Interaction between two large particles in a dry granular shear flow

The interaction between two large spherical particles, called intruders, in a dry granular flow down an incline is brought to light and studied experimentally and numerically. Several parameters are varied, namely, the size ratio between the intruders and the small flowing particles, the thickness of the granular flow, the incline slope and roughness, and the densities of the intruders with respect to the small-particle density. In all cases, intruders get aligned with the flow. A thorough parametric study shows that a transition occurs between attractive and repulsive regimes of interaction: at steady state, intruders either flow at a defined longitudinal distance, which may be zero with intruders in contact, or stand as far apart as possible. The mean longitudinal and vertical distances between the intruders are found to be tightly linked, with all points plotting the pairs on a single, master curve. The wake and shear effects are shown to control the relative position of the intruders. They may be modulated due to the weight and buoyancy of the intruders, and to local modifications of the collisions between intruders and small flowing particles because of the proximity of the incline bottom or the flow surface.