Mean Free Path Calculation for Hard Spheres and Viscous GasThe mean free path of molecules in a gas can be modeled with the assumption that the molecules are hard spheres, and it can be modeled based on the viscosity of the gas. The intent here is to compare the results. This calculation is just an investigation to see how closely the modeling of mean free path by hard sphere geometry and by projection from gas viscosity agree. For the projection from viscosity, standard air is used as the default, so that the values for air from the table above are substituted if no values are entered for the relevant parameters. Those values can be changed. As an example of model parameters, if you use 760mmHg for gas pressure, 0.3nm for molecular diameter, and 524.07R for temperature (the standard temperature for air in the table), the hard sphere calculation gives a mean free path of 99nm. If for the same temperature and using the standard value for air viscosity, 0.01827 centiPoise, the calculated mean free path is 65nm. If you adjust the hard sphere diameter to 0.3697nm, you bring the two estimates of mean free path into agreement, but I have no idea whether you can attach physical significance to this agreement. I would be interested in any physical data which might bear on this question. References:

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