Simulations enhance understanding of flow of granular materials with powdered lubricant

The flowability of granular materials can be improved by adding tiny, frictionless particles — they adhere to the surface of the bulk granular particles and act as a lubricant. While prior studies have primarily focused on determining the optimal lubricant-to-bulk particle size ratio and ideal lubricant particle size, the underlying physical mechanism for lubrication between granular particles is not well understood.

Sayali Chaudhary and Ashish Orpe incorporated discrete element method simulation technique to better understand the flow of spherical granular particles mixed with lubricant particles. The simulations were performed for a wide range of lubricant concentrations but for a fixed lubricant to particle size ratio.

The results showed that the presence of the lubricant particles modified the interaction between colliding bulk particles during flow. The flow rate was found to initially increase with added lubricant content, but after an intermediate lubricant content point, it decreases.

The increase of flow rate is due to reduction of friction coefficient between lubricant coated bulk particles. At higher lubricant concentrations, the increased presence of lubricant particles increases the packing density of the particles in the flowing layer, resulting in damping and reduction in flow rate. The findings align with previous experimental observations. The authors believe their success achieved in this work can be built upon further to apply to more complicated materials and systems.

“The outcome of the work offers valuable implications for optimizing lubricant concentrations in pharmaceutical and other industries where powder processing is critical,” Orpe said.

“We hope our findings will lead towards improvement in theory and design of equipment handling such materials,” Chaudhary added.

Source: “Surface coating induced lubrication in flowing granular materials,” by Sayali V. Chaudhary and Ashish V. Orpe, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0310815 .