Active viscosity of e-coli suspensions
&

Elastic flow instabilities in curved microchannels

 


Anke Lindner
PMMH, ESPCI, Paris

Monday, November 5th, 14:15 at MEB10


1)  Active suspensions are fluids laden with self-swimming entities such as bacteria, algae or artificial swimmers. The self-propelled particles inject energy into the suspending fluid, leading to very different properties of active compared to passive suspensions. In particular, it has been predicted theoretically that the viscosity of suspensions of so called pushers can be reduced compared to the viscosity of the suspending fluid.

Pioneering experimental measurements have confirmed that finding, but no bulk rheological experiments at controlled shear rates exist up to date.  Here we present experiments measuring the viscosity of a wild type E-Coli suspension. To this purpose, we use a Y shaped micro-fluidic channel as a rheometer allowing us to resolve small differences between the viscosity of the suspending fluid and the active suspension at low shear rates and with a high resolution.  We systematically vary the shear rate and the bacterial density. In this way we show that in a specific range of parameters the viscosity of the active suspension is lower than the viscosity of the suspending fluid. We discuss our results in the perspective of recent theoretical and experimental work.

 

2) Purely elastic instabilities are known to occur in flows with curved streamlines in viscoelastic fluids at low Reynolds numbers. They have recently attracted renewed interest as they have been shown to increase mixing in wavy microchannels. The onset of instability has been proposed to be a function of the balance between curvature and normal stress effects, but the exact form of this relation is scarcely studied, in particularly for channel flow. Here we report the results of a combined experimental and numerical investigation of variation of the instability threshold with the channel curvature.