Acoustic vaporization of perfluorohexane droplets is induced by heterogeneous nucleation at 1.1 MHz

Abstract

We investigated the ADV of perfluorohexane (PFH) nano and micro-droplets at a frequency of 1.1 MHz, at conditions where there is no superharmonic focusing. Our experiments were performed on suspensions of droplets in glycerol to avoid sedimentation. The ADV pressure threshold was defined as the pressure for which there is half a chance to observe a vaporization event. Our experiments showed that the threshold depends on the number of insonified droplets and the data were fitted by a statistical model, that is independant on the mechanism leading to ADV. The fit allows the derivation of the threshold in the case of a single insonified droplet. We further observed that the value of the ADV pressure threshold of a single PFH droplet decreases as its radius increases. In addition, at a constant droplet radius, the threshold did not vary when the PFH volume inside the droplet decreases, thanks to the encapsulation of an increasing number of water droplets. These results are incompatible with a model of homogeneous nucleation (where a nucleus can appear anywhere into the PFH volume). However, we developed a heterogeneous nucleation model, where the nucleus appears at the inner droplet surface, that successfully predicts our experimental ADV results.