Molecular diffusion

Molecular diffusion is the net transport of molecules from higher to lower concentration, produced by random thermal motion. It occurs without bulk flow and tends toward uniform concentration.

Fick's laws of diffusion: Equations relating diffusion flux to concentration gradients and describing how concentration changes over time. They quantify how molecular diffusion responds to concentration gradients.
Concentration: The amount of a substance present in a specified volume or other defined region. Differences in concentration determine the direction of net molecular transport.
Osmosis: The net movement of solvent through a selectively permeable membrane in response to differences in chemical potential. It is a membrane-mediated transport process driven by molecular diffusion and solvent chemical potential.
Advection: The transport of a substance by the bulk motion of a fluid. Unlike diffusion, advection carries molecules with a moving fluid.
Brownian motion: The irregular motion of particles suspended in a fluid, caused by collisions with its molecules. It provides observable evidence of the random motion underlying molecular diffusion.
Concentration gradient: The rate and direction in which concentration changes across space. Diffusion flux points down this spatial change in concentration.
Gas diffusion: The spreading and mixing of gas molecules through random motion and collisions. It describes molecular diffusion in gases, where molecules travel between frequent collisions.
Convection: Heat or material transport involving bulk fluid motion, often combined with molecular diffusion. It adds fluid-scale circulation to the molecular transport considered here.
Diffusion coefficient: A parameter measuring how rapidly a substance spreads through a medium by diffusion. Its value sets the rate at which a concentration gradient smooths out.
Chemical potential: A thermodynamic measure of how a system's free energy changes when particles are added. In nonideal mixtures, chemical-potential gradients more fundamentally drive diffusion than concentration gradients.

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