Rotating soft-tail millimeter-scaled swimmers with superhydrophilic or superhydrophobic surfaces

Microswimmers capable of propulsion at low Reynolds numbers have great potential for numerous applications. Several kinds of artificial microswimmers, actuated by magnetic fields, with rigid helical tails have been developed. In previous works, we demonstrated that a magnetic swimmer with a soft tail could form a helical shape during rotation to propel itself at low Reynolds numbers. The advantage of compliant structures is that the high softness makes swimmers more adaptive in confined or complex environments. The interfacial friction drags are reduced by using superhydrophobic surfaces. Therefore, this paper aims to study the influence of surface and interface properties, particularly the hydrophilicity and hydrophobicity of the soft tail surface, on the swimming performance. For this, we compared the swimming performance of two millimeter-scaled swimmers with a superhydrophilic soft tail and a superhydrophobic soft tail, respectively, in viscous liquids. The two swimmers exhibit different dynamic morphologies and swimming performance with the same input parameters. The swimmer with a superhydrophobic soft tail shows higher swimming velocity than that with a superhydrophilic soft tail, except in pure glycerol. The present work provides useful informations for selecting surface coating of swimmers with soft tails at low Reynolds numbers.