On the mechanism of acoustic droplet vaporization

It has been observed in the past that micrometer sized droplets can be vaporized into gas bubbles by the application of diagnostic ultrasound. This paper will discuss the possible underlying mechanism of acoustic droplet vaporization (ADV) including acoustic cavitation, acoustic heating, shape oscillations during acoustic irradiation, and hydrodynamic cavitation. Experiments were performed on droplet emulsions as well as on single droplets. The vaporization of emulsions was quantified based on gas bubbles generated in a flow tube using a 10 MHz linear imaging army. Single droplets were monitored optically under a microscope and a high speed video system. The frequency dependence of ADV as well as trials with degassed water (40% of saturation) imply that acoustic cavitation is not the mechanism for ADV. Acoustic heating was investigated by exposing the droplet emulsion to repetitive tone bursts (50 Hz to 5 kHz). No significant change in pressure threshold for ADV was observed, however the yield of gas bubbles was used to calculate the single pulse conversion efficiency of ADV to 26%. Droplet shape oscillations causing a non-uniform Laplace pressure, were found to be 15% or less of the droplet diameter. They could be observed at the beginning and at the end of the acoustic irradiation. Observed was a dipole-type motion (1.3 μm amplitude) of irradiated droplets as well as the spot-like onset of vaporization on the axis of oscillation close to a pole of the droplet. It is concluded that because of the high Reynolds number during ADV (4-5×10⁵), the mechanism of vaporization might be based on hydrodynamic effects.