Negative Intrinsic Viscosity in Graphene Nanoparticle Suspensions Induced by Hydrodynamic Slip

Area of Science:

• Fluid dynamics
• Materials science
• Nanotechnology

Background:

• Nanoparticle suspensions typically increase in viscosity with concentration.
• Atomically thin nanomaterials like graphene exhibit anomalous viscosity behavior, sometimes showing lower viscosity than the solvent.
• This phenomenon, termed negative intrinsic viscosity, has been observed experimentally but lacks molecular-level confirmation.

Purpose of the Study:

• To investigate the shear viscosity of graphene nanosheet suspensions in water using molecular dynamics simulations.
• To confirm and elucidate the molecular mechanisms behind negative intrinsic viscosity in realistic graphene-water systems.
• To explore the influence of graphene aspect ratio and concentration on suspension viscosity.

Main Methods:

• Molecular dynamics (MD) simulations of few-nanometer graphene sheets in water at high Péclet numbers (Pe ≥ 100).
• Varying graphene aspect ratios from 4.5 to 12.0.
• Comparison with continuum boundary integral modeling in the dilute regime.

Main Results:

• Intrinsic viscosity decreases with increasing graphene aspect ratio, becoming negative beyond an aspect ratio of approximately 5.5.
• Hydrodynamic slip at the graphene-water interface was identified as the primary cause, suppressing particle rotation and promoting alignment with flow.
• Viscosity initially decreases with increasing graphene concentration, falling below that of pure water, before increasing due to particle aggregation at higher concentrations.

Conclusions:

• Molecular dynamics simulations robustly confirm negative intrinsic viscosity in graphene-water systems, driven by hydrodynamic slip.
• The findings provide a molecular-level understanding of anomalous viscosity in atomically thin nanomaterial suspensions.
• This research has significant implications for developing novel lubricants, inks, and nanocomposites with tunable rheological properties.