  Acoustically vaporizable droplets are phase-change agents that can improve
the effectiveness of ultrasound-based therapies. In this study, we demonstrate
that the compression part of an acoustic wave can generate tension that
initiates the vaporization. This counter-intuitive process is explained by the
occurrence of Gouy phase shift due to the focusing of the acoustic wave inside
the droplet. Our analysis unifies the existing theories for acoustic droplet
vaporization under a single framework and is supported by experiments and
simulations. We use our theory to identify governing parameters that allow to
vaporize droplets using predominantly compression waves, which are safer in
medical use.
