Intermonolayer friction and shear viscosity of lipid membranes by molecular dynamics simulations

The flow behaviour of lipid bilayer membranes is characterized by a surface shear viscosity for in-plane shear deformations and an intermonolayer friction coefficient for slip between the two leaflets of the bilayer. Experimental measurements of these properties are delicate and rely on theoretical interpretations which are still under debate. We present molecular dynamics simulations of lipid membranes exposed to perpendicular and parallel shear flows, which provide direct access to the shear viscosity and the intermonolayer friction, respectively. 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. Having established the flow properties from non-equilibrium simulations, we are now in a convenient position to validate the available theories coupling flow properties to equilibrium dynamics: the Saffman theory relating tracer-diffusion to the shear viscosity, and the Seifert-Langer theory relating undulatory dynamics to intermonolayer friction. The simulation results are also compared against the available experimental data.