Atomistic mechanisms of viscosity in 2D liquid-like fluids

Viscosity reflects the resistance of a fluid to flow and plays a fundamental role in fluid dynamics across several scales. Still, its microscopic mechanisms at the individual particle level remain a subject of ongoing research. Here, we systematically investigate the shear viscosity of two-dimensional (2D) simple fluids using computer simulations of three different systems. We propose a simple formula for the shear viscosity that is solely determined by the lifetime of local atomic connectivity τ_LC, namely the average time an atom remains bonded with its neighbors, and the average particle velocity. The derived analytical expression shows excellent agreement with the simulation data. We also construct a model for τ_LC based on the local atomic structure and we show that the microscopic length scale associated to viscosity directly determines the propagation limit of collective shear waves in liquids, linking atomic motion to collective dynamics.