Phase-separation models for swimming enhancement in complex fluids

TL;DR

This paper proposes a physical mechanism where phase separation in complex fluids creates low-viscosity layers that enhance microscopic swimmers' propulsion, challenging previous assumptions that such fluids hinder locomotion.

Contribution

It introduces models showing how phase separation near surfaces can significantly increase swimming speeds in complex fluids, supported by theoretical analysis and experimental comparison.

Findings

Phase separation creates low-viscosity layers near swimmers.

Models predict orders of magnitude increase in swimming speed.

Experimental results support the phase-separation enhancement mechanism.

Abstract

Swimming cells often have to self-propel through fluids displaying non-Newtonian rheology. While past theoretical work seems to indicate that stresses arising from complex fluids should systematically hinder low-Reynolds number locomotion, experimental observations suggest that locomotion enhancement is possible. In this paper we propose a physical mechanism for locomotion enhancement of microscopic swimmers in a complex fluid. It is based on the fact that micro-structured fluids will generically phase-separate near surfaces, leading to the presence of low-viscosity layers which promote slip and decrease viscous friction near the surface of the swimmer. We use two models to address the consequence of this phase separation: a nonzero apparent slip length for the fluid and then an explicit modeling of the change of viscosity in a thin layer near the swimmer. Considering two canonical setups for low-Reynolds number locomotion, namely the waving locomotion of a two-dimensional sheet and that of a three-dimensional filament, we show that phase-separation systematically increases the locomotion speeds, possibly by orders of magnitude. We close by confronting our predictions with recent experimental results.