I'm right now working with modelling a system, where atoms diffuse along a coordinate z until they reach a certain point z=z0 at which they are taken out of the system. The diffusion equation is: ∂n/∂t = D∂2n/∂z2 The question is now what boundary conditions to use for the concentration profile n(z) of the system. I want to take n(z0)=0 since for me this would reflect the fact that atoms are taken instantaneously out of the system, when they diffuse to this point. But my intuition also tells me that there can be no flux of atoms out of the system at z=z0 if the concentration at this point is zero. Does anyone have anything that can enlighten me on this problem?