Resonant alignment of microswimmer trajectories in oscillatory shear flows

TL;DR

This study investigates how microswimmers, specifically Dunaliella salina algae, respond to oscillatory shear flows, revealing a resonant alignment phenomenon that could impact active rheology and industrial processes.

Contribution

It uncovers a novel resonant alignment of microswimmer trajectories in oscillatory flows and provides a simple model explaining this behavior.

Findings

Algae align perpendicular to flow-shear plane under oscillatory shear.

Resonance in flow parameters causes ordered swimming trajectories.

Model and simulations support the experimental observations.

Abstract

Oscillatory flows are commonly experienced by swimming microorganisms in the environment, industrial applications and rheological investigations. We experimentally characterise the response of the alga {\it Dunaliella salina} to oscillatory shear flows, and report the surprising discovery that algal swimming trajectories orient perpendicular to the flow-shear plane. The ordering has the characteristics of a resonance in the driving parameter space. The behaviour is qualitatively reproduced by a simple model and simulations accounting for helical swimming, providing the mechanism for ordering and criteria for the resonant amplitude and frequency. The implications of this work for active oscillatory rheology and industrial algal processing are discussed.