Determining the local shear viscosity of a lipid bilayer system by reverse non-equilibrium molecular dynamics simulations.

The parallel shear viscosity of a dipalmitoylphosphatidylcholine (DPPC) bilayer system is studied by reverse non-equilibrium molecular dynamics simulations (RNEMD) with two different united-atom force fields. The results are related to diffusion coefficients and structural distributions obtained by equilibrium molecular simulations. We investigate technical issues of the algorithm in the bilayer setup, namely, the dependence of the velocity profiles on the imposed flux and the influence of the thermostat on the calculated shear viscosity. We introduce the concept of local shear viscosity and investigate its dependence on the slip velocity of the monolayers and the particle density at the headgroup-water interface and the tail-tail interface. With this we demonstrate that the lipid bilayer is more viscous than the surrounding water phase, and that slip takes place near the headgroup region and in the centre of the bilayer where the alkyl tails meet. We also quantify the apparent increase in viscosity of the water molecules entangled at the water-headgroup interface.