Nucleation of Bubbles in Perfluoropentane Droplets Under Ultrasonic Excitation
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Lipid coated perfluorocarbon (PFC) nanodroplets can be vaporized by an external ultrasound pulse to generate bubbles. These bubbles produced in situ have potential applications in tumor imaging and drug delivery. Here we employ the classical nucleation theory (CNT) to investigate the peak negative pressure required to induce acoustic droplet vaporization (ADV). The theoretical analysis predicts that the ADV threshold increases with increasing surface tension of the droplet core and increasing excitation frequency. It decreases with increasing temperature and droplet size. The predictions are in qualitative agreement with experimental observations. We also estimate and discuss the energy required to form critical cluster to argue that nucleation occurs inside the droplet, as was also observed by high-speed cameras. To find the stability of these PFC droplets to temperature fluctuations, their limit of superheat was also calculated. These CNT based theoretical predictions prove to be a promising tool to design phase-shift nano emulsions with low threshold pressures.