On the description of nanoparticle diffusion in gases

• We examine nanoparticle diffusion, ion mobility, and drag in bath gases.
• Experimental and simulation data are considered over a range of Knudsen numbers.
• The bath gas diameter must be accounted for to obtain simple dependencies.
• A new formula is tested for interpolating data from low to high Knudsen numbers.

Analyses are presented using both experimental and simulation data for nanoparticle diffusion and mobility in gases. These analyses show that it is important to include the diameter of the bath gas when calculating effective cross sections, if simple dependencies are desired. This can be especially important when analyzing experimental data for nanoparticles where accurate values are difficult to obtain. In particular, we show there is little evidence for a transition from specular to diffuse in the nature of the scattering for nanoparticles. The accuracy of the Stokes–Millikan equation, commonly used to fit diffusion and mobility data, is tested for hard sphere diffusion and found to give good predictions, albeit with different parameters than typically used for experimental data. In addition, a new empirical formula, introduced by us previously to fit simulation data, was tested against experimental data and found to be adequate. One strength of this formula is its explicit inclusion of experimentally determined quantities and its reduced number of adjustable parameters. Its predictions are most accurate for intermediate and large Knudsen numbers, the values found in practice for nanoparticle systems.