Wrinkles, folds and plasticity in granular rafts

TL;DR

This study explores the mechanical behavior of granular rafts at liquid interfaces, revealing limitations of elastic models and highlighting the roles of discreteness, friction, plasticity, and jamming in their deformation patterns.

Contribution

It provides the first quantitative comparison between granular raft morphology and elastic models, uncovering effects of discreteness and friction not captured by traditional elasticity.

Findings

Identification of a secondary wavelength during compression

Discrepancy between experimental results and elastic model predictions

Observation of hysteresis indicating plasticity and jamming behaviors

Abstract

We investigate the mechanical response of a compressed monolayer of large and dense particles at a liquid-fluid interface: a granular raft. Upon compression, rafts first wrinkle; then, as the confinement increases, the deformation localizes in a unique fold. This characteristic buckling pattern is usually associated to floating elastic sheets and as a result, particle laden interfaces are often modeled as such. Here, we push this analogy to its limits by comparing the first quantitative measurements of the raft morphology to a theoretical continuous elastic model of the interface. We show that although powerful to describe the wrinkle wavelength, the wrinkle-to-fold transition and the fold shape, this elastic description does not capture the finer details of the experiment. We describe an unpredicted secondary wavelength, a compression discrepancy with the model and a hysteretic behavior during compression cycles, all of which are a signature of the intrinsic discrete and frictional nature of granular rafts. It suggests also that these composite materials exhibit both plastic transition and jamming dynamics.