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

微泳者是处于非平衡状态的系统，表现出从密度波、对流卷到湍流的各种有趣的集体运动模式。微泳者、活跃杆和细丝的集体动力学通常基于粗粒连续介质理论或基于粒子的模拟，而忽略了水动力流场。在优先项目的第二阶段，我们将继续我们的研究，水动力学如何影响微泳运动员的集体运动。我们将使用第一阶段开发的并行计算机代码，在该代码中，使用多粒子碰撞动力学(MPCD)的模拟技术在流体环境中模拟一组名为Squirmer的模型游泳运动员。我们将继续研究重力下微型游泳运动员的集体动力学，重点是浮筏的形成，这是在主动乳状液中看到的，以及对齐扭矩的影响(重底泳运动员)。在这里，有趣的集体模式，如对流卷曲和羽流，预计会让人想起微生物的生物对流。然后，我们将蠕动棒粘在一起，形成活性棒和柔韧的细丝。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，以及致密活性生物聚合物系统中液晶缺陷的有趣动力学。真正的微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蠕动者和一群蠕动者的运动。

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.