Light control of the flow of phototactic microswimmer suspensions

Some micro-algae 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 micro-algae 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 hydrogen algal production) and toward the improvement of pollutant bio-detector technology

The random walk of a low-Reynolds-number swimmer

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

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 micro-algae (\textit{Chlamydomonas Reinhardtii}) is far greater than for suspensions containing the same volume fraction of dead cells and suspensions show shear thinning behaviour. 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

Deflection of phototactic microswimmers through obstacle arrays

We study the effect of inhomogeneous environments on the swimming direction of the microalgae \textit{Chlamydomonas Reinhardtii} (CR) in the presence of a light stimulus. Positive or negative phototaxis describe the ability of microorganisms to bias their swimming towards or away from a light source. Here we consider microswimmers with negative phototaxis in a microfluidic device with a microfabricated square lattice of pillars as obstacles. We measured a mean deflection of microswimmers that shows an interesting nonlinear dependence on the direction of the guiding light beam with respect to the symmetry axes of the pillar lattice. By simulating a model swimmer in a pillar lattice and analyzing its scattering behavior, we identified the width of the reorientation distribution of swimmers to be also crucial for the nonlinear behavior of the swimmer deflection. On the basis of these results we suggest in addition an analytical model for microswimmers, where the pillar lattice is replaced by an anisotropic scattering medium, that depends only on a scattering rate and the width of the reorientation distribution of swimmers. This flexible and handy model fits the experimental results as well. The presented analysis of the deflection of light guided swimmers through pillar lattice may be used for separating swimmers having different reorientation distributions

Campesinato de fronteira, pagamentos e serviços ambientais: análise da expressão diferenciada da lógica de mercado em Anapu, Pará.

Analisando quatro estudos de caso no município de Anapu, no Pará, este artigo contrasta duas abordagens para a provisão de serviços ambientais: através do pagamento em efetivo para conservação, ou pagamento por serviços ambientais (PSA), e do apoio a sistemas sustentáveis agrícolas e florestais. As abordagens são avaliadas à luz da teoria do campesinato de fronteira, destacando-se como conclusão que a abordagem de ?PSA de conservação? não se mostra adequada para replicação nesse contexto, enquanto as demais abordagens são mais adequadas no propósito de aliar conservação ambiental e produção sustentável, desde que o acesso aos recursos financeiros e de capital humano se consolide com perspectivas de longo praz

Fluctuations et forces de surface dans les transitions de mouillage

jury composé de: Elisabeth Charlaix, Daniel Beysens, Jean-Francois Joanny, Joseph Indekeu, Francois GalletWe study the effect of surface forces and fluctuations in wetting transitions. We present the first successful experimental attempt to locate the transition point between the standard first-order wetting and the long-range critical wetting: a critical end point, observed in a mixture of pentane and hexane which is deposited on an aqueous solution of glucose. Furthermore, we present the first direct measurement of the contact angle in the intermediate wetting state (frustrated-complete wetting) in the sequential-wetting scenario of hexane on brine and compare to theoretical predictions. Moreover,we show that surface fluctuations can lead to short range critical wetting. Finally, we confine thermal fluctuations into a wetting film leading to an analogous of the Casimir effect.Cette thèse étudie le rôle des forces de surface sur les transitions de mouillage. En particulier, la séquence de deux transitions de mouillage induites par la présence des forces de van der Waals à longue portée a été caractérisée précisément. Les singularités de l'énergie libre de systèmes (hexane / eau salée) ont été déterminées par une mesure directe des angles de contact par interférométrie. De plus, le diagramme de phase de mouillage global a été construit pour le système alcanes / solutions de glucose à l'aide de mesures ellipsométriques. Par aileurs, nous démontrons que la prise en compte des fluctuations de surface peut expliquer la persistence de la transition continue due aux forces à courte portée malgré la présence inévitable des forces à longue portée. Enfin, l'étude du confinement des fluctuations thermiques de volume dans un film de mouillage a été entamée durant cette thèse, il s'agit de l'analogue de l'effet Casimir pour le cas de systèmes critiques

La lumière au bout du tunnel, ou comment concentrer et séparer des microalgues en combinant lumière et écoulement

La microalgue verte Chlamydomonas reinhardtii (Chlamy) est capable de se déplacer au sein de son environnement liquide à l’aide de ses deux flagelles. Chlamy possède un « stigma » – l’oeil le plus simple du monde biologique – qui lui permet d’être phototactique, c’est-à-dire de pouvoir se diriger vers une source lumineuse. Nous décrirons ici le phénomène de photofocalisation par lequel, grâce à un écoulement et de la lumière, les microalgues sont amenées à se concentrer spontanément au centre d'un canal. En outre, nous montrerons comment les propriétés de ce micronageur peuvent être généralisées à d’autres systèmes et ainsi contribuer à une compréhension plus globale des écoulements de matière active