Single-particle stress for a graphene particle

We studied the dynamics of graphene freely-suspended in simple shear flow, using a combination of molecular dynamics simulations and boundary integral simulations. Graphene is a plate-like colloidal particle, whose nanoscopic thickness is comparable to that of a typical liquid solvent molecule. Unusual for plate-like particles (e.g. clays) the thickness is also smaller than the hydrodynamic slip length characterising the relatively velocity at the fluid-solid boundary. This talk will examine the consequence of this geometric regime on the dynamics of graphene particles in shear flow, and on the ensuing rheology. For slip lengths larger than the particle thickness, the rotational dynamics predicted by Jeffery's theory ceases to occur. We recently demonstrate this behaviour by simulating 2D rigid particles that rotate only in the flow-gradient plane as well as 3D particles that have fully 3-dimensional orbits. This effect is also seen for mildly flexible particles and for particle concentrations beyond the classical dilute regime. Our simulations also indicate a substantial drop in suspension viscosity in this slip regime.In the talk, the physical origin of the drop in viscosity will be discussed in the dilute limit for the rigid case, based on the analysis of the single-particle stresslet (moment of hydrodynamic traction) and a further integral term involving the integral of the slip velocity over the particle surface; this last term is usually ignored, as it is zero for non-slip particles, . The results show a non-trivial dependence of both these terms on the ratio of slip length and particle thickness, and on the geometric aspect ratio. We discuss whether negative contributions of the particle to the suspension viscosity are physically possible.