Diffusion is a continuum transport phenomenon driven by microscopic fluctuations in molecular velocity that generate net flux across concentration gradients when the macroscopic length scale significantly exceeds the mean free path. The mechanism relies on the statistical imbalance of molecules crossing an interface, described quantitatively by Fick's Law where the diffusion coefficient is derived from the root-mean-square fluctuating velocity and the mean free path. This concept operates within the domain of fundamental transport processes as a specific subset of irreversible thermodynamics, distinguishing itself from convection (mean flow) and coupling with other fluxes via reciprocity relations in multi-component systems.
Diffusion is a continuum transport phenomenon driven by microscopic fluctuations in molecular velocity that generate net flux across concentration gradients when the macroscopic length scale signific…