Predicting the spontaneous vaporisation of superheated perfluorocarbon droplets

Superheated perfluorocarbon droplets have been widely explored as agents for ultrasound imaging and therapy, as well as for other applications such as radiation dosimetry. Submicrometre, or “nano” droplets offer a number of potential advantages over microbubbles, e.g., longer circulation half-lives, higher surface area-to-volume ratio and the ability to perfuse the microvasculature more easily. A key challenge in the use of nanodroplets, however, is the need to avoid spontaneous vaporisation whilst keeping the energy required for acoustic activation within the range of pressures that can be used safely in humans. This is especially important for imaging applications. Perfluoropropane (C;3F;8) microbubbles can be condensed to form liquid nanodroplets that offer a good trade-off between thermal stability and acoustic vaporisation threshold. Anecdotal reports, however, suggest that C;3F;8 droplets can spontaneously vaporise, and may therefore pose a potential safety risk, especially if bubble coalescence occurs. The aim of this study was to build on recent theoretical models of droplet vaporisation and investigate the probability of vaporisation as a function of temperature and interfacial tension. The results are compared with experimental measurements of vaporisation rates for different droplet formulations.