State changes in lipid interfaces observed during cavitation

Here we investigate the cavitation phenomenon at a lipid interface of multilaminar vesicles (MLVs) subjected to acoustic shock waves. The lipid membranes contain a fluorescent dye, Laurdan, which produces a fluorescence emission sensitive to the thermodynamic state of the interface. Fluorescence emissions were measured at 438nm and 470nm using two photomultiplier tubes (with 8 MHz bandwidth) from which the temporal evolution of the interface’s thermodynamic state was determined with submicrosecond resolution. Acoustic emissions were recorded simultaneously in order to detect the presence of cavitation. Different lipids were used to prepare the MLVs in order to observe cavitation phenomenon as a function of the state of the interface. It was deduced that the interface behaves as an adiabatic system decoupled from the bulk, where the entropy increase due to vaporization during cavitation is compensated by the entropy decrease resulting from condensation and dehydration of the lipids. These results show that cavitation physics critically depends on the thermodynamics of the interface. While applied here on a simple system of pure lipid MLVs, the thermodynamic approach is applicable to native biological membranes and cavitation phenomenon in general. [Work supported by UK EPSRC EP/L024012/1.]