Tuning vaporization threshold of perfluorocarbon by interfacial melting in endoskeletal droplets

Perfluorocarbon (FC) droplets have been extensively used as phase-change contrast agents for biomedical ultrasound imaging and therapy. Several studies have aimed at understanding the vapor embryo nucleation and vaporization behavior of these droplets. However, these studies have not looked at tuning the thermodynamic limit of stability (spinodal) by using multiphase mixtures. We investigated the vaporization behavior of endoskeletal perfluoropentane (C₅F₁₂) droplets by incorporating solid FC and solid hydrocarbon (HC) skeletons. Multiple geometries were generated, including endoskeletal (solid-in-liquid) as well as exoskeletal (liquid-in-solid) droplets. Vaporization of these droplets was measured over a range of temperatures both optically using a microscope and acoustically with clinical ultrasound scanner. We show that C₅F₁₂ stability can be tuned by controlling the intermolecular interactions, as captured quantitatively by the exchange parameter. Using a simple statistical thermodynamics lattice model, we demonstrate that the presence of the FC strengthens the intermolecular attraction and the presence of HC breaks the intermolecular attraction between the liquid molecules, making it possible to finely tune the spinodal and consequently the vaporization temperature.