Granular materials such as sand can either be jammed and rigid, or yield and flow. Puzzling changes in the forces between the grains deepen the mystery surrounding this basic, but poorly understood, transition. Why does sand mimic a solid when we walkon it but emulate a fluid when it is in an hourglass? Why does salt flow only when its shaker is tipped far enough? These simple questions are without a clear answer, and have in recent years inspired investigations into what exactly happens when otherwise jammed granular media lose rigidity and yield.
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