Fick's laws of diffusion

Fick's first law relates diffusive flux to the concentration gradient; the second describes how concentration changes over time as diffusion proceeds.

Fick's first law: A constitutive equation stating that diffusive flux is proportional to the negative concentration gradient. It gives the steady or instantaneous flux relation that forms one half of the pair.
Diffusion: The net spreading of particles or other quantities from regions of higher concentration toward lower concentration. Fick's laws provide a mathematical description of this broad transport process.
Diffusion couple: A joined pair of materials with different compositions, used to study interdiffusion across their interface. Its evolving concentration profile can be analyzed with Fick's second law.
Maxwell–Stefan diffusion: A framework describing multicomponent diffusion through frictional interactions among species. It handles coupled mixtures where independent Fickian flux laws can be inadequate.
Fick's second law: A diffusion equation describing how concentration changes over time as spatial flux varies. It derives concentration evolution from flux conservation and the first law.
Flux: The amount of a quantity crossing a unit area per unit time, with direction. The first law expresses diffusion through flux, rather than particle motion alone.
Diffusion in semiconductors: The thermally driven movement of dopant atoms through semiconductor crystals. Fickian models estimate how dopant concentration changes during fabrication.
Anomalous diffusion: Transport in which mean-square displacement does not grow linearly with time. Its nonstandard time scaling cannot generally be captured by classical Fickian diffusion.
Diffusion coefficient: A parameter measuring how rapidly particles spread through a medium under concentration gradients. It sets the proportionality between concentration gradients, flux, and diffusion rate.
Conservation of mass: The principle that mass is neither created nor destroyed in ordinary physical and chemical processes. Particle conservation turns spatial differences in flux into concentration change.

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