Dynamics and ordering of weakly Brownian particles in directional drying

Drying of particle suspensions is an ubiquitous phenomenon with many natural and practical applications. In particular, in unidirectional drying, the evaporation of the solvent induces flows which accumulate particles at the liquid/air interface. The progressive build-up of a dense region of particles can be used, in particular, in the processing of advanced materials and architectures while the development of heterogeneities and defects in such systems is critical to their function. A lot of attention has thus been paid to correlate the flow and particles dynamics to the ordering of particles. However, dynamic observation at the particle scale and its correlation with local particle ordering are still missing. Here we show by measuring the particle velocities with high frame rate laser scanning confocal microscopy that the ordering of weakly Brownian particles during directional drying in a Hele-Shaw cell opened on one side depends on the particle velocity. Under the ambient and experimental conditions presented in the following, the particle velocities accumulate in two branches. A higher degree of ordering is found for the branch of faster particle velocity which we explain by an increase in the pressure drop which drags the particles into a denser packing as the flow velocity increases. This counter-intuitive behaviour is the opposite to what is found with Brownian particles, which can reorganize by Brownian motion into denser packing during drying, as long as the flow velocity is not too high. These results show that different kinetic conditions can be used to obtain dense, defect-free regions of particles after drying. In particular, it suggests that rapid, directional drying could be used to control the crystallinity of particle deposits.Comment: 10 pages, 12 figure

Lipids behaviour in aqueous solution of disrupted microalgae cultivated under nitrogen starving conditions: molecular simulation compared to experimental study of representative synthetic mixtures

International audienceThis work deals with the microalgae culture and biorefinery for biofuel production. Microalgae transform CO 2 into biomass and valuable molecules, including lipids. The downstream processing to valorise intracellular compounds necessitates innovative, efficient and clean processes for biomass harvesting, concentration, cell disruption and fractionation. Membrane filtration is a promising process for the concentration and the purification of lipids in a wet pathway to produce biofuel. The behaviour of the biomolecules and particles during the filtration processes is unknown. Yet it drives their performances. For example, strong interactions between lipids and polar molecules released during the cell disruption may prevent an efficient separation and valorisation of both compounds because of the water-oil interface stabilisation. Consequently, to achieve oil droplets coalescence, the interface must be disturbed during filtration. The understanding of the local organisation on the target molecules is a necessary step to develop innovative fractionation strategies and optimize the coupling of different processes. To bring understanding, a multiscale approach is proposed. The use of synthetic mixtures drives the link between the different scales. These synthetic mixtures are simplified emulsions that mimic a supernatant of a real grinded biomass, centrifugated to get rid of cell fragments. The composition is based on the characterisation of disrupted cells samples, from a P. kessleri culture, under nitrogen starving conditions. The synthetic mixtures contain water, triglycerides and polar lipids. On the one hand, coarse-grained molecular simulations were carried out, to study the behaviour of lipids in the solution. Interfacial tension of the water-lipids interface was calculated for several compositions and validated with experiments. On the other hand, the interfacial properties of complex mixtures were related to molecular organisation deduced from molecular simulation. The perspective of this project is at first the characterization of water-lipid interfaces with real products, to compare to the results with synthetic solutions. This necessitates a large-scale culture in starving conditions to recover enough lipids in the supernatant. The acquired knowledge on the behaviour and organisation of lipids at the interfaces will help the optimization of membrane processes for the concentration and coalescence of lipids before valorisation into biofuels

Light induced flows opposing drainage in foams and thin-films using photosurfactants

International audienceWe study the influence of UV light on the drainage flows of foams and thin-liquid films stabilized by photoswitchable azobenzene surfactants, whose shape and hydrophobicity can be modified using UV illumination. This model system, the dynamics of which was well characterized in a previous study, enables us to trigger a controlled variation of the surface excess and surface tension. In both geometries we observe light-induced flows which are able to suppress the drainage flow induced by gravity. However, we show that the physical origin of the flows is different in both geometries. At the scale of a few films in the so-called 'two-bubble' experiment the comparisons of the physical length scales, i.e. the radius of the meniscus and the film thickness, to the chemical "reservoir length" (Γ/c) show that the flux of the surfactant at the interface in the presence of UV light is different in the films and in the meniscus, inducing a Marangoni flow from the meniscus to the film, which is stronger than gravity and capillary suction. The velocity of this flow can be tuned by the light intensity and the surfactant concentration. In the real foams, however, we show that the above mechanism is not relevant because the radii of curvature of the Plateau borders are orders of magnitude lower than in the two-bubble experiment, thus the capillary suction prevents such transfer between the films and the Plateau borders. Instead, the decrease of the drainage velocity is shown to be due to a gradient of the surface tension in the illuminated zone hence to a local variation of the capillary pressure. This study underlines the importance of characterizing the radius of the Plateau borders for the understanding of foams, as this key parameter sets the order of magnitude of capillary pressure, film thickness and amount of available surfactant. We also show that this photosurfactant is a new toolbox for the understanding of foam stability

Cross-flow filtration for the recovery of lipids from microalgae aqueous extracts: membrane selection and performances

International audienceThe biorefinery of microalgae necessitates innovative choices of soft and energy-efficient processes to guarantee the integrity of fragile molecules and develop eco-friendly production. A wet processing of biomass is proposed, which avoids expensive drying steps. It includes harvesting, cell disruption, and fractionation of the target compounds. Membrane filtration is a promising clean fractionation step. In this paper, the recovery of lipids from starving Parachlorella kessleri aqueous extracts by cross-flow filtration was studied. A model solution was formulated to test four membranes of different materials (PVDF, PES, PAN) and cut-offs (200 kDa - 1.5 µm). The hydrophilic PAN 500kDa membrane presented the best performance (flux stability, permeate flux, lipid retention, and cleanability) and was therfore selected for filtrating real aqueous extracts. Similar permeation fluxes were obtained with model and real products: 34 -41Lh-1m-2 respectively. The coalescence of lipid droplets was observed with model solutions but not with real products, less concentrated. The lipids from the real products were wholly retained by the PAN membrane, whereas some of the polysaccharides and proteins were able to permeate. An optimization of the coupling between culture, cell disruption, clarification, and filtration would allow a good concentration and purification of the lipids from microalgae

Adsorption of soluble polymers at liquid interfaces and in foams

International audienceWe review some results from the literature describing the structure and dynamics of polymer molecules adsorbed at liquid interfaces as well as their behavior in foams. The first part describes theoretical and experimental results concerning the structure and dynamics of model polymer systems, i.e. homopolymers and copolymers. In the second part, we review experimental results concerning mixtures of polymers and surfactants that are widely used to stabilize foams. In such mixtures, the surfactants and the polymer molecules form complexes in the bulk solution and at liquid interfaces which help stabilizing the foams. We review the physicochemical parameters that influence the adsorption dynamics and the viscoelastic properties of the interfaces as well as the foam properties of such mixtures. (C) 2014 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved

Adsorption et rhéologie interfaciale de complexes polyélectrolytes-tensioactifs

F. Brochard (président), J-M. di Meglio (rapporteur), P. Claesson (rapporteur), C. Williams, J. Meunier, V. Bergeron, L. Vovelle (invité)The first part of this thesis is an experimental study of the adsorption of oppositely charged surfactant/polyelectrolyte complexes at the air-water interface using ellipsometry, surface tension and observation of foam-film drainage. The complexes can form hydrophobic globules that adsorb in heterogeneous and thick layers rearranging very slowly. The structure and thickness of these layers depends on the ratio of surfactant to polyelectrolyte concentrations as well as the hydrophobicity of the surfactant which influence the affinity of the complexes for the air-water interface. Moreover the polyelectrolyte molecular weight is a parameter that controls the size of the complexes and the thickness of the layers. The second part deals with the surface rheology of these adsorbed layers using both shear and dilatational deformations. The measurements show the formation of soft physical gels at the interface. Lastly, we present the details of the construction of a surface magnetic viscometer.La première partie de cette thèse est une étude expérimentale de l'adsorption de complexes polyélectrolytes/tensioactifs de charge opposée à l'interface eau-air à l'aide la tensiométrie, l'ellipsométrie et l'observation de drainage de films de savon. Ces complexes peuvent former des globules hydrophobes s'adsorbant en des couches épaisses et hétérogènes, aux temps de réarrangement très longs. La structure et l'épaisseur des couches dépend fortement du ratio des concentrations tensioactif/polyélectrolyte et de l'hydrophobie du tensioactif, qui permettent de modifier l'affinité des complexes pour la surface. De plus, la masse moléculaire du polyélectrolyte contrôle la taille des complexes formés. La deuxième partie traite de la rhéologie de surface en cisaillement et en dilatation de ces couches de complexes. Les mesures montrent la formation de gels physiques mous de surface. Enfin, nous présentons les détails de la construction d'un viscosimètre magnétique de surface

Congélation d’émulsions : de la mayonnaise à la métallurgie

Si congeler de la mayonnaise n’est pas recommandé, cela pourrait toutefois nous aider à comprendre la fabrication d’alliages métalliques, la cryopréservation des cellules ou encore la congélation des sols en hiver. Nous nous intéressons ici au cas des gouttes d’huile dans une émulsion, observées par microscopie confocale au cours de la congélation. De nombreux phénomènes physiques (transport, diffusion, solidification, instabilités) prennent place lors de ce processus, offrant aux physicien.ne.s un problème inédit aux multiples ramifications. Ces études pourraient améliorer notre compréhension de plusieurs phénomènes de solidification, naturels comme technologiques

Solute effects on dynamics and deformation of emulsion droplets during freezing

International audienceSoft or rigid particles, suspended in a liquid melt, interact with an advancing solidification front in various industrial and natural processes, such as fabrication of particle-reinforced-composites, growth of crystals, cryopreservation, frost heave, and growth of sea ice. The particle dynamics relative to the front determine the microstructure as well as the functional properties of the solidified material. The previous studies have extensively investigated the interaction of foreign objects with a moving solid-liquid interface in pure melts while in most real-life systems, solutes or surface active impurities are almost always present. Here we study experimentally the interaction of spherical oil droplets with a moving planar ice-water interface, while systematically increasing the surfactant concentration in the bulk liquid, using in situ cryo-confocal microscopy. We demonstrate that a small amount of surfactant in the bulk liquid can instigate long-range droplet repulsion, extending over a length scale of 40 to 100µm, in contrast to the short-range predicted previously (<1µm). We report on the droplet deformation, while they are in contact with the ice-water interface, as a function of the bulk surfactant concentration, the droplet size, and the crystal growth rate. We also depict the dynamic evolution of solute-enriched premelted films (~5µm). Our results demonstrate how an increasing concentration of surfactant in the bulk and its subsequent segregation during solidification can dramatically alter the solidification microstructures. We anticipate that our experimental study can serve for the development of theoretical models incorporating solute effects

Multiple objects interacting with a solidification front

International audienceThe interaction of objects suspended in a liquid melt with an advancing solidification front is of special interest in nature and engineering sciences. The front can either engulf the object into the growing crystal or repel it. Therefore, the object-front confrontation can have a strong influence on the microstructure and mechanical or functional properties of the solidified material. The past theoretical models and experimental studies have mostly investigated the interaction of isolated, spherical, and hard objects in pure melts. However, the outcome of object-front interactions in complex (more realistic) systems, where multiple objects and solutes are present, is still poorly understood. Here we show the interaction of multiple oil droplets with an ice-water front in the absence and presence of solute effects using in situ cryo-confocal microscopy. We report on how the object size, number of objects, and bulk solute concentration influence the the object-front interaction and the front morphology, as well as the subsequent object spatial distribution. We suggest that the volume fraction of objects suspended in a liquid melt in conjunction with the amount of bulk solute concentration are two important parameters to be incorporated in the development of object-front interaction models