where erf(x) is the Gauss error function. Note that C converges to 1 2 , since after sufficient time the solute molecules will be distributed evenly over the two halves of the channel. The above equation is the concentration at y = L/2, whereas in the experiment we use a slit with a finite width to spatially select the top part of the channel. Hence, we should actually integrate c(y,t) from y = L/2w to L/2 (with w the slit width); but numerical calculation shows that if the slit is not too wide (w ≤ L/8) the integral can be approximated to within less than 1% by the concentration at y = L/2, so in the global fit of eq. ( ) explained below we use eq. ( ) instead of the more cumbersome integral.