Rolling and Stumbling: A novel route to self-organized structures and crystallization in 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. It is marked by a sharp leading edge at the potential minimum and a curved rear. The area of the structure obeys a power-law scaling with the number of spheres. Imaging shows that the regulation of motion of individual spheres occurs via toggling between two types of motion, namely, rolling and sliding. Weakly frictional rollers congregate near the sharp leading edge whereas a dense rear comprises highly frictional sliders. Experiments further suggest that because the rolling and sliding friction coefficients differ substantially, the spheres acquire a local time-averaged coefficient of friction within a large range of intermediate values in the system. The various sets of spatial and temporal configurations of the rollers and sliders constitute the internal states of the system. Experiments demonstrate and simulations confirm that the global features of the structure are maintained robustly by auto-tuning of friction through these internal states, providing a route to self-organization of a many-body system. Recent results show that when the system is restricted to a quasi-2 dimensional space, this self-organization leads to crystallization. *Work done at TIFR in collaboration with Deepak Kumar, Anit Sane, Soham Bhattacharya, Nitin Nitsure and Shankar Ghosh