Particle induced lubrication during the flow of a granular material

We have studied the flow behavior of a granular material by coating the surface of individual particles in the material with very tiny, frictionless, lubricant particles. Gravity driven flow of such lubricant coated granular material was initiated on an inclined chute for different concentrations of the lubricant employed. Experiments were performed for a fixed particle size and constant inclination and inlet gate opening of the chute while varying the lubricant concentration (weight ratio of $10^{-5}$ to $10^{-3}$). The measured volumetric flow rate was observed to exhibit a non-monotonic behavior with increasing lubricant concentration. The flow rate initially increased for small enough lubricant concentration and subsequently decreased at much larger lubricant concentration. The increase in flow rate was attributed to the reduced inter-particle friction while the decrease resulted due to damping of the inter-particle collisions caused by excessive coating of the lubricant thereby reducing the inter-particle momentum transfer. We conjecture that the non-monotonicity arises from the relative dominance between inter-particle friction and collision which is altered with respect to lubricant concentration employed. The overall observed behavior suggests an alternative, yet simple, method to characterize the effect of inter-particle interactions on the granular flow behavior.