sheets). In 3D, it will be either stretched into one of the principal strain direction and compressed in the two others (thus leading to the formation of a filament), or compressed into one direction and elongated in the two others (leading to the formation of sheets). This stretching can drastically accelerate the homogenization of the tracer because the flow has the effect of increasing the exchange surface and maintaining strong concentration gradients in the compression direction(s), thus enabling efficient diffusive transport of the tracer. The effect is important if the stretching time (the inverse of the stretching rate) is smaller than the diffusion timescale based on the initial blob size. For example, if an heterogeneity is stretched into a sheet of thickness $s(t)$ at a constant rate $\dot \epsilon = \mathrm{d}(\ln s)/\mathrm{d}t$, the homogenization timescale $t_{h}$ is given by $$t_{h} \sim \frac{1}{2\dot{\epsilon}}\mathrm{ln}\left[2 \frac{\dot{\epsilon}s_0^2}{K}+1\right],
\label{eq:t12_Ranz}$$ where $s_0$ the initial thickness of the heterogeneity, and $K$ the tracer diffusivity (*e.g.* @ranz_1979; see also @kellogg_1987). If the stretching time $1/\dot \epsilon$ is large compared to the diffusion time $s_0^2/K$ (*i.e.* if ${\dot{\epsilon}s_0^2}/{K}\ll 1$), a Taylor expansion of equation \[eq:t12\_Ranz\] shows that $t_{h}\to s_0^2/K$, which shows that in this limit the homogenization is controlled 