The Impact of Inelastic Collisions on the Mean Free Path in Air

DIMITRIOS TSALIKIS, Vlasis Mavrantzas, Sotiris Pratsinis, ETH Zurich, Switzerland

     Abstract Number: 22
     Working Group: Aerosol Physics

Abstract
The mechanics of molecular collisions in air are investigated by thoroughly-validated atomistic molecular dynamics that treat oxygen and nitrogen as true diatomic molecules accounting for their shape and force field, for the first time to our knowledge. Most of these collisions involve two molecules, but few involve three and even four molecules, a direct result of accounting for their force field and non-spherical shape. Furthermore, colliding molecules can spend appreciable time together before splitting apart, resulting in even spurious collisions when they briefly separate and collide repeatedly without interacting with any other molecule. By excluding such collisions through Hazard plot analysis and systematically recording the distribution of molecular free paths (distance traveled between collisions), the mean free path, λ, in air is determined as 38.5 ± 1 nm at 300 K and 1 atm. This is about 43 % smaller than the 67.3 nm widely accepted λ value today at these conditions in textbooks from the classic kinetic theory of gases. Accounting only for their force field (Lennard-Jones) but assuming spherical oxygen and nitrogen molecules yields a λ of 42.3 ± 1.1 nm, again about 37% smaller than that from kinetic theory. So, accounting for the force field of the molecules is more important than their shape, which is not insignificant either.

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