Colloidal Microworms Propelling via a Cooperative Hydrodynamic Conveyor Belt

Abstract

We study propulsion arising from microscopic colloidal rotors dynamically assembled and driven in a viscous fluid upon application of an elliptically polarized rotating magnetic field. Close to a confining plate, the motion of this self-assembled microscopic worm results from the cooperative flow generated by the spinning particles which act as a hydrodynamic "conveyor belt." Chains of rotors propel faster than individual ones, until reaching a saturation speed at distances where induced-flow additivity vanishes. By combining experiments and theoretical arguments, we elucidate the mechanism of motion and fully characterize the propulsion speed in terms of the field parameters. DOI: 10.1103/PhysRevLett.115.138301 PACS numbers: 82.70.Dd, 87.85.gj Propulsion in viscous fluids plays a key role in many different contexts of biology, physics, and chemistry. From a fundamental perspective, the transport of microscopic objects in a liquid medium poses the appealing challenge to find an adequate swimming strategy due to the negligible role of inertial force compared to viscous one. At low Reynolds number the Navier-Stokes equations become time reversible [1], and any strategy based on reciprocal motion, i.e., a motion composed by symmetric backward and forward displacements, will fail to produce net propulsion [2].Facing this challenge, the last few years have witnessed the theoretical propositions of several suitable geometries and procedures to propel micromachines in viscous fluids [3][4][5][6][7][8][9]. Parallel advances in miniaturization have led to the generation of new classes of chemically powered [10,11] or externally actuated [12][13][14][15] prototypes with exciting applications in emerging fields such as microsurgery [16,17] or lab-on-a-chip technology [18,19].In contrast to reaction driven microswimmers, actuated magnetic micropropellers do not present the autonomous behavior that distinguishes force-and torque-free motion of biological organisms [20], but instead avoid efficiency reduction due to fuel shortage or directional randomization. To date, three main approaches have been developed to transport microscopic particles in a viscous fluid using uniform magnetic fields [21], namely, by actuating flexible magnetic tails [12,22], by rotating helical shaped structures [23,24], or by using the close proximity to a bounding wall [25,26]. In the latter case, it is well established that in the Stokes regime the rotational motion of a body close to a surface can be rectified into net translation due to the hydrodynamic interaction with the boundary [27]. Surface rotors are optimal to work in confined geometries such as microfluidic channels or biological networks characterized by narrow pores.In this Letter we show that an ensemble of rotors dynamically self-assembled via attractive dipolar interactions can be propelled by a hydrodynamic "conveyor-belt" effect generated by the cooperative flow of the spinning particles close to a surface. Recent theoretical works [28][29][30] have addressed the rich dynamic...

Emergent propagation modes of ferromagnetic swimmers in constrained geometries

“…[9][10][11][12][13][14][15][16][17][18][19] Magnetic actuation enables propulsion to be controlled wirelessly without affecting biological viability, an essential requirement of many biomedical applications, with the added benefit that the magnetic field also determines the direction of motion. Simple magnetic attraction of particles can be effective, but requires the magnetic field to have both a strong magnitude and gradient, so this solution is only viable when the particles are close to the magnetic poles (e.g.…”

“…[9][10][11][12][13] Alternatively, swimming has been demonstrated using interacting superparamagnetic particles, either to produce flagellum-like motion when the particles are physically connected 22 or to produce swarm-like collective motion of individual particles. [14][15][16][17] Our group proposed a third strategy based on two ferromagnetic particles with differing size and magnetic anisotropies, connected by an elastic link. 18,19 Experimental demonstration of a prototype magneto-elastic swimmer can be found in the supplementary material of reference 19.…”

“…Indeed, many swarm-type swimmers cannot propagate in bulk fluids and require the interface to generate propulsion. [15][16][17] While necessary for establishing proof of principle for swimming, this approach has neglected to consider the effects of encountering a geometrical barrier on motion of otherwise free swimmers. However, biomedical and microfluidic applications will require swimmers initially moving in bulk fluids to interact with surface barriers, such as the cell membrane, blood vessel wall or the sides of a microfluidic chamber.…”

Unstable fronts and motile structures formed by microrollers

“…In all of our experiments, ω < ω c . In contrast with recent experiments on Quincke rollers 14 , the rotation direction is prescribed and does not arise from the system dynamics.Hydrodynamics is the dominant inter-particle interaction in this system, which is distinctly different from many other systems of rotating magnetic particles, where dynamics is found to be a strong function of inter-particle magnetic interactions [15][16][17][18] . Many ferromagnetic particles with a small remnant moment could produce the same behaviour.…”

Flexible mechanism of magnetic microbeads chains in an oscillating field

“…[6][7][8][9][10][11] Magnetic actuation enables motion of the magnetic micro devises to be controlled wirelessly without affecting biological viability but with the extra benefit that the direction of motion can be determined by the field. 12,13 One of the challenges in proposing the micro devices to move in the low-Reynolds-number regime should be that a microswimmer must deform without structural instability in a way that is not invariant under time-reversal.…”