Anders Malthe-Sørenssen - Negative coefficient of friction in graphene-based metamaterials

The friction force between two objects in contact typically increases linearly with the applied normal force, and the constant of proportionality is called the coefficient  of friction. Most materials have a positive coeffcient of friction, but the friction force as a function of normal force does not have to be strictly increasing. In regions where the friction force decreases with increasing normal force, the local or differential coeffcient of friction is negative. Materials with negative coefficients of friction have been found, but typically with local coefficient of frictions that are much smaller than 0.01. Here, we show that by introducing systematic cuts in a thin graphene sheet, the frictional behavior of the graphene sheet becomes strongly dependent on the strain applied to the sheet. By constructing a simple nanomachine this effect can be turned into a system with a local negative coeffcient of friction with a coefficient of friction of about -0.1. We argue that this can be explained by changes in the commensurability between the graphene sheet and the substrate. These results indicate that elaborately constructed sheet-based metamaterials can have surprising and noval frictional behaviors.