Understanding if and how the chirality of biomolecules is transferred across scales into larger components and active processes remains elusive. For instance, flagellated bacteria swim in helical trajectories but chirality seems absent from the active turbulence dynamics they display in dense suspensions. We address this question by examining multiscale dynamics in Paenibacillus vortex colonies. We find active turbulence without manifest chirality in the bulk, but wide, clockwise (viewed from the air side) circulation all along the tortuous centimeter-scale external boundary, while similar but counterclockwise flows follow internal boundaries. We trace the origin of these robust edge flows to an unexpected asymmetry at the individual level that is amplified by local interactions. We rationalize our findings with a model of noisy self-propelled particles immersed in a Stokes fluid that accounts faithfully for our observations. Our modeling and experimental efforts reveal that local nematic alignment and hydrodynamic interactions amplify the weak chiral bias in individual motion, promoting the formation of strong edge flows. The likely topological protection of these flows provides robust transport mechanism over large scales. Such robust boundary phenomena in weakly chiral active fluids may inspire new control strategies for active and biological matter.

中文翻译：

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成群细菌菌落中的稳健边缘流


了解生物分子的手性是否以及如何跨规模转移到更大的组分和活性过程中仍然难以捉摸。例如，鞭毛细菌在螺旋轨迹中游泳，但它们在致密悬浮液中表现出的主动湍流动力学中似乎不存在手性。我们通过检查 Paenibacillus 涡旋菌落中的多尺度动力学来解决这个问题。我们发现活跃的湍流在体中没有明显的手性，但沿着曲折的厘米级外部边界沿顺时针方向（从空气侧观察）循环很宽，而类似但逆时针方向的流动遵循内部边界。我们将这些稳健的边缘流的根源追溯到个体层面上意想不到的不对称性，这种不对称性被局部相互作用放大了。我们用一个浸没在斯托克斯流体中的嘈杂自走粒子模型来合理化我们的发现，该模型忠实地解释了我们的观察结果。我们的建模和实验工作表明，局部向列排列和流体动力学相互作用放大了个体运动中的弱手性偏差，促进了强边缘流的形成。这些流的可能拓扑保护提供了大尺度的稳健传输机制。弱手性活性流体中这种稳健的边界现象可能会激发活性和生物物质的新控制策略。