Friction anisotropy and asymmetry in self assembled monolayers

Experiments with molecularly thin self assembled monolayers have suggested existence of friction force anisotropy and asymmetry. Here molecular dynamics simulations are used to study friction anisotropy and asymmetry between two self assembled monolayers (SAM) of n-alkanethiols [CH3(CH2)(n-1)S, n = 14] attached on Au (111) substrates. The simulations show the existence of friction anisotropy and asymmetry in agreement with some of the experimental works in literature. Friction shows dependence on the relative direction of sliding and monolayers tilt direction. The origin of this friction anisotropy is discussed. The friction asymmetry depends on the direction of sliding and is significant in the direction of the tilt. On this "molecular carpet" interestingly, it is easier to slide against the nap (tilt direction) than in the direction of the nap (tilt direction). (C) 2016 Elsevier Ltd. All rights reserved.