I want to set an interior boundary condition as 'evaporation' (Crank's terminology) meaning: j= -D dc/dx = k(c1 - B c2) where k is a surface exchange coefficient, c1 and c2 are values just before and after boundary, and B takes care of cases where concentrations in equilibrium may differ (consider a chunk of salt in equilibrium with a saturated salt solution - the concentration in solution is much less than in solid, but j=0). I don't see any way to do it ; have tried using 'up' and 'down' functions to access the values of c just before and after a boundary but I cant figure out how to display these values to check them (and in the meantime they always seem to come out 0).

this is in comsol 3.2 by the way