Unraveling the Relevance of Graphene-Fluid Hydrodynamic Coupling on the Exfoliation of Graphite in Water.

Liquid-phase exfoliation via shear flow is a widely adopted technique for the large-scale production of graphene. However, the underlying nano- and microscale exfoliation mechanisms remain poorly understood. In this work, we address this issue by performing hybrid nonequilibrium hydrodynamic simulations of coarse-grained defect-free graphite nanoplatelets immersed in a mesoscopic water fluid via the lattice Boltzmann method. This approach enables us to investigate graphene exfoliation up to 100 nm in length. Nonequilibrium effects, such as tumbling, alignment, and bending, are demonstrated. In particular, we reveal that due to the graphene-fluid hydrodynamic coupling, the graphite dynamics distorts the surrounding shear flow and reduces the local shear stress, thereby leading to an increase in the critical shear rate by a factor of 2 ∼ 4. This statement is fully supported by a theoretical analysis using a force-based criterion, i.e., overcoming the maximum interlayer van der Waals attraction, and hierarchical simulations: athermal and no coupling; athermal and hydrodynamic coupling; and thermal and hydrodynamic coupling. Our work unravels the paramount relevance of hydrodynamic coupling on graphene exfoliation and paves the way toward achieving large-scale nonequilibrium graphene simulations reminiscent of experiments.