Dynamic Morphology and Swimming Properties of Rotating Miniature Swimmers With Soft Tails

Abstract

Helical microswimmers are suitable for propelling at low Reynolds numbers. To date, artificial microswimmers with rigid helical tails have been significantly developed. However, swimmers made of soft materials, which are more adaptive in confined or complex environments, are still lack of adequate investigation. In our previous studies, we demonstrated that a magnetically actuated swimmer with a plate soft tail could form a helical structure in a rotating magnetic field to propel itself at low Reynolds numbers. In this paper, we report the different dynamic morphologies formed by the soft tail of the swimmers during a simple rotation. We prove that the pitch length of the formed helical structure dynamically decreases with the rotation frequency. The formed helical structure collapses down to a cylindricallike structure, when the pitch gap of the soft tail decreases to zero. The corresponding rotation frequency is defined as a morphological step-out frequency, which is different from the magnetic step-out frequency in previous works. At the morphological step-out frequency, the swimmers performs a pure rotation without a forward propulsion, which is defined as a “dynamic stop.” The pitch length and the morphological step-out frequency increase with the rigidity of the tail. More interestingly, we found that if the rotation frequency keeps increasing, the swimmer may achieve a reverse propulsion without changing any other input parameters. The dynamic stop and reverse swimming cannot be observed if the soft tails are too long. We expect that the dynamic swimming phenomena found in the swimmers with soft tails can be used for agile and delicate motion control of soft miniature swimmers, which will be the base of numerous biomedical applications.