Short-term memory effects in the phototactic behavior of microalgae

International audiencePhototaxis, the directed motion in response to a light stimulus, is crucial for motile microorganisms that rely on photosynthesis, such as the unicellular microalga \textit{Chlamydomonas reinhardtii}. It is well known that microalgae adapt to ambient light stimuli. On time scales of several dozen minutes, when stimulated long enough, the response of the microalga evolves as if the light intensity were decreasing~[Mayer, \textit{Nature} (1968)]. Here, we show experimentally that microalgae also have a short-term memory, on the time scale of a couple of minutes, which is the opposite of adaptation. At these short time scales, when stimulated consecutively, the response of \textit{C. reinhardtii} evolves as if the light intensity were increasing. Our experimental results are rationalized by the introduction of a simplified model of phototaxis. Memory comes from the interplay between an internal biochemical time scale and the time scale of the stimulus; as such, these memory effects are likely to be widespread in phototactic microorganisms

Evaluation du cisaillement et de l'élongation pour l'étude du mélange en écoulement de Dean laminaire pulsé

Colloque avec actes et comité de lecture. Internationale.International audienceL'objectif est de l'amélioration du mélange par superposition d'une pulsation à un écoulement de Dean en régime laminaire. La modification de l'écoulement secondaire va augmenter les étirements et repliements et donc le mélange. Les effets de chaque paramètre sont discutés en comparant les valeurs de vorticité et de taux de déformation. Les grandeurs sont calculées avec les champs de vitesse secondaire obtenus par PIV et ceux obtenus lors d'une étude numérique. L'étape suivante sera de réaliser cette étude dans un écoulement de Dean alterné pulsé, générant de l'advection chaotique

Secondary flow patterns and mixing in laminar pulsating flow through a curved pipe

Mixing by secondary flow is studied by particle image velocimetry (PIV) in a developing laminar pulsating flow through a circular curved pipe. The pipe curvature ratio is eta = r (0)/r (c) = 0.09, and the curvature angle is 90A degrees. Different secondary flow patterns are formed during an oscillation period due to competition among the centrifugal, inertial, and viscous forces. These different secondary-flow structures lead to different transverse-mixing schemes in the flow. Here, transverse mixing enhancement is investigated by imposing different pulsating conditions (Dean number, velocity ratio, and frequency parameter); favorable pulsating conditions for mixing are introduced. To obviate light-refraction effects during PIV measurements, a T-shaped structure is installed downstream of the curved pipe. Experiments are carried out for the Reynolds numbers range 420 a parts per thousand currency sign Re-st a parts per thousand currency sign 1,000 (Dean numbers 126.6 a parts per thousand currency sign Dn a parts per thousand currency sign 301.5) corresponding to non-oscillating flow, velocity component ratios 1 a parts per thousand currency sign (beta = U (max,osc)/U (m,st)) a parts per thousand currency sign 4 (the ratio of velocity amplitude of oscillations to the mean velocity without oscillations), and frequency parameters 8.37 \u3c (alpha = r (0)(omega/nu)(0.5)) \u3c 24.5, where alpha(2) is the ratio of viscous diffusion time over the pipe radius to the characteristic oscillation time. The variations in cross-sectional average values of absolute axial vorticity (|zeta|) and transverse strain rate (|epsilon|) are analyzed in order to quantify mixing. The effects of each parameter (Re-st, beta, and alpha) on transverse mixing are discussed by comparing the dimensionless vorticities (|zeta (P) |/|zeta (S) |) and dimensionless transverse strain rates (|epsilon (P) |/|epsilon (S) |) during a complete oscillation period

Mixing enhancement by pulsating chaotic advection

The purpose of this study is to investigate transverse mixing enhancement by superposition of periodic time dependence, in the form of pulsation, on a twisted pipe flow in which the fluid particles trajectories are spatially chaotic. The pulsation makes the secondary flow structure more complex, resulting in stronger velocity gradients that enhance stretching and folding, the main mechanisms of chaotic mixing. Here, the chaotic configuration is six alternating 90 curved pipes. The imposed pulsating conditions range as follows: steady Reynolds numbers 420 \u3c= Re-st \u3c= 1000, velocity component ratios 1 \u3c= (beta = U-max,U-osc/U-m,U-st) \u3c= 4 and frequency parameters 8.37 \u3c (alpha = ro(w/v)(0.5))\u3c 24.5. The secondary velocity fields are measured by particle image velocimetry. The axial vorticity and transverse strain rate at the outlet of each curved pipe in pulsatile flow are compared with those of the steady flows. Analysis of these criteria for mixing assessment shows that beta \u3e= 2 and alpha \u3c= 15 are favourable pulsating conditions for transverse mixing enhancement. Moreover, in some pulsation conditions, the cell centres visit a zone in the flow cross-section that is much larger than in the steady case, implying that pulsation also contributes to mixing homogenization. (c) 2013 Elsevier B.V. All rights reserved

SECONDARY FLOW VELOCITY FIELD IN LAMINAR PULSATING FLOW THROUGH CURVED PIPES - PIV MEASUREMENTS

Effects of different parameters on the secondary flow pattern have been studied experimentally by particle image velocimetry (PIV) for a developing laminar pulsating flow through a circular curved pipe. The curvature ratio is eta=r(c)/r(0) = 11 and the curvature angle is 90 degrees. As different secondary flow patterns formed by oscillation cause different transverse mixings, the enhancement of transverse mixing is investigated here. A T-shaped structure installed downstream of the curved pipe allowed PIV measurements obviating light diffraction effects. From knowledge of the velocity components of the secondary flow, the variation in axial vorticity (xi) and transverse strain (epsilon) were calculated. The experiments were carried out for the range of stationary Reynolds numbers 420 \u3c= Re(st)\u3c= 1000 (corresponding to Dean numbers 126.6 \u3c= Dn \u3c= 301.51), velocity component ratios 1 \u3c=(beta=U(max,osc)/U(m,st))\u3c= 4 and frequency parameters 8.37\u3c(alpha=r(0)(omega/nu)(0.5))\u3c24.5. To guarantee being in the laminar regime, the higher values of beta(beta=3 and 4) were studied just for Re(st)=420. The effects of each parameter ((Re(st), beta and alpha) on transverse mixing are discussed by comparing the dimensionless vorticities (vertical bar zeta(P)vertical bar/vertical bar zeta(S)vertical bar) and dimensionless transverse strains (vertical bar epsilon(P)vertical bar/vertical bar epsilon(S)vertical bar) during a complete oscillation period

Phototactic Behavior of Micro-Swimmers: Light Effects on Cell Swimming in Oscillating Flow

International audienceAbstract In this investigation, the phototactic behavior of the swimmer alga Chlamydomonas reinhardtii was studied in oscillatory flows. Two light-emitting diodes (LED) were placed next to one of the lateral walls of a long PDMS microchannel of rectangular cross-section (width = 500 μm, height = 85 μm) which was enclosed in a dark box. Oscillating flow of algal suspension was generated using a pressure-driven control system. Both low and high concentrations of the cells were considered. The short- and long-term effects of light exposure on the cells swimming, and the influence of amplitude and frequency of flow oscillation in the presence of light stimulus were determined. It was shown that if the period of flow oscillation is shorter than the response time of the cells to a light stimulus (about 1 second), the cells cannot react immediately to the light and thus, the increase in the percentage of cells population showing a negative phototaxis (Nup) occurs later compared to the cases where the oscillation period is long enough (low frequencies). Moreover, it was shown that a smaller oscillation amplitude results in a sharper and earlier increase in Nup since the cells are less affected by the flow advection and respond more efficiently to the light stimulus. Regarding the long-term effects of light exposure, it was shown that the phototactic behavior of the cells is a function of time, in both low and high concentrated suspensions; however, this evolution with time depends on the concentration. Over a cycle of light exposure on the dilute suspensions, Nup increases in the beginning of the cycle until reaching a peak; then it decreases to the initial value indicating that the effect of light is no longer perceptible by the cells

Memory effects in the phototactic behaviour of Chlamydomonas reinhardtii

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Flows induced by a capsule of microalgae

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Photosynthetic Micro-Swimmers in Oscillating Flow

International audienceAbstract Swimming and transport of the photosynthetic alga Chlamydomonas reinhardtii in steady and oscillatory shear flows were investigated. Sinusoidal flow of algal suspension was generated in the microchannels using a microfluidic pressure-driven controller. The oscillating flow was expressed as Q (t) = A. sin (ωt) where Q (t), t, A, and ω, represent the instantaneous flow rate, time, amplitude and angular frequency of the oscillating flow, respectively. The different amplitudes considered in the study were A = 0.075, 0.15, 0.22, 0.3 and 0.5 μl/min. Maximum mean velocities corresponding to this range of amplitudes varies between 14.7 μm/s and 392 μm/s depending on the microchannel used in the experiment. At each amplitude, four frequencies were applied: f = 0.5, 1, 2 and 3.3 Hz. The motion of algal cells was recorded and their trajectories in the flow were analyzed. It was shown that the dynamics of cells in the steady flow varies from a random motion to a zigzag motion depending on the flow shear rate. At low shear rates, the average value of the angle (α) between the cell trajectory and flow stream was 40°, and a zigzag motion of the cells was observed between the lateral walls. However, at higher flow shear rates, the mean value of this angle was about 10° with a smaller distribution range (0° < α < 30°) compared to the low shear regime (10° < α < 70°). When the flow condition was turned to a sinusoidal flow, two strong symmetric peaks appeared close to ± 90° in the probability distribution of α. The visualizations of cell trajectories showed that the cells swim perpendicular to the flow direction in wavy-form trajectories such that regardless of the oscillation conditions, their average vertical velocity (Vy) is equal to their motility (130 μm/s)

Phototactic microswimmers in pulsatile flow: Toward a novel harvesting method

International audiencePhototactic behavior is coupled with pulsatile flow features to reveal the advantages of pulsation for separating motile algae cells in a double Y-microchannel. The underlying mechanism is as follows: during half of the pulsation cycle, when the flow rate is low, the phototactic microswimmers are mainly redirected by the external stimulation (light); while, during the rest of the cycle, the flow effects become dominant and the microswimmers are driven toward the desired outlet. The results show that in the absence of light source, the pulsatile flow has no advantage over the steady flow for separation, and the microswimmers have no preference between the exit channels; the separation index (SI) is around 50%. However, when the light is on, SI increases to 65% and 75% in the steady and pulsatile flows, respectively. Although the experiments are conducted on the well-known model alga, Chlamydomonas reinhardtii, a numerical simulation based on a simple model demonstrates that the idea can be extended to other active particles stimulated by an attractive or repulsive external field. Thus, the potential applications can go beyond algae harvesting to the control and enhancement of separation processes without using any mechanical component or chemical substance