Non-Equilibrium Phase Behaviour in Giant Lipid Vesicles Following Very Rapid Temperature Changes

Abstract

Artificial lipid bilayers are useful models of biological membranes, appealing for applicative purposes, and an interesting example of quasi-2D liquid systems on which to investigate new physics. It is well known that ternary-component lipid membranes, depending on the relevant thermodynamic parameters (composition, temperature and pressure), can show phase coexistence between two liquid phases, Lo and Ld, that can be imaged by fluorescence microscopy.We have performed experiments observing both equilibrium and non-equilibrium morphology of lipid phases. Specifically we have investigated a new dynamical regime in which we follow the diffusive mixing of miscible phases, and observe pattern formation out of equilibrium. This has been possible thanks to the development of a method to induce rapid temperature changes, by infrared irradiation. In this fashion, a temperature change can be imposed faster (in about 1s) than the diffusive time over relevant lengthscales (several 10s).Our observations are that the line tension rapidly vanishes upon heating the system above the miscibility transition temperature. After a few seconds, the spectrum of the interface fluctuations becomes very different from the equilibrium capillary waves, growing in amplitude and losing the characteristic 1/wave-vector2 equilibrium form. The interface from then blurs out, resembling fractal growth fronts, until the phases are fully mixed.The same fast temperature change allows us to very rapidly cool the system. This extends previous measurements by other groups, giving insight into processes that take place over the first few seconds of phase separation.Investigating these non-equilibrium conditions is relevant because biological membranes in living systems are not in a steady state: they are continually subject to rapid changes in composition, temperature and curvature, and their response is known to couple into a variety of biochemical processes by mediating protein binding and interactions.