Philippe Peyla

Effective viscosity of microswimmer suspensions.

The measurement of a quantitative and macroscopic parameter to estimate the global motility of a large population of swimming biological cells is a challenge. 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 dead cells. In addition, suspensions show shear thinning behavior. We relate these macroscopic measurements to the orientation of individual swimming cells under flow and discuss our results in the light of several existing models.

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.

Random walk of a swimmer in a low-Reynolds-number medium.

Swimming at a micrometer scale demands particular strategies. Indeed when inertia is negligible as compared to viscous forces (i.e. Reynolds number

Amoeboid motion in confined geometry.

Re

Effective viscosity of non-gravitactic Chlamydomonas Reinhardtii microswimmer suspensions

is lower than unity), hydrodynamics equations are reversible in time. To achieve propulsion at low Reynolds number, swimmers must then deform in a way that is not invariant under time reversal. Here, we investigate dispersal properties of self propelled organisms by means of microscopy and cell tracking. Our system of interest is the micro-alga \textit{Chlamydomonas Reinhardtii}, a motile single celled green alga about 10 micrometers in diameter that swims with to two front flagella. In the case of dilute suspensions, we show that tracked trajectories are well modeled by a correlated random walk. This process is based on short time correlations in the direction of movement called persistence. At longer times, correlations are lost and a standard random walk characterizes the trajectories. Moreover, high speed imaging enables us to show how the back-and-forth motion of flagella at very short times affects the statistical description of the dynamics. Finally we show how drag forces modify the characteristics of this particular random walk.

Intriguing viscosity effects in confined suspensions: A numerical study

Many eukaryotic cells undergo frequent shape changes (described as amoeboid motion) that enable them to move forward. We investigate the effect of confinement on a minimal model of amoeboid swimmer. A complex picture emerges: (i) The swimmers nature (i.e., either pusher or puller) can be modified by confinement, thus suggesting that this is not an intrinsic property of the swimmer. This swimming nature transition stems from intricate internal degrees of freedom of membrane deformation. (ii) The swimming speed might increase with increasing confinement before decreasing again for stronger confinements. (iii) A straight amoeoboid swimmers trajectory in the channel can become unstable, and ample lateral excursions of the swimmer prevail. This happens for both pusher- and puller-type swimmers. For weak confinement, these excursions are symmetric, while they become asymmetric at stronger confinement, whereby the swimmer is located closer to one of the two walls. In this study, we combine numerical and theoretical analyses.

Magneto-optical study of CdTe/Cd1-xMnxTe multiple quantum wells with low potential barriers

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 algae are bottom-heavy swimmers, i.e., their centre of mass is not located in the centre of the body. In this way, the algae affect the vorticity of the flow when they are perpendicularly 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 Chlamydomonas reinhardtii and could not identify a preferred orientation, i.e., our specific strain of 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 Chlamydomonas reinhardtii is negligible. This finding reopens the question of whether the origin of persistence in the orientation of cells is actually responsible for the increased viscosity of the suspension.

Elastic Interaction between Surface Defects in Thin Layers

The effective viscosity of dilute and semi-dilute suspensions in a shear flow in a microfluidic configuration is studied numerically. The suspension is composed of monodisperse and non-Brownian hard spherical buoyant particles confined between two walls in a shear flow. An abrupt change of the viscosity behaviour occurs with strong confinements: when the wall-to-wall distance is below five times the radius of the particles, we obtain a change of the sign of the contribution of the hydrodynamic interactions to the effective viscosity. This effect is the macroscopic counterpart of the peculiar micro-hydrodynamics of confined suspensions due to the influence of walls. In addition, for higher concentrations (above 25%), we find that the viscosity meets a minimum when the inter-wall distance is around five times the sphere radius. This phenomenon is reminiscent of the Fahraeus-Lindqvist effect for blood confined in small capillaries. However, we show that for sheared confined semi-dilute suspensions, the physical origin of this minimum is not due to a migration effect but to the change of hydrodynamic interactions.

Deposition and growth with desorption in molecular-beam epitaxy

The authors present an optical study of multiple quantum wells of CdTe/Cd1-xMnxTe grown with a low manganese concentration in the barrier layers, typically x approximately=0.05. The structures grow pseudomorphically to the substrate so that the strains in the CdTe and CdMnTe layers are precisely known. In an attempt to determine the valence band offset the energy difference between the light- and heavy-hole excitons has been carefully analysed. They find that in these quantum wells with small barrier heights, and hence with small confinement energies, the exciton binding energies may account for a substantial fraction of the splitting. Thus a precise determination of the valence band offset would need a very accurate calculation of these binding energies. The transition from a type I to a type II band structure with an applied magnetic field has been investigated. Various simulations show that the large decrease of energy expected for the heavy-hole excitonic gap at this transition can be almost completely compensated by a simultaneous decrease of the exciton binding energy, which explains why no significant features at the transition were observed in their samples.

Amoeboid swimming in a channel

› defect separation), we discover that defects in thin layers may either attract or repeleach other depending on the direction (though elastic deformation is isotropic) with respect to the localgeometric force distribution caused by the defect. Moreover, the force distribution fixes the exponent inthe power law 1y