Emergence of macroscopic directed motion in populations of motile colloids

摘要

集体运动在所有尺度上都可以在自然界看到， 从成群的鸟类到成群的鱼类和成群的细菌都包括在内，但在简单的物理模型中却难以捕捉到这种行为。能够表现出集体行为的人造"活性 物"系统通常都依靠碰撞，从而使得对相互作用的描述复杂化。现在，Denis Bartolo及同事建立了一个独特的实验系统，它由自行推进的滚动球组成，它们自我组织，数百万之众向一个方向运动。这些球通过直截了当的液力相互作用来"感觉"彼此，这样所有参数都可以很容易被计算和调整。这项工作显示，在个体层面上真正的物理相互作用足以让均匀的活动群体进行稳定的定向运动。该系统有可能被用来 模拟自然集体运动和设计新的自组织材料及成群运动的微型机器人。%From the formation of animal flocks to the emergence of coordinated motion in bacterial swarms, populations of motile organisms at all scales display coherent collective motion. This consistent behaviour strongly contrasts with the difference in communication abilities between the individuals. On the basis of this universal feature, it has been proposed that alignment rules at the individual level could solely account for the emergence of unidirectional motion at the group level. This hypothesis has been supported by agent-based simulations. However, more complex collective behaviours have been systematically found in experiments, including the formation of vortices, fluctuating swarms, clustering and swirling. All these (living and man-made) model systems (bacteria, biofilaments and molecular motors, shaken grains and reactive colloids) predominantly rely on actual collisions to generate collective motion. As a result, the potential local alignment rules are entangled with more complex, and often unknown, interactions. The large-scale behaviour of the populations therefore strongly depends on these uncontrolled microscopic couplings, which are extremely challenging to measure and describe theoretically. Here we report that dilute populations of millions of colloidal rolling particles self-organize to achieve coherent motion in a unique direction, with very few density and velocity fluctuations. Quantitatively identifying the microscopic interactions between the rollers allows a theoretical description of this polar-liquid state. Comparison of the theory with experiment suggests that hydrodyn-amic interactions promote the emergence of collective motion either in the form of a single macroscopic 'flock', at low densities, or in that of a homogenous polar phase, at higher densities. Furthermore, hydrodynamics protects the polar-liquid state from the giant density fluctuations that were hitherto considered the hallmark of populations of self-propelled particles. Our experiments demonstrate that genuine physical interactions at the individual level are sufficient to set homogeneous active populations into stable directed motion.