Non-Fickian diffusion in viscous aerosol particles

Abstract

Slow condensed phase diffusion in organic aerosol particles can impede many chemical and physical processes associated with atmospheric aerosol (e.g. gas-particle equilibrium partitioning). The characteristic times associated with these high viscosity particles are typically modelled using a concentration-dependent diffusivity within a purely Fickian framework. In that model, the medium in which diffusion is taking place is treated as being inviscid as far as mass transport is concerned. In this report, we investigate the validity of assuming that the viscosity is equal to zero by using a transport model that includes viscous pressure gradients. It is found that the effect of viscosity is negligible for particles with radii that are larger than 100 nm but, below that radius, it can delay water uptake and loss by orders of magnitude for physically realistic viscosities. However, if the Stokes-Einstein relation is obeyed then, even for nanosized particles, viscosity can be ignored. In addition to numerical calculations, a dimensionless Deborah number is defined that indicates the significance of Fickian diffusion compared to the rheological response during water transport.

Geometric Analysis of Free and Accessible Volume in Atmospheric Nanoparticles

“…50 On the other hand, the gradient observed in the unoccupied and accessible volumes may also suggest a gradient in water transport, which is thought to be more significant for nanosized particles. 51,52 Additionally, the dispersion of pores/cavities as described in this work, particularly at low to medium RH values, is consistent with the hopping process described by Song et al, 21 and it could be potentially connected with particle morphology and phase state.…”

Equilibration times in viscous and viscoelastic aerosol particles

“…We have previously shown that, when v̄ α are constant, the mass average velocity can be expressed in terms of mass fluxes according to 39 …”

Viscosity of aqueous ammonium nitrate–organic particles: Equilibrium partitioning may be a reasonable assumption for most tropospheric conditions

“…To prepare the sample for the rFRAP experiments, first, Rhodamine B isothiocyanate dextran (Sigma-Aldrich, r dye = 5.8 nm, (Paës et al, 2017)) was added as a fluorescent dye to the bulk solutions of aqueous respectively). The time required for the droplets to equilibrate with the gas phase was estimated using the characteristic mixing time τ of a droplet of radius r droplet (Preston, 2022)…”