Diffusion in gases is a transport mechanism driven by molecular fluctuating motion and concentration gradients across mean-free paths, fundamentally distinct from convective bulk flow which requires net fluid velocity. The theoretical framework defines the diffusion coefficient as proportional to the root-mean-square molecular velocity multiplied by the mean free path, establishing that mass diffusivity and momentum diffusivity (viscosity) share similar magnitudes in dilute gases due to identical underlying collisional physics. This concept operates within non-equilibrium thermodynamics and fluid dynamics, explaining why surface transport perpendicular to impenetrable boundaries relies exclusively on diffusive fluxes regardless of the dominant macroscopic convection regime.
Diffusion in gases is a transport mechanism driven by molecular fluctuating motion and concentration gradients across mean-free paths, fundamentally distinct from convective bulk flow which requires …