Spontaneous Stratification in Granular Mixtures

TL;DR

This study investigates spontaneous stratification in granular mixtures, revealing that grain shape and roughness influence layering patterns during pouring, with avalanches playing a key role in the process.

Contribution

It demonstrates a new spontaneous stratification phenomenon in granular mixtures based on grain shape and roughness, linked to avalanche dynamics.

Findings

Stratification occurs when large grains are rougher than small grains.

Segregation occurs when large grains are rounder than small grains.

Avalanches induce layering through a kink mechanism.

Abstract

Granular materials size segregate when exposed to external periodic perturbations such as vibrations. Moreover, mixtures of grains of different sizes spontaneously segregate in the absence of external perturbations: when a mixture is simply poured onto a pile, the large grains are more likely to be found near the base, while the small grains are more likely to be near the top. Here, we report a spontaneous phenomenon arising when we pour a mixture between two vertical plates: the mixture spontaneously stratifies into alternating layers of small and large grains whenever the large grains are rougher than the small grains. In contrast, we find only spontaneous segregation when the large grains are more rounded than the small grains. The stratification is related to the occurrence of avalanches; during each avalanche the grains comprising the avalanche spontaneously stratify into a pair of layers through a "kink" mechanism, with the small grains forming a sublayer underneath the layer of large grains.