The sinking dynamics and splitting of a granular droplet

TL;DR

This study investigates the sinking and splitting behavior of dense granular droplets in a bed of lighter particles, revealing unique physical mechanisms and conditions that lead to droplet fragmentation.

Contribution

It introduces a novel understanding of granular droplet splitting driven by immobilized zones and identifies key physical requirements for the phenomenon.

Findings

Granular droplets cause immobilized zones that lead to splitting.

Splitting involves inclined sinking trajectories of fragments.

Three conditions are necessary for droplet splitting: friction, density contrast, and size.

Abstract

Recent experimental results have shown that vibro-fluidized, binary granular materials exhibit Rayleigh-Taylor-like instabilities that manifest themselves in rising plumes, rising bubbles and the sinking and splitting of granular droplets. This work explores the physics behind the splitting of a granular droplet that is composed of smaller and denser particles in a bed of larger and lighter particles. During its sinking motion, a granular droplet undergoes a series of binary splits resembling the fragmentation of a liquid droplet falling in a miscible fluid. However, different physical mechanisms cause a granular droplet to split. By applying particle-image-velocimetry and numerical simulations, we demonstrate that the droplet of high-density particles causes the formation of an immobilized zone underneath the droplet. This zone obstructs the downwards motion of the droplet and causes the droplet to spread and ultimately to split. The resulting fragments sink at inclined trajectories around the immobilized zone until another splitting event is initiated. The occurrence of consecutive splitting events is explained by the re-formation of an immobilized zone underneath the droplet fragments. Our investigations identified three requirements for a granular droplet to split: 1) frictional inter-particle contacts, 2) a higher density of the particles composing the granular droplet compared to the bulk particles and 3) and a minimal granular droplet diameter.