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Irreversibility and chaos in active particle suspensions
Physical Review Fluids ( IF 2.7 ) Pub Date : 2021-01-26 , DOI: 10.1103/physrevfluids.6.013104
Sergio Chibbaro , Astrid Decoene , Sebastien Martin , Fabien Vergnet

We investigate the collective behavior of active suspensions of microswimmers immersed in a viscous fluid through numerical studies. We consider the two kind of organisms generally studied in experimental and theoretical studies, namely the pullers and the pushers, which differ in the way they are able to swim. We model them such that the body shape and the flagella used to swim are mathematical described. We tackle the problem using a new numerical approach, based on fictitious domains, which allows to fully solve the fluid-structure interaction and therefore exactly captures both the far- and near-field interactions between swimmers. Considering a two-dimensional domain for simplicity, we first investigate the state of bacterial turbulence in an unbounded domain, analyzing also their dynamical properties in terms of chaos. Then we analyze the rheological properties of the suspension by varying the different parameters. Our main findings are the following: (i) At variance with some previous studies, we show that pullers are able to trigger bacterial turbulence, although there is an asymmetry between ellipsoidal puller and pusher swimmers when elongated. (ii) Moreover, spherical pullers and pushers are found to be indistinguishable from a macroscopic point of view. (iii) We show that the complex dynamics, in particular the difference between pushers and pullers in collective properties, is related to a spontaneous breaking of the time-reversal symmetry. The key ingredients for breaking the symmetry are the concentration of the suspension and the shape of the organisms. (iv) The suspensions are chaotic in all cases, but that plays no role with regard to the macroscopic properties of the suspension. (v) Rheological signatures, already known in experiments, are analyzed and explained within this framework. We unambiguously show that our results are due to hydrodynamic interactions, whereas collisions or contacts do not play a role. The present results should be taken into account for the proposition of simplified models.

中文翻译:

活性颗粒悬浮液的不可逆性和混乱

我们通过数值研究调查了浸没在粘性流体中的微泳者的主动悬浮液的集体行为。我们考虑了在实验和理论研究中通常研究的两种生物,即拉动器推动器,它们的游泳方式不同。我们对它们进行建模,以使其形状和鞭毛用来游泳是数学上的描述。我们使用基于虚拟域的新数值方法来解决该问题,该方法可以完全解决流体与结构之间的相互作用,因此可以准确地捕获游泳者之间的远场和近场相互作用。为了简单起见,考虑二维域,我们首先研究无界域中细菌湍流的状态,并根据混沌来分析它们的动力学特性。然后我们通过改变不同的参数来分析悬浮液的流变特性。我们的主要发现如下:(i)与以前的一些研究不同,我们显示出拉拔器能够触发细菌湍流,尽管拉长的椭圆形拉拔器和推拉式游泳者之间存在不对称性。(ii)此外,从宏观的角度来看,球形拉拔器和推杆是无法区分的。(iii)我们发现复杂的动力学,特别是推动器和牵引器之间的集体属性差异,与时间反转对称性的自发破坏有关。破坏对称性的关键因素是悬浮液的浓度和生物体的形状。(iv)悬浮液在所有情况下都是混乱的,但是对于悬浮液的宏观性能而言,它并不起作用。(v)在此框架内分析和解释了实验中已知的流变学特征。我们明确地表明,我们的结果是由于流体动力相互作用而引起的,而碰撞或接触并不起作用。
更新日期:2021-01-26
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