Viscous peeling of a graphene sheet*

To get insights into the process of liquid-phase exfoliation of graphite into graphene, we study numerically and theoretically the dynamics of a peeling front in a system of two adhered carbon nanosheets immersed in water. The crack propagation is induced by lifting one of the edges with an assigned velocity v. A continuum model based on the equation of the elastica coupled with a model for the hydrodynamic pressure is compared to non-equilibrium molecular dynamics (MD) simulations of a graphene-water system. We quantify the viscous-dependent contribution to the external peeling force as a function of v, as separated from the adhesive contribution.
Contrary to primary expectations, we find that the lubrication forces have a negligible effect on the peeling force owing to the large slip length characterising our system. The inclusion of an entrance pressure drop due to converging flow streamlines upstream of the peeling edge is found to be necessary. Despite this inclusion, while the shape of the sheet in MD agrees with that in the continuum model, the viscous contribution to the peeling force predicted by MD is larger than that predicted by the continuum model. The mechanism underlying this discrepancy is explained by studying the drag forces on the peeling edge.