Get relative normal velocity of particles at a distance x0 between them

Hi everyone,

I am simulating wet particles. Their recovery coefficients are related to a kind of relative normal velocity at a distance x0 between them, and x0 is then related to the thickness of an assumed uniform liquid film on a particle. If x0 is known, how to get the relative normal velocity at a distance x0 between particles in the check_solids_dem_mod code file.

Thank you.