Diffusion
Definition
Spreading of molecules driven by concentration gradients. Mixing time scales as t ≈ L²/2D, so it is slow across wide channels.
Explanation
Diffusion is the spreading of molecules by random thermal motion, from high toward low concentration. Without turbulence it is the only mixing mechanism in laminar microflows.
The characteristic time to diffuse a distance L is t ≈ L²/(2D); the square dependence is why mixing across wide channels is slow and why folded mixers are used.
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Further reading
- Diffusion & mixing cheat sheet — SemitreeVerify the current edition.