Flocking ferromagnetic colloids.
Flocking ferromagnetic colloids.
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DOI：
10.1126/sciadv.1601469
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发表时间：
2017-02
期刊：
影响因子：
13.6
通讯作者：
Aranson IS
中科院分区：
文献类型：
--
作者：
Kaiser A;Snezhko A;Aranson IS
在垂直交变磁场的作用下，胶体颗粒滚动并聚集在一起。当受到电场或磁场的激励时，微观胶体颗粒的集合体表现出令人着迷的集体运动。行为范围从相干涡旋运动到相分离和动态自组装。尽管胶体系统相对简单，但了解它们的集体反应，特别是在非平衡条件下，仍然难以捉摸。我们报告了由垂直交变磁场激励的滚动铁磁微粒系统中出现的聚集和整体旋转。通过结合实验和离散粒子模拟，我们确定了导致大规模集体运动出现的主要物理机制：顺时针/逆时针粒子旋转的自发对称破缺、粒子速度的碰撞排列以及由于形状缺陷而导致的随机粒子重新定向。我们还表明，粒子之间的流体动力学相互作用对集体动力学没有定性影响。我们的发现揭示了一大类活动系统中空间和时间一致性的开始，包括合成的（胶体、机器人群和生物聚合物）和活体（细菌的悬浮液、细胞集落和鸟群）。
Energized by a vertical alternating magnetic field, colloidal particles roll and flock together. Assemblages of microscopic colloidal particles exhibit fascinating collective motion when energized by electric or magnetic fields. The behaviors range from coherent vortical motion to phase separation and dynamic self-assembly. Although colloidal systems are relatively simple, understanding their collective response, especially under out-of-equilibrium conditions, remains elusive. We report on the emergence of flocking and global rotation in the system of rolling ferromagnetic microparticles energized by a vertical alternating magnetic field. By combing experiments and discrete particle simulations, we have identified primary physical mechanisms, leading to the emergence of large-scale collective motion: spontaneous symmetry breaking of the clockwise/counterclockwise particle rotation, collisional alignment of particle velocities, and random particle reorientations due to shape imperfections. We have also shown that hydrodynamic interactions between the particles do not have a qualitative effect on the collective dynamics. Our findings shed light on the onset of spatial and temporal coherence in a large class of active systems, both synthetic (colloids, swarms of robots, and biopolymers) and living (suspensions of bacteria, cell colonies, and bird flocks).