Comparison of the propulsion of helical robots moving at low reynolds numbers based on hard- and soft-magnetic elements

Abstract

This study compares the propulsion of scaled-up helical microrobot models, based on hard- and soft-magnetic elements under rotating magnetic fields. The experiments were performed at the millimeter scale and interpreted using hydrodynamic scaling laws to predict microscale behavior. Results show that hard-magnetic microrobots achieved step-out frequencies and maximum propulsion speeds 4.5 times higher than soft-magnetic microrobots. Below saturation magnetization, soft-magnetic microrobots demonstrated similar performance irrespective of magnetic susceptibility, highlighting that torque generation in these materials is purely geometry-dependent. Employing a tapered ribbon design increased propulsion speed by a factor of 3.5 compared to regular helical designs. These results show that the impact of using soft rather than hard magnets is manageable, allowing for biodegradable magnets such as pure iron. The theory and experiments in this paper provide a quantitative basis for selecting materials and designs.