Quirk of microswimmer propellers found to enable bimodal swimming and different applications

Microswimmers, tiny speedy robots, have a range of potential applications from delivering drugs in a body to acting as sensors or contrast agents. However, microswimmers to date are limited in their mobility.

The most common microswimmers use helicoidal propellers, which give them a linear velocity-frequency dependence but typically only allow unidirectional motion. Propellers with nonlinear velocity-frequency relationships were previously avoided due to their wobbling behaviors, but Bachmann et al. found a branching solution, so the wobbling could enable bimodal swimming.

Building on their previous study, which predicted dumbbell-shaped propellers could be used as a frequency-controlled bidirectional microswimmer, the authors 3D-printed a dumbbell microswimmer to characterize its response both experimentally and with numerical hydrodynamic calculations.

The authors found the magnetic moment of the dumbbell shape, which has a nonlinear frequency-velocity dependence, allowed the microswimmer’s orientation and its swimming direction to be manipulated with constant magnetic field components. This worked before actuation and during actuation when it could be used to direct the microswimmer.

“The most exciting part of our results is the message that microswimmers with nonlinear frequency-velocity dependence can be controlled and can therefore be utilized,” said author Felix Bachmann. “This hopefully breaks up some previously set rules about microswimmer design and enables new concepts from cargo transport to sensing and swarm control.”

The authors hope their results will inspire researchers to study different shapes that can further enhance control of their movement. They are continuing to work on developing a range of movements by combining different shapes, magnetic moment orientations, branch determination and tumble-like step-out behavior.

Source: “Opportunities and utilization of branching and step-out behavior in magnetic microswimmers with a nonlinear response,” by Felix Bachmann, Joshua Giltinan, Agnese Codutti, Stefan Klumpp, Metin Sitti, and Damien Faivre, Applied Physics Letters (2021). The article can be accessed at https://doi.org/10.1063/5.0045454 .

This paper is part of the Mesoscopic Magnetic Systems: From Fundamental Properties to Devices Collection, learn more here .