Diffusion
= Diffusion
{title2=$\mathbf j=-D\nabla C$}
Diffusion is the spreading of heat or material by microscopic random motion. A <concentration> gradient produces flux down that gradient; with uniform <diffusion coefficient> $D$, conservation gives the <diffusion equation> $C_t=D\nabla^2C$. In a moving carrier, <advection> must also be included. Physical diffusion is a transport mechanism, distinct from the general probabilistic class of <Markov diffusion>.