Hello strawberryshortcake,
Good question. Denser particles can accumulate in vortices or eddies. If there is too little energy to lift or agitate the dense particles, then the particle will settle via gravity. If the eddy has a low energy spot (quiescent) then there will be particles settling.
If you can have a higher velocity and/or fluid viscosity, this will help minimize particles from settling.
I hope this helps,
Eugene