Abstract
Microswimmers are smart devices with potential applications in medicine and biotechnology at the micrometer-scale. Magnetic micropropellers with their remote control via rotating magnetic fields are especially auspicious. Helicoidal propellers with a linear velocity–frequency dependence emerged as the standard propulsion mechanism over the last decade. However, with their functions becoming more pivotal on the way to practical uses, deviations in shape and swimming behavior are inevitable. Consequently, propellers with nonlinear velocity–frequency relationships arise that not only pose different challenges but also offer advanced possibilities. The most critical nonlinearities are the wobbling behavior with its solution branching that has potential for bimodal swimming and the swimming characteristics in the step-out regime that are essential for selection and swarm control. Here, we show experimentally and with numerical calculations how the previously unpredictable branching can be controlled and, thus, becomes utilizable with an example 3D-printed swimmer device. Additionally, we report how two step-out modes arise for propellers with a nonlinear velocity–frequency dependence that have the potential to accelerate future microswimmer sorting procedures.
Highlights
Have positive outcomes as they can be utilized for picking up or releasing cargo or drugs,[26] sensing,[27] changing swimming directions,[12,22,23,28] or the control of propeller swarms via their step-out behavior.[20,29–31] Step-out occurs above a certain frequency fso and describes a nonlinear decline in propulsion velocity the actuation frequency is increasing.[13,32,33] Overall, nonlinearity of the velocity–frequency dependence will become a more prominent challenge in future generation of magnetic micropropellers, a challenge that offers additional opportunities by utilizing these special characteristics
The focus will be on influencing the occurring branching behavior, as it is in our view both the main challenge and the main opportunity when it comes to utilizing nonlinear magnetic micropropellers
Branching describes the existence of two swimming velocity responses at the very same actuation frequency caused by two different stable solutions for the propeller dynamics.[21,23,35]
Summary
Have positive outcomes as they can be utilized for picking up or releasing cargo or drugs,[26] sensing,[27] changing swimming directions,[12,22,23,28] or the control of propeller swarms via their step-out behavior.[20,29–31] Step-out occurs above a certain frequency fso and describes a nonlinear decline in propulsion velocity the actuation frequency is increasing.[13,32,33] Overall, nonlinearity of the velocity–frequency dependence will become a more prominent challenge in future generation of magnetic micropropellers, a challenge that offers additional opportunities by utilizing these special characteristics. The most critical nonlinearities are the wobbling behavior with its solution branching that has potential for bimodal swimming and the swimming characteristics in the step-out regime that are essential for selection and swarm control.
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