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Aerosol Particle Diffusivity in the Free Molecule Regime.

Katerina S Karadima1,2,3, Dimitris G Tsalikis3, Vlasis G Mavrantzas1,2,3

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Molecular dynamics simulations reveal nanoparticle diffusivity in the critical crossover regime. Tiny nanoparticles below 3 nm deviate from classic models, highlighting the importance of atomic interactions.

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Area of Science:

  • Aerosol science
  • Nanotechnology
  • Computational physics

Background:

  • The diffusivity of aerosol nanoparticles (NPs) in the crossover regime (molecules to <5 nm NPs) is crucial for nanotechnology and aerosol processes like nucleation and transport.
  • Previous experiments using large particles at low pressures did not capture atomic-level interactions dominant at the nanoscale.
  • Understanding NP diffusivity is key for accurate modeling of aerosol behavior in the free molecular regime.

Purpose of the Study:

  • To determine the diffusion coefficients of tiny fullerene and silica nanoparticles (0.4–7 nm) in air using molecular dynamics (MD) simulations.
  • To investigate the validity of existing diffusivity equations (Epstein, SCM) in the critical nanoscale crossover regime.
  • To compare simulation results with experimental data and other literature models for NP diffusivity.

Main Methods:

  • Atomistic molecular dynamics (MD) simulations were employed, considering both NPs and gas molecules with their full force fields and shapes.
  • Diffusion coefficients were calculated for NPs ranging from 0.4 nm to approximately 7 nm in diameter.
  • MD-derived diffusivities were systematically compared against established theoretical models and experimental findings.

Main Results:

  • Below 3 nm, MD-derived diffusivities closely matched experimentally based gas diffusivity equations but deviated significantly from Epstein and SCM equations.
  • These deviations were most pronounced as NP size approached gas molecule size, emphasizing atomic interaction effects.
  • Above 5 nm, the MD-derived diffusivities converged with the predictions of the classic Epstein and SCM equations.

Conclusions:

  • Classic diffusivity equations (Epstein, SCM) are inadequate for NPs below 3 nm due to the dominance of atomic-level interactions.
  • Molecular dynamics simulations provide accurate NP diffusivity data in the critical nanoscale crossover regime.
  • The findings necessitate revised models for NP transport and behavior at the molecular to nanoscale interface.