Attraction and repulsion between objects in a granular flow.

The objective of this work is the understanding of lift forces exerted on two side-by-side intruders within a granular flow that is uniform upstream. To this end, we have performed experiments that reveal long-range attraction and repulsion depending on flow velocity and the distance between intruders. Additionally, a hydrodynamic model based on kinetic theory for inelastic particles was solved numerically. The model reproduces remarkably well the general qualitative features of the experiments which shows that the system behaves similarly to a pair of hot intruders immersed in a liquid of temperature-dependent viscosity. Our results confirm what we had found previously through numerical simulations in a similar geometry; namely, that attraction or repulsion exists between the intruders depending on the granular flow velocity and the distance separating the obstacles. In particular, an empirical equation relating the difference in mean flow velocity at both sides of the obstacles.