Martinez-Pedrero Fernando, Navarro-Argemí Eloy, Ortiz-Ambriz Antonio, Pagonabarraga Ignacio, Tierno Pietro
Departament de Física de la Matèria Condensada, Universitat de Barcelona, Barcelona, Spain.
Departamento de Química Física I, Universidad Complutense de Madrid, Madrid, Spain.
Sci Adv. 2018 Jan 26;4(1):eaap9379. doi: 10.1126/sciadv.aap9379. eCollection 2018 Jan.
Hydrodynamic interactions (HIs), namely, solvent-mediated long-range interactions between dispersed particles, play a crucial role in the assembly and dynamics of many active systems, from swimming bacteria to swarms of propelling microrobots. We experimentally demonstrate the emergence of long-living hydrodynamic bound states between model microswimmers at low Reynolds numbers. A rotating magnetic field forces colloidal hematite microparticles to translate at a constant and frequency-tunable speed close to a bounding plane in a viscous fluid. At high driving frequency, HIs dominate over magnetic dipolar ones, and close propelling particles couple into bound states by adjusting their translational speed to optimize the transport of the pair. The physical system is described by considering the HIs with the boundary surface and the effect of gravity, providing an excellent agreement with the experimental data for all the range of parameters explored. Moreover, we show that in dense suspensions, these bound states can be extended to one-dimensional arrays of particles assembled by the sole HIs. Our results manifest the importance of the boundary surface in the interaction and dynamics of confined propelling microswimmers.
流体动力相互作用（HIs），即分散粒子之间由溶剂介导的长程相互作用，在许多活性系统的组装和动力学中起着关键作用，从游动的细菌到成群的推进微型机器人。我们通过实验证明了在低雷诺数下模型微型游泳器之间出现了长寿命的流体动力束缚态。旋转磁场迫使胶体赤铁矿微粒在粘性流体中以恒定且频率可调的速度靠近边界平面平移。在高驱动频率下，流体动力相互作用比磁偶极相互作用占主导，靠近的推进粒子通过调整其平移速度以优化对的传输而耦合成束缚态。通过考虑与边界表面的流体动力相互作用和重力的影响来描述物理系统，这与所探索的所有参数范围内的实验数据都能很好地吻合。此外，我们表明在密集悬浮液中，这些束缚态可以扩展为由唯一的流体动力相互作用组装而成的一维粒子阵列。我们的结果表明了边界表面在受限推进微型游泳器的相互作用和动力学中的重要性。
