Diffusivity of Small (< 10 nm) Nanoparticles in Air by Fully Atomistic Molecular Dynamics

Katerina Karadima, Dimitrios Tsalikis, Vlasis Mavrantzas, SOTIRIS E. PRATSINIS, ETH Zurich, Switzerland

     Abstract Number: 30
     Working Group: Aerosol Physics

Abstract
Despite rigorous treatments of nanoparticle (NP) dynamics in gases, the NP diffusivity in the crossover regime from molecules to small NPs remains still poorly understood. Experiments in the past have attempted to address this regime by employing micron-sized particles and operating at low pressures in order to maintain the large Knudsen numbers (Kn > 10) in this regime. Here diffusion coefficients of tiny (from 0.4 to about 7 nm in size) fullerene and silica particles in air are obtained from molecular dynamics (MD) simulations wherein both particles and gas molecules are considered in their full atomistic representation [1]. The MD-derived diffusivities show systematic deviations from the Stokes-Cunningham-Millikan equation which become most pronounced as the nanoparticle size approaches that of gas molecules. A better description of the simulation results for particles larger than 2 nm is offered by the Epstein-based modification of Fernández de la Mora et al. [2], which makes use of the collision diameter instead of the equivalent spherical diameter of the particle and assumes inelastic collisions of rigid bodies captured through an arbitrary accommodation coefficient. The diffusivities for particles below 2 nm are not described well by the Tammet [3] and other formulations either. A modification of the Stokes-Cunningham-Millikan expression is proposed, which can describe NP diffusivity accurately from the free molecular to continuum regime.

[1] Tsalikis, D.G., Mavrantzas, V.G., and Pratsinis, S.E. (2023). Dynamics of molecular collisions in air and its mean free path. Phys. Fluids 35, 097131.
[2] Fernández de la Mora, J., De Juan, L., Liedtke, K., and Schmidt-Ott, A. (2003). Mass and size determination of nanometer particles by means of mobility analysis and focused impaction. J. Aerosol Sci. 34, 79–98.
[3] Tammet, H. (1995). Size and mobility of nanometer particles, clusters and ions. J. Aerosol Sci. 26, 459–475.