenomena such as cavitation induced by high-frequency acoustic waves. This is one of the reasons for the large discrepancy between the experimental estimate of the cavitation threshold and the theory's predictions. Our investigation aims to identify nucleation thresholds in various substances characterized by different physical properties. The experiments were performed by initiating nucleation through ultrasound at 24 kHz. The cavitation onset was studied using a novel procedure based on high-speed imaging and acoustic measurements with a hydrophone. Combining these two techniques allowed us to define the exact instant cavitation occurred in the liquid medium. The bubble nucleation was framed at 200,000 fps with a spatial resolution in the order of micrometers. Such fine temporal and spatial resolutions allowed us to track the expansion of the cavitation bubble right after its onset. We tested five different substances and tracked the amplitude of the transducer oscillation to reconstruct the pressure field when cavitation occurs. This allows us to identify the liquid's acoustic cavitation threshold (tensile strength). The data collected confirmed that the vapor pressure is not a good indicator of the occurrence of cavitation for acoustic systems. Furthermore, all substances exhibit similar behavior despite their different physical properties. This might seem counterintuitive, but it sheds light on the nucleation mechanism that originates cavitation in a lab-scale acoustic system.