Colloidal Magnus effect in polymer solutions

TL;DR

This paper explains the inverse Magnus effect observed in colloids within polymer solutions as a result of stress-gradient-induced polymer density inhomogeneities, providing a new mechanistic understanding of colloidal motion in complex fluids.

Contribution

It introduces a novel mechanism based on polymer density inhomogeneities that explains the inverse Magnus effect in colloids, aligning with experimental observations without fitting parameters.

Findings

The inverse Magnus effect arises from stress-gradient-induced polymer density inhomogeneities.

Classical viscoelastic features like normal-stress differences do not explain the phenomenon.

The proposed model quantitatively matches experimental data.

Abstract

Rotating particles moving in fluids undergo a transverse migration via the inertia-induced Magnus effect. This phenomenon vanishes at colloidal scales because inertia is negligible and the fluid flow is time reversible. Yet, recent experiments discovered an inverse Magnus effect of colloids in polymeric and micellar solutions supposedly because their viscoelasticity breaks the time reversibility. Our study shows that classical viscoelastic features -- normal-stress differences and/or shear-thinning cannot explain this phenomenon. Instead, it originates from local polymer density inhomogeneities due to their stress-gradient-induced transport, a mechanism increasingly important at smaller scales -- indeed relevant to colloidal experiments. Incorporating this mechanism into our model leads to quantitative agreement with the experiments without fitting parameters. Our work provides new insights into colloidal motion in complex fluids with microstructural inhomogeneities, offers a simple mechanistic theory for predicting the resulting migration, and underscores the necessity of assimilating these findings in future designs of micro-machinery including swimmers, actuators, rheometers, and so on.