Modeling viscoelastic rheology

Hi all!! I am a student working with FEniCS to model the flow of a viscoelastic solid using Stokes flow. I am using a mixed formulation (Taylor-Hood) as demonstrated here. I am trying to model little pockets of liquid within the solid. Is there a way to change the viscosity of the material in specific cells of the mesh? Either that, or to remove cells within the domain in such a way that the boundaries between the empty space and the solid are non-rigid? Thank you very much!

See for instance: How to define different materials / importet 3D geometry (gmsh)