# Mechanics of a granular skin

## TL;DR

This paper investigates the failure mechanisms of underwater magic sand columns, revealing that a self-generated cohesive skin with multi-scale properties governs their mechanical behavior and failure modes.

## Contribution

It uncovers the role of a self-formed cohesive skin in magic sand's mechanics and characterizes its multi-scale properties and failure modes.

## Key findings

- The cohesive skin encapsulates the material and influences failure modes.
- Failure transitions from brittle to ductile as skin rigidity decreases.
- The skin's properties span from contact-line dynamics to sample-scale responses.

## Abstract

Magic Sand, a hydrophobic toy granular material, is widely used in popular science instructions because of its non-intuitive mechanical properties. A detailed study of the failure of an underwater column of magic sand shows that these properties can be traced to a single phenomenon: the system self-generates a cohesive skin that encapsulates the material inside. The skin, consists of pinned air-water-grain interfaces, shows multi-scale mechanical properties: they range from contact-line dynamics in the intra-grain roughness scale, plastic flow at the grain scale, all the way to the sample-scale mechanical responses. With decreasing rigidity of the skin, the failure mode transforms from brittle to ductile (both of which are collective in nature) to a complete disintegration at the single grain scale.