A model for shear-induced breakup of layered particles and application to graphene production by turbulent exfoliation and microplastics erosion

Suspending particles in a turbulent flow can lead to shear-induced particle breakup. While models for breakup of colloidal aggregates in turbulence exist, models for exfoliation of layered material, in which each particle is composed of stacks of sheets, are not available. The development of such models is crucial to understand the formation of graphene sheets in the process of liquid-phase exfoliation. By analysing data from Molecular Dynamic simulations of graphene multilayers and boundary integral calculations in Stokes flow, we have recently proven that for relatively short particles a mechanism of exfoliation of graphene is the sliding of parallel sheets induced, primarily, to the effect of viscous shear forces on the planar surfaces of the particles and normal stresses on the edges of the particles (Gravelle et al, J. Chem. Phys. 152, 2020). Such model is accurate for particles whose length is in the rage of a few tens of nanometers, but gives predictions for the critical shear rate for exfolaition that are abnormally large in comparison to those seen in experiments when applied to micron sized particles. This talk will discuss an alternative model based on shear-induced peeling that predicts a dependence of the critical shear rate for exfoliation on the bending rigidity of the particle and smaller values of the critical shear rate. We discuss this model based on both purely theoretical calculations and experiments carried out in a shear cell by attaching flexible sheets to a rigid wall . Both experiments and theory predict a critical shear rate that depends very weakly on the adhesion energy between the layers. We show that this surprising effect is due to the large dependence of the curvature of the sheet on the applied viscous shear rate in the neighbourhood of a critical shear rate value, and the associated strong dependence of the elastic energy release rate of the deformable sheet. Our models can be used for point-particle simulations of layered material breakup in turbulent flows, where the shear rate will be the ambient shear rate of the small eddies where each layered particle instantaneously resides. Such models could also be used to predict flaking of microplastics in the ocean.