Sedimentation of C60 and C70: Testing the Limits of Stokes' Law

The Stokes-Einstein-Sutherland Equation at the Nanoscale Revisited

The Stokes-Einstein-Sutherland (SES) equation is at the foundation of statistical physics, relating a particle's diffusion coefficient and size with the fluid viscosity, temperature, and the boundary condition for the particle-solvent interface. It is assumed that it relies on the separation of scales between the particle and the solvent, hence it is expected to break down for diffusive transport on the molecular scale. This assumption is however challenged by a number of experimental studies showing a remarkably small, if any, violation, while simulations systematically report the opposite. To understand these discrepancies, analytical ultracentrifugation experiments are combined with molecular simulations, both performed at unprecedented accuracies, to study the transport of buckminsterfullerene C60 in toluene at infinite dilution. This system is demonstrated to clearly violate the conditions of slow momentum relaxation. Yet, through a linear response to a constant force, the SES equation can be recovered in the long time limit with no more than 4% uncertainty both in experiments and in simulations. This nonetheless requires partial slip on the particle interface, extracted consistently from all the data. These results, thus, resolve a long-standing discussion on the validity and limits of the SES equation at the molecular scale.

Properties and tribological evaluation of graphene and fullerene nanoparticles as additives in oil lubrication

In this study, SAE-0W20 engine oil was mixed with graphene and fullerene nanoparticles. The goal of this study was to evaluate and compare the effects of different carbon nanoparticles on the thermal, rheological, and tribological properties of engine oil, such as thermal degradation, viscosity, friction, and wear. Using a two-step process, graphene and fullerene nanostructures were dispersed in low-viscosity SAE-0W20 engine oil at a concentration of 0.05 wt.%. The friction and wear characteristics were evaluated in a customized cylindrical block-on-ring tribology test according to the ASTM G77 standard. Graphene and fullerene nanoparticles protect contact surfaces by forming a very thin protective film between moving mechanical parts thus resulting in wear and friction reduction. The results showed graphene nanoparticles have improved significantly the tribological performance of SAE-0W20 engine oil.