Suspensions of plate-like particles with slip can display negative intrinsic viscosity*

It is generally accepted in suspension rheology that the effective viscosity of a suspension of rigid particles increases with solid loading. Challenging this assumption by designing solute particles that reduce the viscosity of the solvent, i.e. negative intrinsic viscosity suspensions, would enable to add useful properties to a fluid without the penalty of increased friction and viscous loss. Through a combination of MD simulations and micro-hydrodynamic theory, we have demonstrated a new class of viscosity-reducing suspensions, obtained by a mere change of particle shape and a judicious choice of surface hydrodynamic slip properties and aspect ratio of the particles. Key features are a plate-like particle shape and a hydrodynamic slip length larger than the thickness of the particle. The effective viscosity reduction, consequence of the change in rotational dynamics [1] (Kamal et al,., Nat. Comm., 11(1), 2020; Kamal et al., J. Fluid Mech. (19, 2021), has been analyzed in our group via i) numerical calculation of the (dilute limit) particle-stress for plate-like particles with Navier slip at their surface, ii) molecular dynamics simulations of few-layer graphene nanoparticles in water and other solvents, and iii) dynamic multi-particle Boundary Integral simulations for increasing solid fractions. The results indicate that there is a practically realizable geometric and concentration range in which a suspension of plate-like particles can have a viscosity significantly smaller than that of the suspending fluid, and that this range may be realizable with suspensions of 2D nanomaterials such as graphene.