Swimming by reciprocal motion at low Reynolds number

TLDR

Microorganisms normally swim by nonreciprocal flagellar motions at low Reynolds numbers, a requirement imposed by Purcell’s scallop theorem, but this constraint is relaxed in non‑Newtonian fluids where the theorem does not apply. The study aims to demonstrate that a microswimmer can achieve propulsion through reciprocal body‑shape changes in non‑Newtonian fluids, offering a simple actuation strategy for biomedical microdevices. A single‑hinge micro‑scallop was shown to propel in both shear‑thickening and shear‑thinning fluids via reciprocal motion at low Reynolds numbers, with experimental results matching theory and confirming that viscosity modulation drives the net propulsion.

Abstract

Abstract Biological microorganisms swim with flagella and cilia that execute nonreciprocal motions for low Reynolds number (Re) propulsion in viscous fluids. This symmetry requirement is a consequence of Purcell’s scallop theorem, which complicates the actuation scheme needed by microswimmers. However, most biomedically important fluids are non-Newtonian where the scallop theorem no longer holds. It should therefore be possible to realize a microswimmer that moves with reciprocal periodic body-shape changes in non-Newtonian fluids. Here we report a symmetric ‘micro-scallop’, a single-hinge microswimmer that can propel in shear thickening and shear thinning (non-Newtonian) fluids by reciprocal motion at low Re. Excellent agreement between our measurements and both numerical and analytical theoretical predictions indicates that the net propulsion is caused by modulation of the fluid viscosity upon varying the shear rate. This reciprocal swimming mechanism opens new possibilities in designing biomedical microdevices that can propel by a simple actuation scheme in non-Newtonian biological fluids.