# Aerosol Particle Diffusivity in the Free Molecule Regime

## TL;DR

This paper studies how tiny aerosol particles move in air using simulations, revealing how their movement differs from traditional models at very small sizes.

## Contribution

The study provides new molecular dynamics-derived diffusivity data for nanoparticles below 3 nm, showing deviations from classical models.

## Key findings

- MD simulations show diffusivity deviations from classical equations for particles below 3 nm.
- Below 3 nm, MD results align with experimentally based gas diffusivity equations.
- Above 5 nm, MD results match classical Epstein and SCM equations.

## Abstract

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.