Effects of Acoustic Parameters and Bulk Fluid Properties on Acoustic Droplet Vaporization Threshold of Perfluoropentane Droplets

Phase shift droplets vaporizable by acoustic stimulation are better alternatives to conventional ultrasound contrast agents in terms of higher stability and possibility of achieving smaller sizes. This study determines the threshold pressures required to induce acoustic droplet vaporization (ADV) and inertial cavitation (IC) of a suspension of droplets with a perfluoropentane (PFP) core (diameter 400-3000 nm) in a novel tubeless setup using acoustic methods. We further investigate the effects of excitation frequency, pulse lengths, temperature and dissolved gas concentration on both ADV and IC thresholds. We have found that ADV threshold decreases with temperature and increases with frequency. ADV thresholds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholds indicating that phase transition precedes inertial cavitation. The ADV and the IC thresholds did not change when the experiments were performed in degassed conditions. The scattered response from droplets above the ADV threshold was also found to qualitatively match with that of independently prepared lipid-coated microbubble suspensions in magnitude as well as trends.