The molecular dynamics simulation of boundary-layer lubrication

Molecular dynamics simulations have been performed on bilayers of dioctadecyldimethyl-ammonium chloride adsorbed between two solid surfaces. The separation of the layers is reduced so that the alkyl chains overlap to produce a repulsive normal pressure Pzz . The layers are then sheared at a constant velocity of between 1 ms-1 and 100 ms-1 and the resulting pressure Pxz across the boundary and in the direction of the shear, is calculated. This method provides a measure of the friction coefficient μ = |Pxz |/Pzz . The friction coefficient has been calculated as a function of the surface density of the amphiphiles, the relative shearing velocity and the applied normal force. This friction coefficient decreases with increasing normal force, increases with decreasing amphiphile density, and increases with increasing shearing velocity. Results are given for the translational, orientational and conformational structure of the bilayer under shear.