Anomalous flocking in nonpolar granular Brownian vibrators_RawData_ParticlesPositions_Velocity

Using Brownian vibrators, we investigated the structures and dynamics of quasi-2D granular materials, with packing fractions ($\phi$) ranging from 0.111 to 0.832. Our observations revealed a remarkable large-scale flocking behavior in hard granular disk systems, encompassing four distinct phases: granular fluid, flocking fluid, poly-crystal, and crystal. Anomalous flocking emerges at $\phi$ = 0.317, coinciding with a peak in local density fluctuations, and ceased at $\phi$ = 0.713 as the system transitioned into a poly-crystal state. The poly-crystal and crystal phases resembled equilibrium hard disks, while the granular and flocking fluids differed significantly from equilibrium systems and previous experiments involving uniformly driven spheres. This disparity suggests that collective motion arises from a competition controlled by volume fraction, involving an active force and an effective attractive interaction resulting from inelastic particle collisions. Remarkably, these findings align with recent theoretical research on the flocking motion of spherical active particles without alignment mechanisms.

本研究采用布朗振动器（Brownian vibrators），研究了填充率（packing fraction）φ介于0.111至0.832之间的准二维（quasi-2D）颗粒材料的结构与动力学行为。观测结果显示，硬颗粒圆盘系统（hard granular disk systems）中存在显著的大规模集群行为（flocking behavior），其涵盖四种截然不同的物相：颗粒流体相、集群流体相、多晶相以及单晶相。异常集群行为出现在φ=0.317处，此时恰好对应局域密度涨落（local density fluctuations）的峰值；当φ=0.713时，随着系统转变为多晶相，异常集群行为随之终止。多晶相与单晶相的行为与平衡态硬圆盘系统相似，而颗粒流体与集群流体则与平衡态系统及过往涉及均匀驱动球体的实验结果存在显著差异。该差异表明，集体运动（collective motion）源于由体积分数调控的竞争机制，其中包含由非弹性粒子碰撞（inelastic particle collisions）产生的主动力（active force）与有效吸引相互作用（effective attractive interaction）。值得注意的是，本研究结果与近期针对无对齐机制（alignment mechanisms）的球形活性粒子（spherical active particles）集群运动的理论研究结果高度契合。