Department of Physics and Shenzhen Research Institute, The Chinese University of Hong Kong, Shatin, Hong Kong, China.
Center for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, China.
Nature. 2017 Feb 9;542(7640):210-214. doi: 10.1038/nature20817. Epub 2017 Jan 23.
Collective oscillatory behaviour is ubiquitous in nature, having a vital role in many biological processes from embryogenesis and organ development to pace-making in neuron networks. Elucidating the mechanisms that give rise to synchronization is essential to the understanding of biological self-organization. Collective oscillations in biological multicellular systems often arise from long-range coupling mediated by diffusive chemicals, by electrochemical mechanisms, or by biomechanical interaction between cells and their physical environment. In these examples, the phase of some oscillatory intracellular degree of freedom is synchronized. Here, in contrast, we report the discovery of a weak synchronization mechanism that does not require long-range coupling or inherent oscillation of individual cells. We find that millions of motile cells in dense bacterial suspensions can self-organize into highly robust collective oscillatory motion, while individual cells move in an erratic manner, without obvious periodic motion but with frequent, abrupt and random directional changes. So erratic are individual trajectories that uncovering the collective oscillations of our micrometre-sized cells requires individual velocities to be averaged over tens or hundreds of micrometres. On such large scales, the oscillations appear to be in phase and the mean position of cells typically describes a regular elliptic trajectory. We found that the phase of the oscillations is organized into a centimetre-scale travelling wave. We present a model of noisy self-propelled particles with strictly local interactions that accounts faithfully for our observations, suggesting that self-organized collective oscillatory motion results from spontaneous chiral and rotational symmetry breaking. These findings reveal a previously unseen type of long-range order in active matter systems (those in which energy is spent locally to produce non-random motion). This mechanism of collective oscillation may inspire new strategies to control the self-organization of active matter and swarming robots.
集体振荡行为在自然界中无处不在，在从胚胎发生和器官发育到神经元网络起搏等许多生物过程中都起着至关重要的作用。阐明产生同步的机制对于理解生物自组织至关重要。生物多细胞系统中的集体振荡通常源于通过扩散化学物质、电化学机制或细胞与其物理环境之间的生物力学相互作用进行的远程耦合。在这些例子中，一些振荡的细胞内自由度的相位被同步。相比之下，这里报告了一种不需要远程耦合或单个细胞固有振荡的弱同步机制的发现。我们发现，密集细菌悬浮液中的数百万个运动细胞可以自我组织成高度稳健的集体振荡运动，而单个细胞则以不规则的方式移动，没有明显的周期性运动，但有频繁、突然和随机的方向变化。个体轨迹如此不规则，以至于要揭示我们微米大小的细胞的集体振荡，需要将个体速度平均到数十或数百微米。在这种大尺度上，振荡似乎处于相位，细胞的平均位置通常描述一个规则的椭圆形轨迹。我们发现，振荡的相位被组织成厘米级的传播波。我们提出了一个具有严格局部相互作用的噪声自推进粒子模型，该模型忠实地解释了我们的观察结果，表明自组织的集体振荡运动是自发手性和旋转对称性破缺的结果。这些发现揭示了活性物质系统（那些通过局部消耗能量来产生非随机运动的系统）中以前未见的长程有序类型。这种集体振荡机制可能会激发控制活性物质自组织和群体机器人的新策略。
