An Atomic-Scale Simulation of Lubricated Motion

Alkyl chain monolayers and bilayers are investigated by static atomistic simulation. Optimisation of the chain conformations by imposing sets of random torsions leads mainly to all-trans structures. The relaxations of Y-configuration bilayers are investigated after imposing mutual displacements to the interface. Energy profiles for sliding motions of the bilayer show periodic behaviour from which the friction associated with the motion is related to the maximum force in each cycle of the profile. This is consistent with the observation that the lubricating action of an alkyl chain increases with the length of the chain. Derived shear strength values are reasonably close to experimental measurement. For small displacement intervals the profiles showed sharp energy spikes that are analogous to the ‘stick-slip’ discontinuities found in low-speed surface force studies of molecular layers. These are discussed in terms of low-lying metastable states from which, on further displacement, the system relaxes to the ground state.