The cost of swimming in generalized Newtonian fluids: Experiments with C. elegans

TL;DR

This study experimentally investigates how the cost of swimming for C. elegans varies in shear-thinning fluids versus Newtonian fluids, confirming that shear-thinning reduces the energy expenditure of undulatory swimming.

Contribution

It provides the first experimental validation that shear-thinning rheology decreases the swimming cost of C. elegans, aligning with theoretical predictions and offering a predictive framework.

Findings

Cost of swimming is lower or equal in shear-thinning fluids compared to Newtonian fluids.

Swimming cost scales with the fluid's effective viscosity and can be predicted from rheology and kinematics.

Results align with theoretical models, enhancing understanding of microorganism locomotion in complex fluids.

Abstract

Numerous natural processes are contingent on microorganisms' ability to swim through fluids with non-Newtonian rheology. Here, we use the model organism Caenorhabditis elegans and tracking methods to experimentally investigate the dynamics of undulatory swimming in shear-thinning fluids. Theory and simulation have proposed that the cost of swimming, or mechanical power, should be lower in a shear-thinning fluid compared to a Newtonian fluid of the same zero-shear viscosity. We aim to provide an experimental investigation into the cost of swimming in a shear-thinning fluid from (i) an estimate of the mechanical power of the swimmer and (ii) the viscous dissipation rate of the flow field, which should yield equivalent results for a self-propelled low Reynolds number swimmer. We find the cost of swimming in shear-thinning fluids is less than or equal to the cost of swimming in Newtonian fluids of the same zero-shear viscosity; furthermore, the cost of swimming in shear-thinning fluids scales with a fluid's effective viscosity and can be predicted using fluid rheology and simple swimming kinematics. Our results agree reasonably well with previous theoretical predictions and provide a framework for understanding the cost of swimming in generalized Newtonian fluids.