Avalanches in 2D granular media

Granular systems are ubiquitous in nature, ranging from sand and colloids to planetary bodies and popcorn and present a very rich behavior. In this work, we focus on a single layer of mono-disperse beads that are shaken horizontally with an orbital motion (no out-of-plane oscillations). We find that a critical velocity exists above which a nucleation phenomenon is triggered: one bead leaves the plane and goes up. This event then triggers the ascension of other beads and ultimately leads to a steady state with two layers (one on the plate and one up in the air) and a permanent flow between them. We investigate this phenomenon using a combination of experiments, state-of-the-art machine learning algorithms and numerical simulations and identify a scaling law to describe it. Since our experiments can be easily visualized, we envisage that they could serve as a platform to gain insight into avalanches and phase transitions in more complex and less accessible systems.