Fluctuation dynamics of spherical vesicles: frustration of regular bulk dissipation into subdiffusive relaxation.

Spherical lipid vesicles obtained by the extrusion method are nonequilibrium membrane structures more curved than the zero spontaneous curvature equilibrium state of the bilayer. Furthermore, these structures are quite rigid as compared to spontaneous vesicles or microemulsion droplets made of soluble surfactants. The dynamical description of the shape fluctuations derived by Milner and Safran (MS) [Phys. Rev. A 36, 4371 (1987)], which is based on the elastic Helfrich energy referred to the equilibrium state, could be misleading in these cases. In the present contribution, shape fluctuations of unilamellar palmitoyl-oleyl-phosphocholine (POPC) vesicles (radius Rh<or=100 nm ) prepared by extrusion are studied by means of neutron spin echo (NSE) in combination with dynamic light scattering (DLS). The relaxation of the fluctuation modes is inferred from the DLS autocorrelation functions and from the intermediate NSE scattering functions measured for several different values of the wave vector. The observed relaxations are compatible with a stretched-exponential decay rather than the single-exponential behavior. Dynamical frustration of the bulk dissipation mechanism in the way described by Zilman and Granek (ZG) for weak amplitude fluctuations [Phys. Rev. Lett. 77, 4788 (1996)] is invoked as a plausible scenario for explaining the subdiffusive nonexponential relaxations experimentally observed. The combined analysis of NSE and DLS data allows a calculation of the bending elastic constant of POPC vesicles kappa=19+/-2 kBT , in excellent agreement with literature data. The ZG approach is revealed as the adequate extension of the MS theory to describe fluctuation dynamics of rigid membranes.