Theoretical and experimental investigation of the vaporisation of superheated perfluorocarbon droplets

Acoustically activatable “nano”droplets consisting of superheated liquid perfluorocarbons have been investigated for both imaging and therapeutic applications. They offer longer circulation half-lives, higher surface area to volume ratio and the ability to perfuse the microvasculature more easily than gas-filled microbubbles. Optimising droplet formulations to avoid spontaneous vaporisation whilst keeping the peak negative pressure required for acoustic activation within the clinically safe range has proved challenging. The aim of this study was to derive a theoretical model to predict the probablity of droplet vaporisation under different conditions, the resulting bubble dynamics and radiated pressure; and compare the results with observations made by optical microscopy including high speed video. It was found that spontaneous vaporisation of perfluoropropane droplets occurred readily in serum at 37 °C in the absence of ultrasound excitation. Higher molecular weight perflurocarbons required acoustic activation and experimentally measured rates were higher than those predicted by modelling of homogeneous nucelation alone. The results suggest that droplet aggregation and heterogeneous nucleation play important roles in droplet vaporisation and should be accounted for in selecting appropriate ultrasound exposure conditions. The prevalence of spontaneous vaporisation may also have important safety implications for clinical applications of low boiling point perfluorocarbon droplets.