Viscoelastic propulsion of a rotating dumbbell

TL;DR

This study demonstrates how normal stress differences in viscoelastic fluids can be exploited for propulsion, using a rotating asymmetric dumbbell in experiments and modeling to understand the mechanism.

Contribution

It introduces a novel propulsion method utilizing viscoelastic normal stress differences with experimental validation and predictive modeling.

Findings

Asymmetric dumbbells achieve propulsion in viscoelastic fluids.

Normal stress differences enable net movement in the direction of the smaller sphere.

The model predicts swimming speed dependence on size ratio.

Abstract

Viscoelastic fluids impact the locomotion of swimming microorganisms and can be harnessed to devise new types of self-propelling devices. Here we report on experiments demonstrating the use of normal stress differences for propulsion. Rigid dumbbells are rotated by an external magnetic field along their axis of symmetry in a Boger fluid. When the dumbbell is asymmetric (snowman geometry), non-Newtonian normal stress differences lead to net propulsion in the direction of the smaller sphere. The use of a simple model allows to rationalise the experimental results and to predict the dependence of the snowman swimming speed on the size ratio between the two spheres.