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Softer than soft: diving into squishy granular matter

Softer than soft, squishy granular matter is composed of grains capable of significantly changing their shape (typically larger than 10% of deformation) 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 \dc{large material} deformations, and ($iii$) the analysis upon the effects the large body distortion has 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.