Diffusive flux
= Diffusive flux
{title2=$\mathbf j=-D\nabla u$}
A diffusive flux transports a density $u$ down its <gradient> according to <Fick's first law>. For scalar <diffusion coefficient> $D>0$, $\mathbf j=-D\nabla u$ and the <conservation law> $u_t+\operatorname{div}\mathbf j=0$ gives a <diffusion equation>. If $D$ depends on $u$, this becomes a <nonlinear diffusion equation>.