Morphologies and swimming characteristics of rotating magnetic swimmers with soft tails at low Reynolds numbers

Helical microswimmers capable of propulsion at low Reynolds numbers have been proposed for numerous applications. Several different kinds of helical swimmers inspired by E. coli bacteria have been proposed by researchers, and most of them have rigid helical tails. However, high softness could make swimmers more adaptive in confined environments for biomedical applications. This paper aims to study the morphologies and the swimming characteristics of magnetically actuated swimmers with belt-like soft tails initially straight at low Reynolds numbers. We found that a swimmer with a soft tail during rotations shows different morphologies: a twisted shape until the input frequency increases to a threshold frequency, and a helical shape until a step-out frequency. Beyond the stepout frequency, the shape of the soft tail becomes irregular. The soft tail swimmers with different lengths show similar swimming velocities at same rotational frequencies. However, their maximal swimming velocities are different because of the varied step-out frequencies. The interactions between the soft tails reduce the swimming velocity, and this influence increases with the rotational frequency. Thus, the swimming performance is not improved by doubling the number of soft tails in our experiments.