How hydrodynamics influences the collective motion of microswimmers: A particle-based simulation study

微游泳者是处于非平衡状态的系统，表现出各种有趣的集体运动模式，从密度波和对流卷到湍流。微游泳体、活性杆和细丝的集体动力学通常是基于粗粒度连续体理论或基于颗粒的模拟来建模的，而这些理论忽略了流体动力流场。在优先项目的第二个阶段，我们将继续我们的研究，水动力学如何影响微型游泳者的集体运动。我们将使用在第一阶段开发的并行计算机代码，其中使用多粒子碰撞动力学（MPCD）模拟技术在流体环境中模拟一组称为蠕动者的模型游泳者。我们将继续研究重力作用下微游泳者的集体动力学，集中研究浮筏的形成，这在活性乳剂中可以看到，以及对齐扭矩的影响（底部重的游泳者）。在这里，有趣的集体模式，如对流卷和羽流，预计会让人想起微生物的生物对流。然后我们把蠕动的纤维粘在一起，形成有活性的杆状纤维和柔韧的细丝。它们将使我们能够探索完整的流体动力学如何影响集体模式，如细菌湍流或密度波，以及密集活性生物聚合物系统中液晶缺陷的有趣动力学。真正的微游泳者通常在粘弹性环境中运动。因此，我们将在MPCD中实现粘弹性流体，并展示这如何改变单个和一组蠕动器的运动。

Microswimmers are systems in non-equilibrium and show a variety of intriguing collective motional patterns ranging from density waves and convection rolls to turbulence. The collective dynamics of microswimmers, active rods, and filaments is often modeled on the basis of coarse-grained continuum theories or with particle-based simulations, where hydrodynamic flow fields are neglected. In the second period of the priority program we will continue with our study, how hydrodynamics influences the collective motion of microswimmers. We will use the parallelized computer code, developed in the first period, where a collection of the model swimmers called squirmers is simulated in a fluid environment using the simulation technique of multi-particle collision dynamics (MPCD).We will continue our investigation of the collective dynamics of microswimmers under gravity concentrating on the formation of floating rafts, which are seen in active emulsions, and on the effect of an aligning torque (bottom-heavy swimmers). Here, intriguing collective patterns such as convection rolls and plumes are expected reminiscent of bioconvection of microorganisms. We then glue squirmers together to form active rods and flexible filaments. They will enable us to explore how the full hydrodynamics influences collective patterns such as bacterial turbulence or density waves and the intriguing dynamics of liquid-crystal defects in systems of dense active bio-polymers. Real microswimmers typically move in a viscoelastic environment. We will thus implement a viscoelastic fluid in MPCD and show how this changes the motion of single and a collection of squirmers.