Kumar Deepak, Nitsure Nitin, Bhattacharya S, Ghosh Shankar
Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai 400005, India;
School of Mathematics, Tata Institute of Fundamental Research, Mumbai 400005, India;
Proc Natl Acad Sci U S A. 2015 Sep 15;112(37):11443-8. doi: 10.1073/pnas.1500665112. Epub 2015 Aug 31.
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. A low density of weakly frictional rollers congregates near the sharp leading edge whereas a denser 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 autotuning of friction through these internal states, providing a previously unidentified route to self-organization of a many-body system.
在圆柱形小瓶中的单层颗粒球，由轨道振荡器持续驱动并受到对称限制离心势的作用，会自组织形成一种仅占据后半空间的独特不对称结构。它的特征是在势最小值处有一个尖锐的前沿和一个弯曲的后部。该结构的面积与球的数量遵循幂律缩放关系。成像显示，单个球的运动调节是通过在两种运动类型之间切换实现的，即滚动和滑动。低密度的弱摩擦滚动体聚集在尖锐的前沿附近，而密度较大的后部则由高摩擦滑动体组成。实验进一步表明，由于滚动摩擦系数和滑动摩擦系数有很大差异，球在系统内的很大范围内获得了局部时间平均摩擦系数。滚动体和滑动体的各种空间和时间配置集构成了系统的内部状态。实验证明并经模拟证实，通过这些内部状态对摩擦进行自动调整，可稳健地维持结构的全局特征，为多体系统的自组织提供了一条此前未被识别的途径。
