We investigate the swimming of microalgae (Chlamydomonas Reinhardtii) toward a light source (phototaxis) within a flow. This fundamental and interdisciplinary study involves experiments, theory and numerical simulations. It is a prelude to new methods for the separation and concentration processes in the production of third generation biofuels (for which CR is a promising candidate). However, the classical concentration and separation processes such as centrifugations are energy consuming. New sustainable methods should be developed where the motile character of the cells and their mutual hydrodynamic interactions must be fully understood. It poses new and fundamental questions that open the door to promising future applications. Our study will concern hydrodynamic interactions in microswimmer suspensions in the classical framework of advection-diffusion in presence or not of light.
Light Control of the Flow of Phototactic Microswimmer Suspensions
Some microalgae are sensitive to light intensity gradients. This property is known as phototaxis : The algae swim toward a light source (positive phototaxis). We use this property to control the motion of microalgae within a Poiseuille flow using light. The combination of flow vorticity and phototaxis results in a concentration of algae around the center of the flow. Intermittent light exposure allows analysis of the dynamics of this phenomenon and its reversibility. With this phenomenon, we hope to pave the way toward new algae concentration techniques (a bottleneck challenge in biofuel algal production) and toward the improvement of pollutant biodetector technology.
We present a numerical investigation of sheared suspensions of non-colloidal spherical particles on which a torque is applied. Since the torque modifies particle rotation, we show that it can indeed strongly change the effective viscosity of suspensions.
L. Jibuti, S. Rafaï and P. Peyla Suspensions with a tunable effective viscosity : a numerical study. Journal of Fluid Mechanics, Available on CJO DOI
Random walk of a swimmer in a low-Reynolds-number medium
Swimming at a micrometer scale demands particular strategies. To achieve propulsion, microswimmers must deform in a way that is not invariant under time reversal. Here, we investigate dispersal properties of the microalga Chlamydomonas reinhardtii by means of microscopy and cell tracking.
M.Garcia, S.Berti, P.Peyla, and S.Rafai (2011), Random walk of a swimmer in a low-Reynolds-number medium, Phys. Rev. E, 83 (3), 035301 (Rapid Comm.). DOI
Effective Viscosity of Microswimmer Suspensions
Experiments on the rheology of active suspensions
have been performed. Effective viscosity of sheared suspensions of live unicellular motile microalgae (Chlamydomonas Reinhardtii) is far greater than for suspensions containing the same volume fraction of
Salima Rafaï, Levan Jibuti, and Philippe Peyla. Effective Viscosity of Microswimmer Suspensions Phys. Rev. Lett. 104, 098102 (2010)
The strong extensional viscosity of viscoelastic polymer solutions allows to suck fluid from a container through a nozzle elevated above the surface of the fluid without any connection between the pumping device and the pool other than the fluid column going up, a phenomenon referred to as tubeless siphon or Fano flow. The resistance to elongation is enough to balance gravity over distances much greater than the capillary length.
When the succion speed is increased, the shear stresses lead to entrainment of more fluid from the pool than is actually aspirated by the nozzle. This results in accumulation of the excess fluid and formation of a growing recirculating zone below the nozzle which periodically falls down to the pool. At even higher succion rates, the recirculation extends over the whole column height leading to the formation of a steady viscoelastic fountain with fluid going up to the nozzle at the center and flowing down outside.
The transitions between the different regimes reveal a subtle interplay between the extensional and shear viscosities in viscoelastic flows. Collaboration : Thomas Podgorski more