Roll and Stumble: A robust mechanism for efficient self-organization of granular matter

A monolayer of granular spheres in a cylindrical vial, driven continuously by an orbital shaker and subjected to a symmetric confining centrifugal potential, self-organizes to form a distinctively asymmetric structure which occupies only the rear half-space. Imaging shows that the regulation of motion of individual spheres occurs via toggling between two types of motion, namely, rolling and sliding. Experiments demonstrate and simulations confirm that the global features of the structure are maintained robustly by an auto-tuning of the effective friction through internal dynamical states of rolling and sliding which provides a protocol-insensitive route to self-organization of a driven many-body system. Recent results show that restricting the motion of the system to a quasi-2 D space leads to efficient crystallization. Relation of two forms of locomotion to more general scenarios of autotuning of friction, as in chemotaxis of bacteria and prevention of stampede in crowd dynamics, will be speculated upon.
*Work done at the Tata Institute of Fundamental Research in collaboration with Deepak Kumar, Anit Sane, Soham Bhattacharya, Nitin Nitsure and Shankar Ghosh.