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Symmetric Mixtures of Pusher and Puller Microswimmers Behave as Noninteracting Suspensions.
Physical Review Letters ( IF 8.1 ) Pub Date : 2020-07-01 , DOI: 10.1103/physrevlett.125.018003
Dóra Bárdfalvy 1 , Shan Anjum 1 , Cesare Nardini 2, 3 , Alexander Morozov 4 , Joakim Stenhammar 1
Affiliation  

Suspensions of rear- and front-actuated microswimmers immersed in a fluid, known respectively as “pushers” and “pullers,” display qualitatively different collective behaviors: beyond a characteristic density, pusher suspensions exhibit a hydrodynamic instability leading to collective motion known as active turbulence, a phenomenon which is absent for pullers. In this Letter, we describe the collective dynamics of a binary pusher–puller mixture using kinetic theory and large-scale particle-resolved simulations. We derive and verify an instability criterion, showing that the critical density for active turbulence moves to higher values as the fraction χ of pullers is increased and disappears for χ0.5. We then show analytically and numerically that the two-point hydrodynamic correlations of the 11 mixture are equal to those of a suspension of noninteracting swimmers. Strikingly, our numerical analysis furthermore shows that the full probability distribution of the fluid velocity fluctuations collapses onto the one of a noninteracting system at the same density, where swimmer-swimmer correlations are strictly absent. Our results thus indicate that the fluid velocity fluctuations in 11 pusher-puller mixtures are exactly equal to those of the corresponding noninteracting suspension at any density, a surprising cancellation with no counterpart in equilibrium long-range interacting systems.

中文翻译:

推杆和拉杆微泳器的对称混合物被视为非相互作用的悬浮液。

浸入流体的后驱动和前驱动微悬浮液的悬浮液(分别称为“推杆”和“拉杆”)在质量上有不同的集体行为:超过特征密度,推杆悬架表现出流体动力学的不稳定性,导致集体运动,称为主动湍流,拉拔器不存在的现象。在这封信中,我们使用动力学理论和大规模粒子分解模拟来描述二元推进器​​-混合器混合物的集体动力学。我们推导并验证了不稳定性判据,表明随着湍流分数的增加,主动湍流的临界密度将移至更高的值。χ 的拉拔器增加并且消失 χ0.5。然后,我们通过分析和数字显示,两点的水动力相关性1个1个混合物等于非互动游泳者悬浮液的混合物。引人注目的是,我们的数值分析还表明,流体速度波动的全部概率分布在相同密度下坍塌到一个非相互作用系统中,其中严格缺乏游泳者与游泳者的相关性。因此,我们的结果表明流体速度在1个1个推拉混合器的混合物在任何密度下都完全等于相应的非相互作用悬浮液的混合物,这是令人惊讶的抵消,在平衡远程相互作用系统中没有对应物。
更新日期:2020-07-01
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