Nonlinear propagation of two dimensional sound waves observed at lipid interfaces

Abstract

Experimental results are presented on the acoustic propagation of mechanical perturbations in a lipid monolayer along the air-water interface. The interface was excited by a piezo-cantilever, and propagating impulses were measured optically using Forster Resonance Energy Transfer (FRET). The velocity of propagation varied from 0.1 to 1 m/s depending on the compressibility of the interface. Near a nonlinearity in the state diagram of the interface, for example, near a phase transition of the lipids, impulses propagated only when the initial mechanical impulse was greater than a certain threshold. In fact, the impulse then propagated as a solitary shock wave causing local adiabatic phase transition of the lipid molecules along the way. Although the phenomenon has been observed here in pure lipid monolayers, which represent a well-documented model for biological membranes, the origin of the observed phenomenon lies in the conservation of the entropy of the interface, determined by the change in state of the interface during the impulse. Given that the state diagrams of biological membranes have nonlinearities near physiological conditions, nonlinear sound waves are expected to be fundamentally involved in inter and intra cellular communication. Indeed, the observed acoustic phenomenon is characteristically similar to nerve impulses.