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.
Highlights
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
Theories on the small amplitude swimming of infinitely long wave-like sheets suggest that elasticity can reduce swimming speed[16,17] and these predictions are consistent with experimental observations of undulatory swimming in C. elegans[20]
Summary
The motility of microorganisms is influenced greatly by their hydrodynamic interactions with the fluidic environment they inhabit. While the dependence of U1 is similar to the Newtonian case, the recovery speed U2 in viscoelastic fluids remains relatively unchanged, and modestly increases with viscosity This raises the possibility that fluid elasticity affects power and recovery strokes very differently - a view supported. Our results show that fluid elasticity influences kinematics and swimming speeds in a manner different from just shear thinning viscosity, which for instance cannot account for the non-monotonic dependence of beating frequency on polymer concentration or viscosity (Fig. 2a) as well as the dramatic increase (
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