在时间周期性流动中，细菌阻碍大尺度输运并增强小尺度混合
Bacteria hinder large-scale transport and enhance small-scale mixing in time-periodic flows
摘要：
了解被动标量在主动流体中的混合和输运对于许多自然（例如藻华）和工业（例如生物燃料，疫苗生产）过程非常重要。在这里，我们研究了一个被动标量（染料）在稀释悬浮液中的游泳大肠杆菌在实验中使用二维（2D）的时间周期性流和在一个简单的模拟混合。结果表明，细菌的存在阻碍了大规模运输，降低了整体混合率。拉伸场，从实验测得的速度场计算，表明细菌活动衰减流体拉伸和降低流动混沌。模拟表明，这种衰减可能是由于细菌沿高拉伸区域的瞬时积累。染料浓度场的空间功率谱和关联函数表明，细菌活动也阻碍了跨尺度的标量方差的传输，导致结构的平均尺寸和寿命的增加。另一方面，在小尺度上，活动似乎增强了局部混合。一个证据是，空间浓度梯度的概率分布几乎是对称的，偏度为零。总体而言，我们的研究结果表明，活动和流量之间的耦合可以导致非平凡的混合和运输的影响。
英文摘要：
Understanding mixing and transport of passive scalars in active fluids is important to many natural (e.g. algal blooms) and industrial (e.g. biofuel, vaccine production) processes. Here, we study the mixing of a passive scalar (dye) in dilute suspensions of swimming Escherichia coli in experiments using a two-dimensional (2D) time-periodic flow and in a simple simulation. Results show that the presence of bacteria hinders large scale transport and reduce overall mixing rate. Stretching fields, calculated from experimentally measured velocity fields, show that bacterial activity attenuates fluid stretching and lowers flow chaoticity. Simulations suggest that this attenuation may be attributed to a transient accumulation of bacteria along regions of high stretching. Spatial power spectra and correlation functions of dye concentration fields show that the transport of scalar variance across scales is also hindered by bacterial activity, resulting in an increase in average size and lifetime of structures. On the other hand, at small scales, activity seems to enhance local mixing. One piece of evidence is that the probability distribution of the spatial concentration gradients is nearly symmetric with a vanishing skewness. Overall, our results show that the coupling between activity and flow can lead to nontrivial effects on mixing and transport.
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