Micron-scale liquid domains appear in ternary lipid membranes containing at least three lipid types as long as the membrane is at a temperature below a miscibility transition. When the miscibility transition occurs at a critical point, distinctive phenomena occur. Just below the critical temperature, the edges of domains fluctuate. Just above the critical point, domains are replaced by submicron fluctuations. We find that the size of the largest fluctuations (the correlation length) and their lipid composition (the order parameter) scale in a way that is consistent with predictions from the two-dimensional Ising model (Honerkamp-Smith et al., Biophysical Journal, 2008). Recently, we measured the time scales over which composition fluctuations persist in lipid membranes (arXiv:1104.2613). In particular, we measured the effective dynamic critical exponent relating the decay time of membrane composition fluctuations to the wavenumber. At temperatures far from the critical
point, the exponent is 2, as expected from diffusion. As temperature approaches the critical point, the exponent increases toward a value of 3, so domains persist for longer times than would be expected if the system were governed solely by diffusion. These values are consistent with recent predictions that treat membranes as quasi 2-dimensional systems that are hydrodynamically coupled to bulk solution (Haataja, Phys. Rev. E 2009, Inaura and Fujitani, J. Phys. Soc. Jpn. 77, 114603, 2008). We can also observe effects of this hydrodynamic coupling when we track diffusion and coarsening of domains across the surface of a lipid vesicle.
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