Softer than soft: Diving into squishy granular matter

Abstract

Softer than soft, squishy granular matter is composed of grains capable of significantly changing their shape (typically a deformation larger than 10%) without tearing or breaking. Because of the difficulty to test these materials experimentally and numerically, such a family of discrete systems remains largely ignored in the granular matter physics field despite being commonly found in nature and industry. Either from a numerical, experimental, or analytical point of view, the study of highly deformable granular matter involves several challenges covering, for instance: (i) the need to include a large diversity of grain rheology, (ii) the need to consider large material deformations, and (iii) analysis of the effects of large body distortion on the global scale. In this article, we propose a thorough definition of these squishy granular systems and we summarize the upcoming challenges in their study.

A soft departure from jamming: the compaction of deformable granular matter under high pressures

“…At high pressures, the behavior of granular materials has been less explored in the literature. Experimentally, various techniques have been used to probe highly compressed granular media 6–9 finding notable changes in behavior including a transformation in the functional shape of distributions of contact forces 6 and a transition to continuum behavior 10 with increasing pressure. Computational studies have leveraged both finite element method (FEM) based models, 11–14 the material point method (MPM), 8,15,16 and bonded particle models (BPM) 15,17,18 to explicitly represent the elastic deformation of grains.…”

“…In this work, we focus on the isotropic linear elastic response of deformable grains under compaction, or what has been termed squishy granular material. 9 In addition to being the primary regime of interest in materials which can sustain significant reversible strains, elastic deformation always serves as a precursor to potential plastic deformation or fracture and is therefore an important limit to establish before overlaying such additional inelastic mechanisms or extending to nonlinear elasticity. The goals of this article are twofold.…”

Compacting an assembly of soft balls far beyond the jammed state: Insights from three-dimensional imaging

“…But what if we consider soft and even squishy grains [15] (i.e., grains having a low stiffness relatively to the applied pressure) while continuing to compress the packing? The grains would deform, changing their shapes elastically or in a more complex manner, without breaking to accommodate for the internal mechanical and geometrical constraints.…”

“…In soft granular assemblies, the macro-and microscale behaviors are no longer uniquely ruled by the steric exclusions and the topological disorder of contacts but are also ruled by the bulk behavior of each constitutive grain. Including the change in grain shape in addition to the inherently multicontact nature of a granular system in realistic three-dimensional (3D) modeling is a vast emerging topic, with challenging experimental, numerical, and theoretical issues [15,16].…”

“…From a numerical point of view, these last few years have seen the advent of new strategies in the form of discrete element methods coupled with different continuum approaches [15]. Although technical challenges still remain, especially in terms of computational time, existing studies, mainly in two dimensions and a few in three dimensions [17][18][19], have allowed us to glimpse new microscopic mechanisms that take place from contact scale to the inside of the grains.…”