Traveling Granular Segregation Patterns in a Long Drum Mixer

TL;DR

This paper reports the discovery of bi-directional traveling wave patterns in granular mixtures within a rotating drum, revealing complex dynamics that precede traditional segregation and challenging existing PDE models.

Contribution

It introduces the observation of traveling wave states in granular segregation, a phenomenon not previously documented in this context.

Findings

Traveling waves precede axial segregation in granular mixtures.

Wave speed decreases as wavelength increases.

Observed dynamics challenge simple first-order PDE models.

Abstract

Mixtures of granular media often exhibit size segregation along the axis of a partially-filled, horizontal, rotating cylinder. Previous experiments have observed axial bands of segregation that grow from concentration fluctuations and merge in a manner analogous to spinodal decomposition. We have observed that a new dynamical state precedes this effect in certain mixtures: bi-directional traveling waves. By preparing initial conditions, we found that the wave speed decreased with wavelength. Such waves appear to be inconsistent with simple PDE models which are first order in time.