Emergent Hydrodynamic Bound States Between Magnetically Powered Micropropellers

TL;DR

This study demonstrates the formation of long-lasting hydrodynamic bound states between magnetically powered micropropellers at low Reynolds number, highlighting the role of boundary surfaces and hydrodynamic interactions in their collective behavior.

Contribution

It experimentally shows hydrodynamic bound states between micro-swimmers driven by magnetic fields, emphasizing the boundary surface's influence and extending to arrays in dense suspensions.

Findings

Hydrodynamic interactions dominate over magnetic dipolar forces at high frequencies.

Bound states form by particles adjusting their speeds to optimize transport.

Dense suspensions can form one-dimensional arrays via hydrodynamic interactions.

Abstract

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. Here we experimentally demonstrate the emergence of long-living hydrodynamic bound states between model micro-swimmers at low Reynolds number. 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 in order 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 onedimensional 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.