In situ observation of soft X-ray-triggered nanoscale phase transitions in perfluorocarbon microdroplets

Microscale liquid-gas phase transitions are common in nature and have significant applications in ecosystems, chemical engineering, and biomedicine. Understanding the mechanisms behind these transitions is crucial. The amorphous intermediate state, with its disordered structures and metastable properties, is key to the phase transition process. However, traditional techniques are ineffective for achieving in situ detection with multiscale, multiphase, and submicron resolution. The thermodynamic and kinetic processes of forming these states from submicron droplets to bubbles are still not well understood. In this study, we utilized a scanning transmission soft X-ray microscopy technique based on a synchrotron accelerator. For the first time, we achieved high-resolution in-situ observation of the growth and evolution of nanobubbles (NBs) within low-boiling-point perfluorohexane (PFH) microdroplets (less than 10 μm). Size effects profoundly influence phase transition dynamics: as the droplet size decreases, the phase transition is initially promoted and then inhibited. Microdroplets near 1 μm exhibit unique bulk vaporization behavior. Additionally, the nucleation rate of PFH-derived vapor NBs at the gas-liquid interface is significantly higher than in non-interface regions, highlighting the critical role of interface effects. Our approach visualizes nanoscale nucleation dynamics and interface effects, challenging traditional assumptions about size-dependent interfacial phenomena. These findings have significant implications for fields such as targeted drug delivery and environmental remediation.