= Concentration gradient
{title2=$\nabla c$}
A <concentration gradient> is the spatial <gradient> $\nabla c$ of a scalar <concentration> field. It points toward the greatest local increase of $c$; its components are spatial <partial derivatives>, so its units are those of <concentration> divided by <length>.
For a one-dimensional profile $c(x)=c_0+ax$, the <concentration gradient> is the constant $a\,\widehat{\mathbf x}$. <Fick's first law> gives a diffusive flux proportional to $-\nabla c$ when that constitutive model applies: the minus sign describes transport toward lower <concentration>. A <fluid> <velocity> can simultaneously advect the constituent in another direction, so a <concentration gradient> alone does not determine its total flux.
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