Shockwave-induced droplet vaporization

Phase-transition of nanometer and micrometer sized droplets to gaseous bubbles using ultrasound (termed acoustic droplet vaporization or ADV) has been studied for a range of biomedical applications, such as targeted drug delivery, transient opening of the blood-brain-barrier, and embolotherapy. Yet, the underlying physics governing ADV is not well understood. It is widely assumed that nucleation in the droplet bulk is caused mainly by the low-pressure portion of the incoming acoustic wave transmitted through the droplet itself. However, recent studies have shown the possibility of phase reversal of spherically converging acoustic waves, which suggest that the compressive part of the acoustic wave may contribute to the droplet vaporization as well. Here, we demonstrate this by performing high-speed videomicroscopy of micrometric droplet vaporization triggered by laser-induced shockwaves, which presents a strong pressure peak and reduced tensile region. These experimental observations, further supported with theoretical modeling, highlight the importance of the compressive field on ultrasound-induced ADV.