  Liquid-phase exfoliation, the use of a sheared liquid to delaminate graphite
into few-layer graphene, is a promising technique for the large-scale
production of graphene. But the micro and nanoscale fluid-structure processes
controlling the exfoliation are not fully understood. Here we perform
non-equilibrium molecular dynamics simulations of a defect-free graphite
nanoplatelet suspended in a shear flow and measure the critical shear rate
$\dot \gamma_c$ needed for the exfoliation to occur. We compare $\dot \gamma_c$
for different solvents including water and NMP, and nanoplatelets of different
lengths. Using a theoretical model based on a balance between the work done by
viscous shearing forces and the change in interfacial energies upon layer
sliding, we are able to predict the critical shear rates $\dot \gamma_c$
measured in simulations. We find that an accurate prediction of the exfoliation
of short graphite nanoplatelets is possible only if both hydrodynamic slip and
the fluid forces on the graphene edges are considered, and if an accurate value
of the solid-liquid surface energy is used. The commonly used "geometric-mean"
approximation for the solid-liquid energy leads to grossly incorrect
predictions.
