= Transport phenomena
<Transport phenomena> are processes that redistribute quantities such as <mass>, <momentum> and <energy>. Bulk motion produces <advection>, while microscopic motion and interactions can produce <diffusion>, <heat> conduction and <viscosity>.
Constitutive laws relate a flux to a driving gradient: <Fourier's law> gives <heat> flux $\mathbf q=-k\nabla T$, and <Fick's first law> gives diffusive flux $\mathbf j=-D\nabla c$. Combining these relations with conservation balances gives evolution equations. The coefficients and the appropriate transported quantity depend on the medium; a formal similarity between two <diffusion equations> does not make their physical diffusivities identical.
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