Diffusive time scale
= Diffusive time scale
{title2=$t_D\sim L^2/\kappa$}
A concentration varying over length $L$ has Laplacian of order its amplitude divided by $L^2$. Balancing the time derivative with $\kappa\Delta\chi$ in the <diffusion equation> gives the displayed relaxation time. A <multiple-scale expansion> with spatial scale $\varepsilon x$ therefore uses slow time $\varepsilon^2t$ when the effective <diffusivity> is of order one.