with D the diffusion coefficient, and with initial concentration profile c(y, 0) = 1 for -L/2 ≤ y < 0 and c(y, 0) = 0 for 0 ≤ y ≤ L/2, where L is the height of the channel. Expressions in summation form for the complete solution are given in the SI. For our experiments, we are only interested in the concentration at the top edge of the channel, c(L/2,t). Introducing a dimensionless time variable τ = Dt/L 2 , we have c(L/2,t) = C(Dt/L 2 ) = C(τ) with C(τ) a function that depends only on the dimensionless parameter τ. Full summation expressions for C are given in the SI, but for practical applications, it suffices to use only the first terms, and to obtain a relative precision of 10 -8 (with respect to the exact solution) at all times, we can use