The flow behavior of lipid bilayer membranes is characterized by a surface viscosity for in-plane shear deformations, and an intermonolayer friction coefficient for slip between the two leaflets of the bilayer. Both properties have been studied for a variety of coarse-grained double-tailed model lipids, using equilibrium and nonequilibrium molecular dynamics simulations. For lipids with two identical tails, the surface shear viscosity rises rapidly with tail length, while the intermonolayer friction coefficient is less sensitive to the tail length. Interdigitation of lipid tails across the bilayer midsurface, as observed for lipids with two distinct tails, strongly enhances the intermonolayer friction coefficient, but hardly affects the surface shear viscosity. The simulation results are compared against the available experimental data.

The flow properties of an amphiphilic bilayer are studied in molecular dynamics simulations, by exposing a coarse grained model bilayer to two shear flows directed along the bilayer surface. The first field, with a vorticity perpendicular to the bilayer, induces a regular shear deformation, allowing a direct calculation of the surface viscosity. In experiments this property is measured indirectly, by relating it to the diffusion coefficient of a tracer particle through the Saffman-Einstein expression. The current calculations provide an independent test of this relation. The second flow field, with a vorticity parallel to the bilayer, causes the two constituent monolayers to slide past one another, yielding the interlayer friction coefficient.

The time correlation functions of the thermal undulations of a lipid membrane have been studied by molecular dynamics simulations of a coarse-grained bilayer model. We observe a double-exponential decay, with relaxation rates in good agreement with the theory by Seifert and Langer, [Europhys. Lett. 23, 71 (1993)]. Intermonolayer friction resulting from local velocity differences between the two monolayers is shown to be the dominant dissipative mechanism for fluctuations with wave lengths below approximately -0.1 microm.

The relaxation processes of height undulations and density fluctuations in a membrane have been studied by molecular dynamics simulations of a coarse grained amphiphilic bilayer model. We observe a double exponential decay in their time correlations, with relaxation rates in good quantitative agreement with the theory by Seifert and Langer [Europhys. Lett. 23, 71 (1993)]. Intermonolayer friction due to slippage between the two monolayers is shown to be the dominant dissipative mechanism at the high wave numbers, q>10 mum(-1), typically encountered in computer simulations. We briefly discuss the ramifications of the slow undulatory relaxation process for the calculation of bending rigidities from the static undulation structure factors. The relaxation rates are sensitive to the surface tension, and at high elongations an oscillatory contribution is observed in the time correlation of the undulations.

A wormlike micelle of coarse-grained amphiphilic molecules is simulated with molecular dynamics. We demonstrate that our worm is inherently stable, i.e., it does not depend on periodic boundary conditions for its continued survival, which sets it apart from some, and perhaps even all, previously simulated worms. The worms are observed to buckle under sufficiently strong compression forces. The persistence length and bending rigidity follow from analyzing the thermal undulations of a tensionless worm. System size dependencies of the elastic modulus of the worm, as reported for amphiphilic bilayers, are eliminated by explicitly calculating the arc length of the worm.