Non-equilibrium activation dynamics of superheated phase-change contrast agents

Phase-change contrast agents, such as superheated perfluorocarbon droplets, potentially offer exciting new opportunities in diagnostic and therapeutic applications of ultrasound. An important advantage of droplets over, e.g., microbubble agents is that they can be “activated” (vaporised) by a reduction in ambient pressure, initiated by an externally applied ultrasound pulse. Controlling the timing and location of droplet activation, however, is vitally important to avoid safety concerns. Current theoretical models for droplet vaporisation assume that a critical stable bubble embryo forms that is in thermal and mechanical equilibrium. Model predictions are, however, inconsistent with some experimental observations. The aim of this study is to develop a non-equilibrium model for droplet activation wherein initial bubble growth rate and vapour pressure are dependent upon the bubble nucleus size. We demonstrate how incorporating nonequilibrium effects into theoretical models of droplet vaporisation dynamics enables better agreement of the droplet expansion with experimental data and thus provide greater insight into control of the activation process.