  The thermodynamic state of lipid interfaces was observed during shock wave
induced cavitation in water with sub-microsecond resolution, using the emission
spectra of hydration-sensitive fluorescent probes co-localized at the
interface. The experiments show that the cavitation threshold is lowest near a
phase transition of the lipid interface. The cavitation collapse time and the
maximum state change during cavitation are found to be a function of both the
driving pressure and the initial state of the lipid interface. The experiments
show dehydration and crystallization of lipids during the expansion phase of
cavitation, suggesting that the heat of vaporization is absorbed from within
the interface, which is adiabatically uncoupled from the free water. The study
underlines the critical role of the thermodynamic state of the interface in
cavitation dynamics, which has mechanistic implications for ultrasound-mediated
drug delivery, acoustic nerve stimulation, ultrasound contrast agents, and the
nucleation of ice during cavitation.
