Formation of a strong negative wake behind a helical swimmer in a viscoelastic fluid

TL;DR

This study explores how the shape of a helical swimmer affects its movement in viscoelastic fluids, revealing the formation of a negative wake and the role of polymer stresses in swimming dynamics.

Contribution

It demonstrates the influence of swimmer shape on speed and flow patterns in viscoelastic fluids, supported by experiments and 3D simulations using the FENE-P model.

Findings

Increasing tail thickness and pitch angle boosts swimming speed.

A strong negative wake forms behind the swimmer in viscoelastic fluids.

Polymer stresses can enhance or hinder swimming performance.

Abstract

We investigate the effects of helical swimmer shape (i.e., helical pitch angle and tail thickness) on swimming dynamics in a constant viscosity viscoelastic (Boger) fluid via a combination of particle tracking velocimetry, particle image velocimetry and 3D simulations of the FENE-P model. The 3D printed helical swimmer is actuated in a magnetic field using a custom-built rotating Helmholtz coil. Our results indicate that increasing the swimmer tail thickness and pitch angle enhances the normalized swimming speed (i.e., ratio of swimming speed in the Boger fluid to that of the Newtonian fluid). Strikingly, unlike the Newtonian fluid, the viscoelastic flow around the swimmer is characterized by formation of a front-back flow asymmetry that is characterized by a strong negative wake downstream of the swimmer. Evidently, the strength of the negative wake is inversely proportional to the normalized swimming speed. Three-dimensional simulations of the swimmer with FENE-P model with conditions that match those of experiments, confirm formation of a similar front-back flow asymmetry around the swimmer. Finally, by developing an approximate force balance in the streamwise direction, we show that the contribution of polymer stresses in the interior region of the helix may provide a mechanism for swimming enhancement or diminution in the viscoelastic fluid.