= Mass diffusivity
{title2=$\kappa$}
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<Mass diffusivity> is a transport coefficient describing how a constituent spreads relative to the mean material motion. For a simple dilute concentration field $c$ with constant isotropic coefficient $D$, <Fick's first law> gives relative flux $\mathbf j=-D\nabla c$. Combining it with local conservation in stationary material gives the <diffusion equation> $\partial_t c=D\nabla^2c$.
The units are length squared per <time>, so spreading over distance $L$ takes a characteristic time of order $L^2/D$. A narrow dye layer in otherwise still fluid provides a motivating measurement: tracking the widening concentration profile can infer $D$ when other transport is negligible. In a moving fluid, <advection> is an additional mechanism, and enhanced spreading need not mean larger molecular <mass diffusivity>. Mixtures, anisotropic media and concentration-dependent response can require a more general flux law.
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