hi, i'm a chemical engineering student with a little problem with Comsol multiphysics; in practice, i have to solve a problem of diffusion in a solid sphere. after drawing the domain, i have to set a boundary condition on sphere's surfaces. this condition, for my problem, is FLUX=Kc(Cb-C) and Cb (bulk concentration) is proportional to an integration coupling variable, that i have previously fixed In particular, Cb is equal to costant * mass_released, where mass_released is equal to volume integral of C0 (initial concentration) minus C After setting all variables and running simulation, it gives me an error referred to evaluation of variable Cb I think the problem is that it's difficult to use in a boundary condition a variable that changes during simulation (in fact it is time dependent).. I hope someone can help me, have a nice day!!!