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Bacteria hinder large-scale transport and enhance small-scale mixing in time-periodic flows [Applied Physical Sciences]
Proceedings of the National Academy of Sciences of the United States of America ( IF 11.1 ) Pub Date : 2021-10-05 , DOI: 10.1073/pnas.2108548118
Ranjiangshang Ran 1 , Quentin Brosseau 1 , Brendan C Blackwell 1 , Boyang Qin 1 , Rebecca L Winter 1 , Paulo E Arratia 2
Affiliation  

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 reduces 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.



中文翻译:

细菌阻碍大规模运输并增强时间周期流动中的小规模混合 [应用物理科学]

了解主动流体中被动标量的混合和传输对于许多自然(例如藻华)和工业(例如生物燃料、疫苗生产)过程很重要。在这里,我们研究了在游泳大肠杆菌的稀释悬浮液中混合被动标量(染料)在使用二维 (2D) 时间周期流和简单模拟的实验中。结果表明,细菌的存在阻碍了大规模运输并降低了整体混合率。从实验测量的速度场计算得出的拉伸场表明,细菌活动会减弱流体拉伸并降低流动混沌性。模拟表明,这种衰减可能归因于细菌沿着高拉伸区域的短暂积累。染料浓度场的空间功率谱和相关函数表明,跨尺度标量变化的传输也受到细菌活动的阻碍,导致结构的平均尺寸和寿命增加。另一方面,在小范围内,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,具有消失的偏度。总的来说,我们的结果表明,活动和流动之间的耦合会对混合和运输产生重大影响。

更新日期:2021-09-28
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