Elastogranular Packing of a Loop

Confined thin structures are ubiquitous in nature. Spatial constraints have led to novel packing strategies at both the micro-scale, as when DNA packages inside a capsid, and the macro-scale, observable in plant root development. Previous work has focused on growing thin structures confined by rigid boundaries. Comparatively, much less is known about the behavior of slender growing structures constrained by deformable boundaries, such as granular materials. By varying the arc length of an elastic loop injected into a granular array of mono-disperse, soft, spherical grains of varying initial number density φ₀, we investigate the resulting behavior of this model elastogranular system. At low φ₀, the elastic loop deforms as though it were hitting a flat surface by periodically folding into the array. Above a critical packing fraction φ_c, local re-orientations of grains cause the elastic loop to deform as though striking a curved surface, leading to the emergence of a distinct circular packing morphology. These results will bring new insight into the packing behavior of wires and thin sheets (where the same morphologies have been observed) and will be relevant to modeling animal burrowing & locomotive strategies, and developing smart, steerable needles.