Propulsion and energetics of a minimal magnetic microswimmer

TL;DR

This paper presents the design, theoretical modeling, and experimental validation of a minimal magnetic microswimmer composed of a ferromagnetic nanorod and a paramagnetic sphere, demonstrating controllable propulsion and efficiency analysis.

Contribution

It introduces a novel minimal hybrid microswimmer design and provides a combined theoretical and experimental study of its propulsion dynamics and efficiency.

Findings

Reversal of microswimmer velocity by nanorod length variation

Theoretical and experimental determination of propulsion efficiency

Identification of parameter space for optimal propulsion

Abstract

In this manuscript we describe the realization of a minimal hybrid microswimmer, composed of a ferromagnetic nanorod and a paramagnetic microsphere. The unbounded pair is propelled in water upon application of a swinging magnetic field that induces a periodic relative movement of the two composing elements, where the nanorod rotates and slides on the surface of the paramagnetic sphere. When taken together, the processes of rotation and sliding describe a finite area in the parameter space, which increases with the frequency of the applied field. We develop a theoretical approach and combine it with numerical simulations, which allow us to understand the dynamics of the propeller and explain the experimental observations. Furthermore, we demonstrate a reversal of the microswimmer velocity by varying the length of the nanorod, as predicted by the model. Finally, we determine theoretically and in experiments the Lighthill's energetic efficiency of this minimal magnetic microswimmer.