Symmetric shear banding and swarming vortices in bacterial superfluids
Symmetric shear banding and swarming vortices in bacterial superfluids
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DOI：
10.1073/pnas.1722505115
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发表时间：
2018-07-10
影响因子：
11.1
通讯作者：
Cheng, Xiang
中科院分区：
文献类型：
--
作者：
Guo, Shuo;Samanta, Devranjan;Cheng, Xiang
细菌悬浮液-活性流体的首要例子-显示出对剪切应力的不寻常的响应。而不是增加悬浮液的粘度，游泳细菌的紧急集体运动可以将悬浮液变成具有零表观粘度的超流体。虽然在体相流变学测量中已经证明了活性超流体的存在，但是这种奇异相的微观起源和动力学还没有被实验探索。在这里，使用高速共聚焦流变仪，我们研究了简单的平面剪切下的浓缩细菌悬浮液的动力学。我们发现，细菌超流体剪切下表现出不寻常的对称剪切带，无视传统的智慧剪切带的复杂流体，稳定的剪切带的形成必然会打破对称性的非剪切样品。我们提出了一个简单的流体动力学模型的基础上局部应力平衡和遍历采样的非平衡剪切配置，定量描述所观察到的对称剪切带结构。该模型还成功地预测了各种有趣的功能，在静止的细菌悬浮液的群集漩涡。我们的研究提供了深入了解集体群集在活跃的流体中的物理特性，并说明了它们对运输过程的深远影响。
Bacterial suspensions-a premier example of active fluids-show an unusual response to shear stresses. Instead of increasing the viscosity of the suspending fluid, the emergent collective motions of swimming bacteria can turn a suspension into a superfluid with zero apparent viscosity. Although the existence of active superfluids has been demonstrated in bulk rheological measurements, the microscopic origin and dynamics of such an exotic phase have not been experimentally probed. Here, using high-speed confocal rheometry, we study the dynamics of concentrated bacterial suspensions under simple planar shear. We find that bacterial superfluids under shear exhibit unusual symmetric shear bands, defying the conventional wisdom on shear banding of complex fluids, where the formation of steady shear bands necessarily breaks the symmetry of unsheared samples. We propose a simple hydrodynamic model based on the local stress balance and the ergodic sampling of nonequilibrium shear configurations, which quantitatively describes the observed symmetric shear-banding structure. The model also successfully predicts various interesting features of swarming vortices in stationary bacterial suspensions. Our study provides insights into the physical properties of collective swarming in active fluids and illustrates their profound influences on transport processes.