Circular Kinks on the Surface of Granular Material Rotated in a Tilted Spinning Bucket

TL;DR

This study investigates the formation of circular kinks on granular surfaces in a tilted spinning bucket, revealing their dependence on rotation speed and tilt angle, and explaining their stability boundary through experiments and modeling.

Contribution

It introduces a new understanding of surface kinks as stability boundaries in granular materials under rotation and tilt, supported by experiments and cellular automata modeling.

Findings

Kinks form when tilt exceeds internal friction angle

Kink radius varies with spinning speed and tilt angle

Kinks mark the boundary between stable and unstable regions

Abstract

We find that circular kinks form on the surface of granular material when the axis of rotation is tilted more than the angle of internal friction of the material. Radius of the kinks is measured as a function of the spinning speed and the tilting angle. Stability consideration of the surface results in an explanation that the kink is a boundary between the inner unstable and outer stable regions. A simple cellular automata model also displays kinks at the stability boundary.