Locomotion of a flexible one-hinge swimmer in Stokes regime

TL;DR

This study demonstrates that flexibility enables a one-hinge swimmer in viscous fluids to break the Scallop theorem and achieve propulsion, with optimal velocity near a specific Sperm number, using a bead-spring model and mesoscopic simulations.

Contribution

It introduces a flexible one-hinge swimmer model that breaks the Scallop theorem without a passive head, showing the importance of flexibility in low Reynolds number propulsion.

Findings

Flexibility breaks time reversal symmetry in the swimmer.

Maximum velocity occurs at Sperm number around 1.7.

Propulsion depends on flexibility, frequency, and amplitude.

Abstract

E. M. Purcell showed that a body has to perform non-reciprocal motion in order to propel itself in a highly viscous environment. The swimmer with one degree of freedom is bound to do reciprocal motion, whereby the center of mass of the swimmer will not be able to propel itself due to the Scallop theorem. In the present study, we are proposing a new artificial swimmer called the one hinge swimmer. Here we will show that flexibility plays a crucial role in the breakdown of Scallop theorem in the case of one-hinge swimmer or two-dimensional scallop at low Reynolds number. To model a one-hinge artificial swimmer, we use bead spring model for two arms joined by a hinge with bending potential for the arms in order to make them semi-flexible. The fluid is simulated using a particle based mesoscopic simulation method called the multi-particle collision dynamics with Anderson thermostat. Here we show that when our swimmer has rigid arms, the center of mass of the swimmer is not able to propel itself as expected from the Scallop theorem. When we introduce flexibility in the arms, the time reversal symmetry breaks in the case of the one-hinged swimmer without the presence of a head contrary to the one-armed super paramagnetic swimmer which required a passive head in order to swim. The reduced velocity of the swimmer is studied using a range of parameters like flexibility, beating frequency and the amplitude of the beat, where we obtain similar scaling as that of the one-armed super paramagnetic swimmer. We also calculate the dimensionless Sperm number for the swimmer and we get the maximum velocity when the Sperm number is around 1.7.