Flagellar Kinematics and Swimming of Algal Cells in Viscoelastic Fluids

TL;DR

This study experimentally investigates how viscoelastic fluids affect the flagellar beating patterns and swimming speeds of the alga Chlamydomonas reinhardtii, revealing complex influences of fluid elasticity on microorganism motility.

Contribution

It provides direct experimental evidence of how fluid elasticity modifies flagellar gait and swimming performance in bi-flagellated algae.

Findings

Fluid elasticity enhances beating frequency and wave speed.

Net swimming speed is reduced in sufficiently elastic fluids.

Elastic stresses influence flagellar cycle dynamics.

Abstract

The motility of microorganisms is influenced greatly by their hydrodynamic interactions with the fluidic environment they inhabit. We show by direct experimental observation of the bi-flagellated alga Chlamydomonas reinhardtii that fluid elasticity and viscosity strongly influence the beating pattern - the gait - and thereby control the propulsion speed. The beating frequency and the wave speed characterizing the cyclical bending are both enhanced by fluid elasticity. Despite these enhancements, the net swimming speed of the alga is hindered for fluids that are sufficiently elastic. The origin of this complex response lies in the interplay between the elasticity-induced changes in the spatial and temporal aspects of the flagellar cycle and the buildup and subsequent relaxation of elastic stresses during the power and recovery strokes.