Helical propulsion in shear-thinning fluids

TL;DR

This study demonstrates that helical microswimmers in shear-thinning fluids swim faster than in Newtonian fluids, with speed increases up to 50%, likely due to viscosity stratification effects rather than local viscosity changes.

Contribution

It provides experimental evidence of enhanced propulsion in shear-thinning fluids and proposes a new hypothesis involving viscosity stratification as the cause.

Findings

Swimming speed increases up to 50% in shear-thinning fluids.

Speed scales linearly with rotation frequency, with fluid-dependent prefactors.

Viscosity stratification may cause the speed enhancement.

Abstract

Swimming microorganisms often have to propel in complex, non-Newtonian fluids. We carry out experiments with self-propelling helical swimmers driven by an externally rotating magnetic field in shear-thinning, inelastic fluids. Similarly to swimming in a Newtonian fluid, we obtain for each fluid a locomotion speed which scales linearly with the rotation frequency of the swimmer, but with a prefactor which depends on the power index of the fluid. The fluid is seen to always increase the swimming speed of the helix, up to 50% faster and thus the strongest of such type reported to date. The maximum relative increase for a fluid power index of around 0.6. Using simple scalings, we argue that the speed increase is not due to the local decrease of the flow viscosity around the helical filament but hypothesise instead that it originates from confinement-like effect due to viscosity stratification around the swimmer.