Thrifty swimming with shear-thinning

TL;DR

This study highlights the importance of accounting for out-of-plane effects in experimental analysis of microscale undulatory swimming in shear-thinning fluids, revealing significant impacts on viscosity and power estimates.

Contribution

It demonstrates that neglecting out-of-plane effects leads to overestimations of viscosity and power in experiments with shear-thinning fluids, emphasizing the need for accurate flow analysis.

Findings

Neglecting out-of-plane effects causes overestimation of fluid viscosity.

Overestimating viscosity leads to inflated power expenditure calculations.

Accurate flow tracking is essential for understanding microscale swimming in non-Newtonian fluids.

Abstract

Microscale propulsion is integral to numerous biomedical systems, for example biofilm formation and human reproduction, where the surrounding fluids comprise suspensions of polymers. These polymers endow the fluid with non-Newtonian rheological properties, such as shear-thinning and viscoelasticity. Thus, the complex dynamics of non-Newtonian fluids presents numerous modelling challenges, strongly motivating experimental study. Here, we demonstrate that failing to account for "out-of-plane" effects when analysing experimental data of undulatory swimming through a shear-thinning fluid results in a significant overestimate of fluid viscosity around the model swimmer C. elegans. This miscalculation of viscosity corresponds with an overestimate of the power the swimmer expends, a key biophysical quantity important for understanding the internal mechanics of the swimmer. As experimental flow tracking techniques improve, accurate experimental estimates of power consumption using this technique will arise in similar undulatory systems, such as the planar beating of human sperm through cervical mucus, will be required to probe the interaction between internal power generation, fluid rheology, and the resulting waveform.