I am also using non-Newtonian fluids for VOF-DPM simulation. For the viscosity of the liquid, I use the Carreau Model, and when the viscosity of the particles is set to a constant value and VOF-DPM is turned on, the following dialog box pops up in the system. I think the viscosity of the particles also needs to use the viscosity properties of non-Newtonian fluids, but I am not sure how to change it.