Minimal geometric requirements for micropropulsion via magnetic rotation

Abstract

Controllable propulsion of microscale and nanoscale devices enhanced with additional functionality would enable the realization of miniaturized robotic swimmers applicable to transport and assembly, actuators, and drug delivery systems. Following biological examples, existing magnetically actuated microswimmers have been designed to use flexibility or chirality, presenting fabrication challenges. Here we show that, contrary to biomimetic expectations, magnetically actuated geometries with neither flexibility nor chirality can produce propulsion, through both experimental demonstration and a theoretical analysis, which elucidates the fundamental constraints on micropropulsion via magnetetic rotation. Our results advance existing paradigms of low-Reynolds-number propulsion, possibly enabling simpler fabrication and design of microswimmers and nanoswimmers.

Controlled Propulsion of Two‐Dimensional Microswimmers in a Precessing Magnetic Field

“…Previously, the forward propulsion of 2D structures was induced by using the so‐called “wobbling regime,” in which the precession angle was highly dependent on the field frequency and strength. [10a] In this case, the wobbling motion was not induced at low frequency, with tumbling motion not resulting in forward propulsion observed instead. However, in a precessing field, the precessing motion of swimmer is introduced not by this wobbling regime, but by the precessing motion of the field itself.…”

“…As there is no external force, F = 0 , and the direction of the forward swimming velocity is collinear with the precession axis, allowing the forward swimming speed to be calculated aswhich means that the rotational axis must have components in both the x‐ and z ‐directions, as previously shown by Cheang et al[10a] Note that the minimal geometric requirement for the swimmer is actually ( B 13 / A 1 + B 31 / A 3 ) ≠ 0, not B ≠ 0 . Herein, we introduced non‐zero Ω 1 and Ω 3 motion by applying a precessing field, which allowed us to tune the wobbling angle independently of the applied rotational frequency.…”

“…However, these swimmers are not capable of swimming away from the surface. Recently, achiral microswimmers comprising three magnetic beads connected by streptavidin–biotin interactions have been reported, being actuated by a rotating magnetic field high enough to induce a wobbling motion for forward propulsion. Similarly, the swimming performance of non‐helical randomly shaped structures has also been investigated .…”

Biohybrid and Bioinspired Magnetic Microswimmers

“…Magnetic actuation. Rotation of magnetic objects with a minimum amount of asymmetry which is able to swim in a rotating magnetic field: the most common and stable form are a,d,e) helical swimmers. Recently, flexible tails in a rotating field similar to bacteria flagella are in the focus of research .…”

Magnetically actuated and controlled colloidal sphere-pair swimmer

“…In some situations, one may be able to employ feedback to obtain desired non-reciprocal motion. Certain magnetically controlled swimming structures such as screws [5], waving tails [6,7], rigidly connected rods [8], and multiple beads connected in various ways [9][10][11][12][13][14] can execute movement required for swimming in a stable way without the need for feedback on relative positions of different parts. It remains unclear, however, if stable swimming of much simpler structures like disconnected colloidal particles free to move independently is possible without any feedback.…”