Aerosol Particle Diffusivity in the Free Molecule Regime

The aerosol nanoparticle (NP) diffusivity in the crossover regime from molecules to tiny (<5 nm) NPs is still in question despite the prime significance of this regime for nanotechnology as well as for aerosol fundamentals: nucleation rate, transport, coagulation, and condensation in the free molecular regime. Experiments in the past have attempted to address this regime by employing micron-sized particles and operating at low pressures to attain the large Knudsen numbers (Kn > 10), characteristic for this regime. However, such efforts miss the atomic level interactions between aerosol particles and surrounding gas molecules. Such interactions are dominant at the low end of the nanoscale. Here, diffusion coefficients of tiny (from 0.4 to about 7 nm in diameter) fullerene and silica particles in air are obtained by molecular dynamics (MD) simulations wherein both particles and gas molecules are considered in their full atomistic representation (force field and shape). Below 3 nm, these MD-derived diffusivities are in excellent agreement with an experimentally based equation for gas diffusivities but show systematic deviations from the classic Epstein and Stokes-Cunningham-Millikan (SCM) equations for particle diffusivity. These deviations become most pronounced as the NP size approaches that of gas molecules. Above 5 nm, the MD-derived diffusivities nicely converge to these equations. These diffusivities are compared also to other literature equations for particle diffusivity in this size regime at ambient conditions.