Suspicious particle movement towards each other

Hi CFDEM,

I am trying to model the transportation of particles in a tapered channel. In this regard, the cfdemSolberIB was used. Particles with 2 different sizes are released every 10000 time step near the inlet and are transported to the outlet as a results of the mean flow velocity and the gravity which is 9.81 m/s2 in the z direction (from inlet toward outlet). While particles are moving toward the outlet, they get stuck at the location where the depth of the channel becomes equal to their diameter. Everything looks great except when the small size particles suspend in the channel and start moving toward each other. I am also running another simulation with 5 different sizes. For that case almost every size are dragged toward another particle. I cannot understand what causes this movement. It seems that there is a attraction force between particles!
Thanks