where 𝑤(𝑧, 𝑡) is the fraction of digested lipids at position 𝑧 and time 𝑡. Assuming lipid diffusion to be much slower than the digestion process, and that the latter takes place at the exposed face of the beaker, we can couple the previous diffusion equation with the boundary conditions 𝑤(0, 𝑡) = 1, 𝜕𝑤 𝜕𝑧 (𝐿, 𝑡) = 0.