Implementation of a viscosity model dependent on solid fraction of fluid cell

Hi, I am simulating a shearing test(Rheology test) of particle suspension. As it is well known that the viscosity of fluid is closely related to the solid fraction of the particles in a particle suspension system. So I was wondering if I want to consider this effect and use variable viscosity dependent on the solid fraction of the fluid cell, which part of the source code I should modify.

Thank you!