How not to sink into highly compressible sand?

What is it about?

Let’s place a pile of sand on a table and slowly start lifting one side of the table. First, at a low tilted angle, the pile will remain a solid block, but if lifted high enough, the pile will destabilize and the sand will start flowing. This transition from solid-like to liquid-like properties under an increasing shear force is called the yielding transition. Now imagine that your grains of sand are very soft and compressible (imagine squishing a kitchen sponge), how would that affect the yielding transition? Our study shows that large deformability helps maintain a pile of sand solid under higher forces, at the condition that the grains are sufficiently pre-compressed. The underlying reason is that, instead of rearranging and flowing, soft grains prefer changing their shape, but only if they can hold on to their neighbors using friction.

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Photo by Utsman Media on Unsplash

Photo by Utsman Media on Unsplash

Why is it important?

Our fundamental study highlights how large shape changes at the grain scale can affect the overall properties of a complex grain assembly, while most previous studies focused on grains that can barely change their shape. Undeformable grains are suited to describe flows of rocks or seeds (crucial for mining and agricultural industries), whereas soft “grains“ are omnipresent in biology. Indeed, living beings are large assemblies of very deformable cells, that sometimes remain as solid tissues, but can also heavily rearrange and flow, to form different organs during the growth of an embryo for instance. The mechanical aspects of cell assemblies appear to be a crucial component of many biological processes, from embryo growth to the spreading of cancer cells. However, in biology, it is challenging to separate the role of mechanical and biochemical processes. By clarifying the role played by large deformations in complex assemblies, we hope to help interpret biological observations and anticipate cell assemblies' mechanical behavior.