On the cross-streamline lift of microswimmers in viscoelastic flows

TL;DR

This paper investigates how microswimmers in weakly viscoelastic flows experience a unique lift force, revealing that their swimming mechanism significantly influences their cross-streamline migration, with potential implications for understanding microscale navigation.

Contribution

The study introduces analytical expressions for the viscoelastic swimming lift on different swimmer types, highlighting the dependence on their hydrodynamic signatures and revealing new effects on their migration.

Findings

Neutral swimmers focus towards the centerline faster by two orders of magnitude.

Force-dipole swimmers show no net change in cross-streamline migration.

Viscoelastic effects depend on the swimmer's hydrodynamic signature.

Abstract

The current work studies the dynamics of a microswimmer in pressure-driven flow of a weakly viscoelastic fluid. Employing the second-order fluid model, we show that the self-propelling swimmer experiences a viscoelastic swimming lift in addition to the well-known passive lift that arises from its resistance to shear flow. Using the reciprocal theorem, we evaluate analytical expressions for the swimming lift experienced by neutral and pusher/puller-type swimmers and show that they depend on the hydrodynamic signature associated with the swimming mechanism. We find that for neutral swimmers focusing towards the centerline is accelerated by two orders of magnitude, while for force-dipole swimmers no net modification in cross-streamline migration occurs.