Dynamics of molecular collisions in air and its mean free path

Abstract

The mechanics and dynamics of molecular collisions in air are investigated by thoroughly validated atomistic molecular dynamics (MD) simulations that treat oxygen and nitrogen as true diatomic molecules accounting for their non-spherical shape and, most importantly, force field. Due to their rotational motion and non-spherical shape, molecules follow complex trajectories at close enough separations experiencing a great variety of collision events. Most of the collisions are bimolecular. However, some can involve up to four molecules as pairs (or even triplets) of molecules that collide repeatedly are observed. Following their initial encounter, these molecules separate briefly, come back, and collide again and again creating even “orbiting” collisions, before they split apart to collide with other molecules. Identifying such rather spurious collisions and filtering them by hazard plot analysis was a key step in correctly determining collision densities and accumulating collision event statistics. By systematically recording the distribution of free paths (distances traveled by molecules between genuine collisions), the mean free path, λ, of air is determined as 38.5 ± 1 nm at 300 K and 1 atm. This is 43% smaller than the 67.3 nm widely accepted λ today at these conditions and quite robust to the employed MD force field as long as it accurately matches the experimentally determined macroscopic properties of air (density, viscosity, and diffusivity).