# Negative Intrinsic Viscosity in Graphene Nanoparticle Suspensions Induced by Hydrodynamic Slip

## TL;DR

Graphene nanoparticle suspensions can have lower viscosity than pure water due to hydrodynamic slip, which could help design better lubricants and inks.

## Contribution

Molecular simulations confirm negative intrinsic viscosity in graphene suspensions due to hydrodynamic slip at the liquid–solid interface.

## Key findings

- Intrinsic viscosity becomes negative for graphene sheets with aspect ratios above ≈5.5.
- Hydrodynamic slip suppresses particle rotation and reduces viscous dissipation.
- Viscosity initially decreases with concentration in the dilute regime before rising due to aggregation.

## Abstract

The viscosity of nanoparticle suspensions is always expected
to
increase with particle concentration. However, a growing body of experiments
on suspensions of atomically thin nanomaterials such as graphene contradicts
this expectation. Some experiments indicate effective suspension viscosities
below that of pure solvent at high shear rates and low solid concentrations,
i.e., the intrinsic viscosity is negative. Using molecular dynamics
simulations, we investigate the shear viscosity of few-nanometer graphene
sheets in water at high Péclet numbers (Pe ≥ 100), for
aspect ratios from 4.5 to 12.0. These simulations robustly confirm
that the intrinsic viscosity decreases with increasing aspect ratio
and becomes negative beyond a threshold ≈5.5, providing a molecular-level
confirmation of this behavior in a realistic graphene–water
system. Comparison with continuum boundary integral modeling shows
quantitative agreement in the dilute regime, confirming the effect
is hydrodynamic in origin. We demonstrate that this anomalous behavior
originates from hydrodynamic slip at the liquid–solid interface,
which suppresses particle rotation and promotes stable alignment with
the flow direction, thereby reducing viscous dissipation relative
to dissipation in pure solvent. This slip mechanism holds for both
fully 3D disc-like and quasi-2D particle geometries explored in the
molecular simulations. As the concentration of graphene particles
increases in the dilute regime, the viscosity initially decreases,
falling below that of pure water. At higher concentrations, however,
particle aggregation becomes significant, leading to a rise in viscosity
after a minimum is reached. Our work has important implications for
the design of lubricants, inks, and nanocomposites with tunable viscosity.