Flow of lubricated granular material on an inclined plane

TL;DR

This study investigates how adding micron-sized lubricants affects the flow of spherical granular materials on an inclined plane, revealing a non-monotonic relationship influenced by friction reduction and collision damping.

Contribution

It provides experimental insights into the non-monotonic flow behavior caused by lubricant concentration, highlighting the competing effects of friction reduction and collision damping in granular flows.

Findings

Flow rate increases then decreases with lubricant concentration.

Lubricant reduces inter-particle friction and collision effects.

Non-monotonic behavior explained by competing mechanisms.

Abstract

We have studied the gravity driven flow of spherical shaped, millimetric sized granular material coated with aspherical, micron-sized, near frictionless lubricant particles. Experiments were performed on an inclined plane using two different sized particles for varying concentrations of the lubricant. The particle volumetric flow rate exhibits a non-monotonic behavior with increasing lubricant concentration. It shows an increase at lower lubricant concentration followed by a decrease at higher lubricant concentration. The lubricant particles adhere to the granular particle surface thereby reducing the inter-particle friction. However, presence of lubricant particles at higher concentration damps out inter-particle collision thereby reducing the interparticle momentum transfer. Non-monotonicity in the observed behavior is then conjectured to arise due to competing effects of inter-particle friction and inter-particle collision. The present work and the overall observed behavior therein provides a simple experimental system to characterize the effects of added lubricant material in pharmaceutical and other relevant industrial applications.