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Phytophthora species cause diseases in a large variety of plants and represent a serious agricultural threat, leading, every year, to multibillion dollar losses. Infection occurs when these biflagellated zoospores move across the soil at their characteristic high speed and reach the roots of a host plant. Despite the relevance of zoospore spreading in the epidemics of plant diseases, characteristics of individual swimming of zoospores have not been fully investigated. It remains unknown about the characteristics of two opposite beating flagella during translation and turning, and the roles of each flagellum on zoospore swimming. Here, combining experiments and modeling, we show how these two flagella contribute to generate thrust when beating together, and identify the mastigonemes-attached anterior flagellum as the main source of thrust. Furthermore, we find that turning involves a complex active process, in which the posterior flagellum temporarily stops, while the anterior flagellum keeps on beating and changes its pattern from sinusoidal waves to power and recovery strokes, similar to Chlamydomonas’s breaststroke, to reorient its body to a new direction. Our study is a fundamental step towards a better understanding of the spreading of plant pathogens’ motile forms, and shows that the motility pattern of these biflagellated zoospores represents a distinct eukaryotic version of the celebrated “run-and-tumble” motility class exhibited by peritrichous bacteria.

International audience; We investigate the nucleation of cavitation bubbles in a conned Lennard-Jones uid submitted to negative pressures in a cubic enclosure. We perform molecular dynamics (MD) simulations with tunable interatomic potentials that enable to control the wettability of the solid walls by the liquid, i.e its contact angle. For a given temperature and pressure, as the solid is taken more hydrophobic, we put in evidence an increase of the nucleation probability. A Voronoi tessellation method is used to accurately detect the bubble appearance and its nucleation rate as a function of the contact angle. We adapt the Classical Nucleation Theory (CNT) proposed for the heterogeneous case on a at surface to our situation where bubbles may appear at walls, edges or corner of the conned box. We nally calculate a theoretical mean expectation time in these three cases. The ratio of these calculated values over the homogeneous case is computed and compared successfully against MD simulations. Beyond the innite liquid case, this work explore the heterogeneous nucleation of cavitation bubble, not only in the at surface case, but for more complex conning geometries.

Background The initial step of a number of human or plant fungal infections requires active penetration of host tissue. For example, active penetration of intestinal epithelia by Candida albicans is critical for dissemination from the gut into the bloodstream. However, little is known about how this fungal pathogen copes with resistive forces upon host cell invasion. Results In the present study, we have used PDMS micro-fabrication to probe the ability of filamentous C. albicans cells to penetrate and grow invasively in substrates of different stiffness. We show that there is a threshold for penetration that corresponds to a stiffness of ~ 200 kPa and that invasive growth within a stiff substrate is characterized by dramatic filament buckling, along with a stiffness-dependent decrease in extension rate. We observed a striking alteration in cell morphology, i.e., reduced cell compartment length and increased diameter during invasive growth, that is not due to depolarization of active Cdc42, but rather occurs at a substantial distance from the site of growth as a result of mechanical compression. Conclusions Our data reveal that in response to this compression, active Cdc42 levels are increased at the apex, whereas active Rho1 becomes depolarized, similar to that observed in membrane protrusions. Our results show that cell growth and morphology are altered during invasive growth, suggesting stiffness dictates the host cells that C. albicans can penetrate.

In the current work, we developed a novel method to fabricate hybrid surfaces consisting of mixed hydrophilic/superhydrophobic properties. These surfaces specifically consist of a regular array of hydrophilic pillars (displaying a receding contact angle lower than 90°) surrounded by a superhydrophobic thinner layer made via the electropolymerization of a fluorinated monomer. Then, we determined the wetting properties of various forms of this complex surface, i.e., displaying different surface properties, by specifically determining their advancing (θa) and receding (θr) contact angles. Two main parameters were varied: the pillar density (from 21.2% to 6.5% based on using a spacing d between pillars varying from 25 to 45 micrometers) and the polymer charge density (from 0 to 100 mC cm-2). We observed that, for low charge density values, only the ground surface was covered by the hydrophobic polymers; while for higher charge density values, polymerization reached higher levels on the lateral surfaces of the nonconductive cylindrical pillars, eventually up to their top surfaces and covering them for the highest charge densities. This feature gave us an additional parameter that we could use to control the surface wettability. We also found that contact angles (advancing and receding) increased markedly with increasing polymer charge density above a critical value (which was higher for receding angles). And we measured advancing and receding contact angles to, respectively, increase and decrease with increasing pillar density. We interpreted qualitatively these behaviors, the main point being the importance of the impalement (null, partial or total).

Biflagellate zoospores of some phytopathogenic Phytophthora species spontaneously aggregate within few minutes in suspension. Depending on species auto-aggregate formation results from bioconvection or from a sequence bioconvection-positive chemotaxis. In this study, we show that P. parasitica zoospores may form aggregates upon application of a K+ gradient in particular geometric arrangements. Based on the use of macro- and microfluidic devices, in addition to time-lapse live imaging both in the vertical and horizontal planes, we defined (i) the spatiotemporal and concentration scale evolution within the gradient in correlation with (ii) cell distribution and (iii) metrics of zoospore motion (velocity, trajectory). The results indicated that K+-induced aggregates result from a single bi-phasic temporal sequence involving negative chemotaxis and then bioconvection in a K+ gradient concentration scale [0-17 mM]. Each K+-sensing cell undergoes a forward-to-backward movement within a threshold range of 1-4mM, thereby forming progressively a swarm. Once a critical population density is achieved zoospores form a plume which undergoes a downward migration leading to aggregate formation on the support surface. We discuss putative sources for K+ gradient generation in natural environment (zoospore population, microbiota, plant roots, soil particles), and implication for events preceding inoculum formation on host plant.

We present here the use of the capillary bridge technique to study the wetting properties (advancing and receding contact angles) of transparent, textured and superhydrophobic surfaces over large wetted area. Apparent contact angles on such surfaces are classically measured using a goniometer in combination with video camera side visualization and a drop shape analysis. Recent experiments of Schellenberger et al. [F. Schellenberger, N. Encinas, D. Vollmer and H. J. Butt, Phys. Rev. Lett., 2016, 116(9), 096101] show that this method can significantly underestimate the apparent advancing contact angle. We use for the first time the capillary bridge setup for such textured surfaces, leading to a large (up to several cm2) wetted area, instead of having a reduced contact zone as in the drop case (mm2 or less). (1) We show here how to use the method and its characteristics to explore the wetting properties of superhydrophobic surfaces. We have developed a new analysis method in order to obtain the value of the contact angle for any position of the substrate. (2) We compare with the classical drop side view method, showing that advancing contact angles are systematically higher. (3) We compare to a few existing models, concluding a good agreement for receding values but not for advancing angles, for which models must be refined.

Numerous scenarios exist for a cavitation bubble growing in a liquid. We focus here on cavitation phenomena within water under static tension in a confined environment. Drawing inspiration from the natural materials in plants, we design a novel experimental setup where a micrometric volume of water is confined by a hydrogel-based material. We show that, submerging the sample in a hypertonic solution, the water within the cavity is placed under tension and the acoustic emission produced by the resulting bubble nucleation is precisely detected. This new experimental procedure is able to strongly reduce the acoustic reflections occurring at the hydrogel/air interface with more classical techniques. We also propose a mathematical model to characterise the pressure wave emitted in order to correctly take into account the dissipation effect induced by the visco-elastic behaviour of the confining hydrogel. Both bubble resonant frequency and damping are captured by the model and quantitatively match the values found in the experiments.

International audience; Poly(tetrafluoroethylene) (PTFE) materials were exposed to low and high-energy oxygen plasma, and the stability of the materials' surface was evaluated using contact angle, surface roughness, and surface chemistry characterizations. Lower-energy oxygen plasma treatments exhibited hydrophilic behavior with contact angles as low as 87°, and the higher-energy oxygen plasma treatments exhibited superhydrophobic behavior with contact angles as high as 151°. The wettability of all the treated samples as stored in air and in water was found to be stable in time as evidenced by the statistically insignificant differences in the advancing, receding, and hysteresis contact angles. Low contact angle hysteresis (θH < 5°) and low sliding angle (α ≈ 4°) were exhibited by the superhydrophobic surface. The surface morphology was found to be responsible for the changes in the wettability of the PTFE samples since (1) there was an increase in the surface rms roughness as the plasma discharge energy was increased, and (2) there were no significant changes in the observed group frequencies of the FT-IR spectra of the treated PTFE from the untreated PTFE.

International audience; We report an experimental study of bubble dynamics in a non-Newtonian fluid subjected to a pressure decrease. The fluid is a hydrogel, composed of water and a synthetic clay, prepared and sandwiched between two glass plates in a Hele-Shaw geometry. The rheological properties of the material can be tuned by the clay concentration. As the imposed pressure decreases, the gas initially dissolved in the hydrogel triggers bubble formation. Different stages of the process are observed : bubble nucleation, growth, interaction and creation of domains by bubbles contact or coalescence. Initially bubble behave independently. They are trapped and advected by the mean deformation of the hydrogel, and the bubble growth is mainly driven by the diffusion of the dissolved gas through the hydrogel and its outgassing at the reactive-advected hydrogel-bubble interface. In this regime, the rheology of the fluid does not play a significant role on the bubble growth. A model is proposed and gives a simple scaling that relates the bubble growth rate and the imposed pressure. Carbon dioxide is shown to be the gas at play and the hydrogel is degassing at the millimeter scale as a water solution does at a smaller scale. Later on, bubbles are not independent anymore. The growth rate decreases and the morphology becomes more anisotropic as bubbles interact because they are separated by a distance smaller than the individual stress field extension. Our measurements show that the interaction distance scales as the bubbles size.

The surface construction to reach super oil non-wetting properties is very complex because of the necessary force for impeding the natural spreading of low surface tension oils. Here, a polymer, which is able to reach the superoleophobicity when it is electrodeposited on smooth surfaces, has been deposited on micro-patterned substrates made of cylindrical arrays (∅: 13 µm, H: 25 µm, distance between cylinders: 40 µm) in order to determine the effect of the pattern on the super oil-repellency properties. The surface analysis using various oils has shown that the pattern used highly decreases the time of deposition and, as a consequence, the required amount of polymer to obtain anti-oil surfaces. This work is the first step in the short term prospects for the elaboration of superoleophobic surfaces combining electropolymerization with lithography.

SUMMARY Most basidiomycete fungi actively eject their spores. The process begins with the condensation of a water droplet at the base of the spore. The fusion of the droplet onto the spore creates a momentum that propels the spore forward. The use of surface tension for spore ejection offers a new paradigm to perform work at small length scales. However, this mechanism of force generation remains poorly understood. To elucidate how fungal spores make effective use of surface tension, we performed a detailed mechanical analysis of the three stages of spore ejection: the transfer of energy from the drop to the spore, the work of fracture required to release the spore from its supporting structure and the kinetic energy of the spore after ejection. High-speed video imaging of spore ejection in Auricularia auriculaand Sporobolomyces yeasts revealed that drop coalescence takes place over a short distance (∼5 μm) and energy transfer is completed in less than 4 μs. Based on these observations, we developed an explicit relation for the conversion of surface energy into kinetic energy during the coalescence process. The relation was validated with a simple artificial system and shown to predict the initial spore velocity accurately (predicted velocity: 1.2 m s–1; observed velocity: 0.8 m s–1 for A. auricula). Using calibrated microcantilevers, we also demonstrate that the work required to detach the spore from the supporting sterigma represents only a small fraction of the total energy available for spore ejection. Finally, our observations of this unique discharge mechanism reveal a surprising similarity with the mechanics of jumping in animals.

This study focuses on the effects of vertical vibrations on sessile drops deposited on hydrophobic substrates. At low amplitudes the contact line remains pinned because of contact angle hysteresis and only drop surface modes areobserved. Above a first threshold the contact line starts to move and exhibits a stick-slip behavior that presents some analogies with the solid friction on amechanical oscillator. At larger amplitudes, non-axisymmetric contour modes show up (modes m=2, 3...). They can be interpreted as a coupling between surface modes and contact line motion. These subharmonic modes are welldescribed within the framework of parametric oscillators. We also discuss here why vibrations can help to measure equilibrium contact angle.

The long evolution of vascular plants has resulted in a tremendous variety of natural networks responsible for the evaporatively driven transport of water. Nevertheless, little is known about the physical principles that constrain vascular architecture. Inspired by plant leaves, we used microfluidic devices consisting of simple parallel channel networks in a polymeric material layer, permeable to water, to study the mechanisms of and the limits to evaporation-driven flow. We show that the flow rate through our biomimetic leaves increases linearly with channel density (1/ d ) until the distance between channels ( d ) is comparable with the thickness of the polymer layer (δ), above which the flow rate saturates. A comparison with the plant vascular networks shows that the same optimization criterion can be used to describe the placement of veins in leaves. These scaling relations for evaporatively driven flow through simple networks reveal basic design principles for the engineering of evaporation–permeation-driven devices, and highlight the role of physical constraints on the biological design of leaves.

Leptosporangiate ferns have evolved an ingenious cavitation catapult to disperse their spores. The mechanism relies almost entirely on the annulus, a row of 12–25 cells, which successively: (i) stores energy by evaporation of the cells’ content, (ii) triggers the catapult by internal cavitation, and (iii) controls the time scales of energy release to ensure efficient spore ejection. The confluence of these three biomechanical functions within the confines of a single structure suggests a level of sophistication that goes beyond most man-made devices where specific structures or parts rarely serve more than one function. Here, we study in detail the three phases of spore ejection in the sporangia of the fernPolypodium aureum. For each of these phases, we have written the governing equations and measured the key parameters. For the opening of the sporangium, we show that the structural design of the annulus is particularly well suited to inducing bending deformations in response to osmotic volume changes. Moreover, the measured parameters for the osmoelastic design lead to a near-optimal speed of spore ejection (approx. 10 m s–1). Our analysis of the trigger mechanism by cavitation points to a critical cavitation pressure of approximately −100 ± 14 bar, a value that matches the most negative pressures recorded in the xylem of plants. Finally, using high-speed imaging, we elucidated the physics leading to the sharp separation of time scales (30 versus 5000 µs) in the closing dynamics. Our results highlight the importance of the precise tuning of the parameters without which the function of the leptosporangium as a catapult would be severely compromised.

Microfluidic separation of magnetic particles is based on their capture by magnetized microcollectors while the suspending fluid flows past the microcollectors inside a microchannel. Separation of nanoparticles is often challenging because of strong Brownian motion. Low capture efficiency of nanoparticles limits their applications in bioanalysis. However, at some conditions, magnetic nanoparticles may undergo field-induced aggregation that amplifies the magnetic attractive force proportionally to the aggregate volume and considerably increases nanoparticle capture efficiency. In this paper, we have demonstrated the role of such aggregation on an efficient capture of magnetic nanoparticles (about 80 nm in diameter) in a microfluidic channel equipped with a nickel micropillar array. This array was magnetized by an external uniform magnetic field, of intensity as low as 6-10 kA/m, and experiments were carried out at flow rates ranging between 0.3 and 30 μL/min. Nanoparticle capture is shown to be mostly governed by the Mason number Ma, while the dipolar coupling parameter α does not exhibit a clear effect in the studied range, 1.4 < α < 4.5. The capture efficiency Λ shows a strongly decreasing Mason number behavior, Λ?Ma-1.78 within the range 32 ≤ Ma ≤ 3250. We have proposed a simple theoretical model which considers destructible nanoparticle chains and gives the scaling behavior, Λ?Ma-1.7, close to the experimental findings. © 2016 American Physical Society.

We study the contour (or contact) line of a large water puddle floating on an organic liquid substrate. The line tension F is a two-dimensional analogue of surface tension. We manipulate the line through a vertical metallic rod. (i) When stretched by a horizontal force f, the line makes a kink characterized by an angle 20 (f = 2 F cos 0), which allows us to measure F. (ii) Moved sinusoidally, we generate "triplons" of wavevectors q along the line. In the gravity regime (qκ - 1 < 1, where κ - 1 is the capillary length), the measured dispersion relation is ω(q) = q 1 . 5 1 γ 0 . 0 5 . An improved version of our previous dynamical model gives an exponent 3/2.

National audience; Dépourvues de muscles, certaines plantes mettent en œuvre des mouvements dont la fulgurance est comparable à celle des animaux. Nous montrons dans cet article que beaucoup de ces mouvements, nécessités par la reproduction ou la nutrition, ont la même base physique : une instabilité mécanique qui libère de l’énergie élastique stockée. Deux grands types d’instabilités mécaniques sont utilisés par les plantes pour amplifier la vitesse de leur mouvement : les ruptures solides ou liquides (cavitation) pour la propulsion des graines ou des spores de fougères, et les instabilités de flambage élastique pour les pièges des plantes carnivores, telles que la Dionée ou l’utriculaire.

A laser is not necessarily a sophisticated device: Pumping energy into an amplifying medium randomly filled with scatterers, a powder for instance, makes a perfect "random laser." In such a laser, the absence of mirrors greatly simplifies laser design, but control over emission directionality or frequency tunability is lost, seriously hindering prospects for this otherwise simple laser. Lately, we proposed a novel approach to harness random lasers, inspired by spatial shaping methods recently employed for coherent light control in complex media. Here, we experimentally implement this method in an optofluidic random laser where scattering is weak and modes extend spatially and strongly overlap, making individual selection a priori impossible. We show that control over laser emission can indeed be regained even in this extreme case by actively shaping the spatial profile of the optical pump. This unique degree of freedom, which has never been exploited, allows selection of any desired wavelength and shaping of lasing modes, without prior knowledge of their spatial distribution. Mode selection is achieved with spectral selectivity down to 0.06nm and more than 10dB side-lobe rejection. This experimental method paves the way towards fully tunable and controlled random lasers and can be transferred to other class of lasers., Comment: 23 pages, 7 figures

We present an innovative mirrorless optofluidic random laser where the optical cavity has been replaced by a random scattering structure. We achieve emission control at any desired wavelength by iteratively shaping the optical pump profile.

International audience; Various plants and fungi have evolved ingenious devices to disperse their spores. One such mechanism is the cavitation-triggered catapult of fern sporangia. The spherical sporangia enclosing the spores are equipped with a row of 12 to 13 specialized cells, the annulus. When dehydrating, these cells induce a dramatic change of curvature in the sporangium, which is released abruptly after the cavitation of the annulus cells. The entire ejection process is reminiscent of human-made catapults with one notable exception: The sporangia lack the crossbar that arrests the catapult arm in its returning motion. We show that much of the sophistication and efficiency of the ejection mechanism lies in the two very different time scales associated with the annulus closure.

We report random laser emission in an innovative structure PDMS microfluidic channel filled with a dye solution. A systematic study on the origin of random lasing and its depedency on pump laser parameters is presented.

We have designed a promising optofluidic laser device based on a microfluidic structured channel filled with an ethanolic dye solution. We report random laser emission attributed to the inherent randomness of the structure. The systematic study of the emission dependence on various pump laser parameters reveals fundamental aspects as well as potential applications of this system.

9 pages; International audience; By means of a photoelastic method, we access the visualization of acoustic waves propagating in a one dimensional array of noncohesive cylinders. As pointed by Nesterenko in the case of spherical grains (V.F. Nesterenko, J. Appl. Mech. Tech. Phys. 24, p. 567 (1983)), the nonlinearity of the contact law between the grains induces a dependence of the wave velocity both on its amplitude and on the confinement force. Our experimental method allows one to access the evolution in time of the internal state of stress of individual grains with an excellent accuracy. We show that the velocity of the sound presents two regimes as a function of the confining force. For low forces, the dependence is strongly nonlinear, while it weakens for higher forces. By means of the direct visualization of the contact zone, we show that both micro- and macroscale imperfections of the surface of contact explain the low forces behavior. We test the consistency of our experimental findings results with both the theoretical expectations and with the experimental determination of the force-displacement dependence. We show moreover that the main damping process originates in solid friction.

International audience; When a water jet impinges upon a solid surface it produces a so called hydraulic jump that everyone can observe in the sink of its kitchen. It is characterized by a thin liquid sheet bounded by a circular rise of the surface due to capillary and gravitational forces. In this phenomenon, the impact induces a geometrical transition, from the cylindrical one of the jet to the bi-dimensional one of the film. A true jet rebound on a solid surface, for which the cylindrical geometry is preserved, has never been yet observed. Here we experimentally demonstrate that a water jet can impact a solid surface without being destabilized. Depending on the incident angle of the impinging jet, its velocity and the degree of hydrophobicity of the substrate, the jet can i) bounce on the surface with a fixed reflected angle, ii) land on it and give rise to a supported jet or iii) be destabilized, emitting drops. Capillary forces are predominant at the sub-millimetric jet scale considered in this work, along with the hydrophobic nature of the substrate. The results presented in this letter raise the fundamental problem of knowing why such capillary hydraulic jump gives rise to this unexpected jet rebound phenomenon. This study furthermore offers new and promising possibilities to handle little quantity of water through “jet micro-fluidics”

We study the motion of a drop lying on a plate simultaneously submitted to horizontal and vertical harmonic vibrations. The two driving vibrations are adjusted to the same frequency, and, according to their relative amplitude and phase difference DeltaPhi, the drop experiences a controlled directed motion with a tunable velocity. We present a simple model putting in evidence the underlying mechanism leading to this ratchetlike motion of the drop. Our model includes the particular case DeltaPhi=pi corresponding to the climbing of a drop on a vertically vibrated inclined substrate, as recently observed by Brunet et al. [Phys. Rev. Lett. 99, 144501 (2007)10.1103/PhysRevLett.99.144501]. This study gives insights in the fundamental issue of wetting dynamics and offers new possibilities of controlled motion in droplet microfluidics applications.

By means of photoelasticity, we success in visualizing in real time the propagation of acoustic waves in a granular packing of cylinders. As previously mentioned by Nesterenko [1] for the case of spherical grains, the nonlinearity of the contact law between grains induces a dependence of the wave velocity both on its amplitude and on the confinement force. Our experimental procedure allows an access to the local state of stress of individual grains as a function of time with a good accuracy. Our results concerning the wave velocity as a function of the amplitude, and of the confinement force, are compared to theoretical predic tions and to the spherical beads case.

We study the inertial dewetting of water films (A) (thickness e) deposited on highly hydrophobic liquid substrates (B). On these ideal surfaces, thin films can be made which dewet at large velocities obeying under those conditions the Culick law for the bursting of soap films. The rim collecting the water film can become coupled to the surface waves characterized by a surface tension gamma(B) upstream of the rim (coated substrate) and gamma = gamma(B) downstream, where the water film has dried. Upon decreasing the thickness, we observe a sequence of two hydraulic shocks during the dewetting inducing gravity waves behind the rim, and capillary waves ahead.

Free fluctuations of the contact line of large drops ("puddles") of wavelength lambda > kappa(-1), the capillary length, cannot be seen on a solid substrate because even a small but finite hysteresis is enough to block these slow modes. We show here that vertical vibrations of the substrate (at frequency omegaE, acceleration Lambda) above a threshold amplitude Lambda(c) release the line and excite contour oscillations (triplons). We observe harmonic modes and parametric excitations at omegaE/2. We construct the phase diagram (Lambda, omegaE) of these subharmonic modes and we study their growth dynamics: they slow down near the threshold of the contour instability.

International audience; As recently studied, a sub-millimetric liquid jet can bounce on sufficiently hydrophobic surfaces [F.Celestini et al., Soft Matter 6(23), 5872-5876 (2010); A. Kibar et al., Exp. Fluids 49(5), 1135-1145 (2010)]. As the hydrophobicity is reduced, the reflection angle (\theta_r) increases and the jet rebound deviates more and more from specular reflection. In the present study, we vary the wetting properties of the substrate using the electrowetting effect to induce a change in the reflection angle. A liquid jet is sent toward a metallic surface coated by an insulating, hydrophobic layer. Applying an ac voltage between the metallic nozzle and the electrode below the insulating layer, we can precisely control the reflection angle of the jet. The effects of the amplitude and the frequency of the applied voltage are analyzed. This study can find applications for the control of jet dynamics.

Yeasts are unicellular fungi, which can be useful (bread, beer) or harmful to humans. Candida albicans is an opportunistic human pathogen, which can cause superficial as well as systemic infections, often life-threatening. One of the virulence factors of this organism is its ability to transit between an ovoid form and a filamentous form (hypha), capable of penetrating cellular tissues. During my thesis I studied the growth of C. albicans hyphae (diameter approximately 2 microns) in PDMS elastomers of various rigidity. I have observed that these hyphae are able to penetrate such environments with rigidities comparable to human tissues. My work was held into two parts.On one hand, I examined the penetration in cross-linked PDMS of a macroscopic one-millimeter diameter probe to better understand the mechanisms and forces involved, by analogy with the perforation of hyphae of microscopic size. I measured the force-strain relationships up to the point where the probe protrudes from the PDMS sample, which is a completely original aspect compared to the state of the art. I was able to show how the critical breaking force varied, but also the forces resistant to penetration up to the exit of the probe and to quantify the effects of the rigidity of the PMDS, but also of the speed.On the other hand, I worked with wild and mutated C. albicans strains. I developed and measured their growth in microfabricated PMDS membrane with chambers using real-time imaging. I first did a lot of work designing these membranes, most of the observations required a short working distance, which added a strong challenge on the manufacture of these membranes. The penetration of rigid media imposes on the filaments resistive stresses to their growth, as close as possible to the real situation. This makes the proposed approach an original one. Thanks to the results of the first part, we were able to determine the turgor pressure displayed in the hyphae using a recent growth model.; Les levures sont des champignons unicellulaires, qui peuvent être utiles (pain, bière) ou délétères pour l’homme. Candida albicans est une levure pathogène opportuniste de l’Homme, pouvant provoquer des infections superficielles mais aussi systémiques, pour lesquelles le pronostic vital est souvent engagé. Un des facteurs de virulence de cet organisme est sa capacité à transiter entre une forme ovoïde et une forme filamenteuse (hyphe), capable de pénétrer les tissus cellulaires. J’ai étudié pendant ma thèse la croissance d’hyphes de C. albicans (diamètre 2 microns environ) dans des élastomères de PDMS plus ou moins rigides. J’ai pu observer que ces hyphes sont capables de pénétrer de tels milieux aux rigidités comparables aux tissus humains. Mon travail s’est développé en deux parties. Dans une première partie, j’ai étudié la pénétration dans du PDMS réticulé d’une tige macroscopique millimétrique pour mieux comprendre les mécanismes et forces en jeu, par analogie avec la perforation des hyphes de taille microscopique. J’ai mesuré les relations forces-déformation jusqu’au point où la tige ressort de l’échantillon de PDMS, ce qui constitue un aspect complétement original par rapport à l’état de l’art. J’ai pu montrer comment variait la force critique de rupture, mais aussi les forces résistantes à la pénétration jusqu’à la sortie de la tige, et quantifier les effets de la rigidité du PMDS, mais aussi de la vitesse. Dans une deuxième partie, j’ai manipulé des souches de C. albicans sauvage et mutées. J’ai observé et mesuré leur croissance dans des dispositifs en PMDS microfabriqués à puits par imagerie en temps réel. J’ai d’abord réalisé un travail important de conception de ces dispositifs, sachant que la plupart des observations nécessitaient une distance de travail faible, ajoutant une contrainte forte sur la fabrication de ces dispositifs. La pénétration de milieux de rigidité et contrainte de rupture contrôlables imposent aux filaments des forces résistantes à leur croissance, au plus proche de la situation réelle, qui fait de l’approche proposée une démarche originale. Grâce aux résultats de la première partie, nous avons pu déterminer la pression de turgescence générée dans les hyphes en utilisant un modèle récent de croissance.

International audience; Chondrites are considered as the most primitive meteorites and represent a tangible record of astrophysical and geologic processes that occurred during the first 5-10 Ma of solar system history within the protoplanetary disk that surrounded the proto-Sun after its birth 4,567 Ga ago. They are presently our best witness-samples to understand the formation and evolution of the early solar system. They consist in lithified samples of materials that originate from small asteroid bodies from within the main asteroid belt ~ 3 AU (astronomical units) from the Sun or possibly further out from the Sun. Chondrules are one of the major components of primitive meteorites. Their sphericity indicates they formed as molten fragments or droplets but conditions and mechanisms of chondrule formation remain unknown. A possible scenario is their formation during hypervelocity impacts and ejections. I will show the result from an experiment that reproduces analogous of iron metal -rich chondrules by impact between a glassy silicate projectile and a metallic steel target. Silicate ejecta share several similarities with chondrules. The silicate ejecta formed in the ejecta plume contains numerous small size spherical iron metal beads as also observed in numerous natural chondrules. We attribute the morphology of ejecta and their small metal beads to the blast dynamics: for a same velocity and surface tension, fragments of silicate liquid are stable when iron liquid fragments of similar size are separated into smaller droplets. Once chondrules and the various other chondrite components (refractory inclusions, matrices) had formed, they accreted into meteorite parent bodies within the disk. If few chondrites still preserve a pristine record of this early solar nebular evolution, the great majority has been affected by secondary processes occurring prior or after their accretion in their asteroid parent bodies. These secondary thermal processes, involved fluid/solid interaction (Darcy percolation) and include gas-solid interactions, aqueous alteration, metasomatism and metamorphism. They have all acted to modify the very early record of these chondrites to different degrees from a chemical/mineralogical point of view and by changing their physical properties (i.e. pore-filling (precipitation) versus leaching and their consequences on the porosity). Here I will show that the petrological record of chondrite and discuss the future challenges to understand the modification of pristine meteorites by reacting complex fluids and build a comprehensive history of the early evolution of the first planetesimals.

In this PhD work, we focus on the biomechanics of two motions among the fastest in plant kingdom. The first part is a theoretical study of the motion leading to a prey capture by the bladderwort’s traps, elastic millimeter-sized bladders closed by a flexible door. A dynamical model, based on mechanical, elastic and hydrodynamic ingredients, links the pressure difference between the trap and its surroundings with the door position by the means of two coupled ordinary differential equations. The model captures the dynamics of the trap and predicts all the range of behaviors found in nature by including stochastic noise in the system. The second part focuses on the fern sporangium motion allowing the spores dispersal. Our experimental and theoretical studies point out the remarkable character of the annulus, a specialized structure of the sporangium, as it behaves as an autonomous catapult due to its poroelastic nature. The different stages of the motion: opening, triggering by cavitation, fast closure and eventual reloading are observed using ultra-fast imaging. The annulus curvature is measured experimentally and then compared to the theoretical predictions for each motion phase. This study enables us to identify the physical processes governing the complete motion and to determine the characteristic parameters of the annulus: stiffness, permeability and internal osmotic pressure, and also the negative cavitation pressure.; Dans cette thèse, nous nous intéressons à la biomécanique de deux mouvements parmi les plus rapides du règne végétal. La première partie porte sur l’étude théorique du mouvement permettant la capture de proies par les pièges de l’utriculaire, des outres déformables de taille millimétrique refermées par une porte flexible. Un modèle dynamique, élaboré à partir d’ingrédients mécaniques, hydrodynamiques et élastiques, permet de relier la différence de pression entre l’intérieur et l’extérieur du piège à la position de la porte via deux équations différentielles couplées. Le modèle permet de capturer la dynamique de fonctionnement du piège et de prédire l’intégralité des comportements observés dans la nature via l’ajout d’un bruit stochastique. La seconde partie est consacrée au mouvement d’éjection des spores par les sporanges de fougères. Notre étude à la fois expérimentale et théorique permet de révéler le caractère remarquable de l’anneau, une structure spécialisée du sporange, dont la nature poroélastique lui confère un comportement comparable à celui d’une catapulte autonome. Les différentes phases du mouvement : ouverture, déclenchement par cavitation, fermeture rapide et recharge éventuelle sont observées via imagerie ultra-rapide. La courbure de l’anneau au cours du mouvement est mesurée expérimentalement puis comparée aux prédictions théoriques pour chacune des phases du mouvement. Cette étude nous permet d’identifier les différents processus physiques à l’origine du mouvement et de déterminer les paramètres caractéristiques de l’anneau : raideur, perméabilité membranaire et pression osmotique interne, ainsi que la pression négative de cavitation.