Effective viscosity of non-gravitactic Chlamydomonas Reinhardtii microswimmer suspensions

Abstract

Active microswimmers are known to affect the macroscopic viscosity of suspensions in a more complex manner than passive particles. For puller-like microswimmers an increase in the viscosity has been observed. It has been suggested that the persistence of the orientation of the microswimmers hinders the rotation that is normally caused by the vorticity. It was previously shown that some sorts of algaes are bottom-heavy swimmers, i.e. their centre of mass is not located in the centre of the body. In this way, the algae affects the vorticity of the flow when it is perpendicular oriented to the axis of gravity. This orientation of gravity to vorticity is given in a rheometer that is equipped with a cone-plate geometry. Here we present measurements of the viscosity both in a cone-plate and a Taylor-Couette cell. The two set-ups yielded the same increase in viscosity although the axis of gravitation in the Taylor-Couette cell is parallel to the direction of vorticity. In a complementary experiment we tested the orientation of the direction of swimming through microscopic observation of single \textit{Chlamydomonas reinhardtii} and could not identify a preferred orientation, i. e. our specific strain of \textit{Chlamydomonas reinhardtii} are not bottom-heavy swimmers. We thus conclude that bottom heaviness is not a prerequisite for the increase of viscosity and that the effect of gravity on the rheology of our strain of \textit{Chlamydomonas reinhardtii} is negligible. This finding reopens the question of whether origin of persistence in the orientation of cells is actually responsible for the increased viscosity of the suspension.

References in corpus (4)

• The Mechanics and Statistics of Active Matter
• Direct measurement of the flow field around swimming microorganisms
• Effective viscosity of microswimmer suspensions
• Sheared active fluids: thickening, thinning and vanishing viscosity

Cited by in corpus (12)

• Physics of Microswimmers - Single Particle Motion and Collective Behavior
• Emergent behavior in active colloids
• Particle diffusion in active fluids is non-monotonic in size
• Swimming in Complex Fluids
• Collective dynamics in systems of active Brownian particles with dissipative interactions
• Van't Hoff's law for active suspensions: the role of the solvent chemical potential
• Viscoelastic propulsion of a rotating dumbbell
• Multi-species dynamical density functional theory for microswimmers: derivation, orientational ordering, trapping potentials, and shear cells
• Particle-scale statistical theory for hydrodynamically induced polar ordering in microswimmer suspensions
• Effective medium model for a suspension of active swimmers
• Self-organisation of auto-phoretic suspensions in confined shear flows
• Exact results for sheared polar active suspensions with variable liquid crystalline order