Symmetric Mixtures of Pusher and Puller Microswimmers Behave as Noninteracting Suspensions.,Physical Review Letters

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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

χ \geq 0.5

. We then show analytically and numerically that the two-point hydrodynamic correlations of the

1 ∶ 1

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

1 ∶ 1

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。