osmotic pressure

Hi all,

we're two Politecnico di Milano's students using Comsol software (4.2a version) for our master thesis.
On the Comsol webpage we've found an article of S. A. Conrad "Finite Element Analysis of Fluid and Solute Transport in Hemodiafiltration Membranes" and we've tried to implement the Landis-Pappenheimer equation as a volume force in the Brinkman equation (in order to include the effect of osmotic pressure).
Our problem is the fact that if we write the volume force expression as a function of proteine's concentration, the result is a warning. Can we insert only constant values of volume force?

Thanks,
Roberta and Manuela