Wetting and Contact Lines of Micrometer-Sized Ellipsoids

We experimentally and theoretically investigate the shapes of contact lines on the surfaces of micrometer-sized polystyrene ellipsoids at the water-air interface. By combining interferometry and optical trapping, we directly observe quadrupolar symmetry of the interface deformations around such particles. We then develop numerical solutions of the partial wetting problem for ellipsoids, and use these solutions to deduce the shapes of the corresponding contact lines and the values of the contact angles, Θc(k), as a function of the ellipsoid aspect ratio k. Surprisingly, Θc is found to decrease for increasing k suggesting that ellipsoid microscopic surface properties depend on ellipsoid aspect ratio

The flow of a very concentrated slurry in a parallel-plate device: influence of gravity

We investigate, both experimentally and theoretically, the fow and structure of a slurry when sheared between 2 horizontal plates. The slurry, otherwise called a "wet granular material", is made of non-Brownian particles immersed in a viscous fluid. The particles are heavier than the fluid, consequently, gravity influences the structure and flow profiles of the sheared material. Experiments are carried out in a plane Couette device, with a model slurry composed of approximately monodisperse spherical PMMA particles in oil, at high average solid concentration (about 58%). Optical observation reveals a typical 2-phase configuration, with a fluidized layer in contact with the upper plate and on top of an amorphous solid phase. We provide data on velocity profiles, wall-slip and shear stress versus the average shear rate. To interpret the data, we propose a model for the ideal case of infinite horizontal flat plates. The model, of mean field type, is based on local constitutive equations for the tangential and normal components of the stress tensor and on expressions relating the material viscometric coefficients (the shear viscosity eta and the normal viscosity psi) with the local concentration (phi) and the local shear rate. 1-,2- and 3-phase configurations are predicted, with non linear flow and concentration profiles. We conclude that the model equations correctly describe the experimental data, provided that appropriate forms are chosen for the divergence of eta and psi near the packing concentration (phi_max), namely a (phi_max-phi)^-1 singularity.Comment: 26 pages, 12 figures ; submitted to Physics of Fluid

Ecoulement et ségrégation dans des pâtes granulaires modèle

Nous étudions les phénomènes de ségrégation par taille dans des suspensions concentrées de particules en écoulement. Les particules sont plus lourdes que le fluide, comme dans la plupart des situations pratiques (boues, bétons, propergols solides). Le matériau modèle est transparent, et nous pouvons mesurer les champs de vitesse et de concentration des espèces sous cisaillement. Dans les deux géométries étudiées (Couette et plan-plan), on observe que 'écoulement principal (azimutal) est en général localisé. De plus, nous montrons l'existence systématique d'une composante non azimutale des trajectoires. Expérimentalement, nous établissons les relations entre figure de ségrégation, écoulements primaire et secondaire. Sur cette base, nous discutons les mécanismes possibles de la ségrégation.BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF

Air Ingestion by a Buckled Viscous Jet of Silicone Oil Impacting the Free Surface of the Same Liquid

As frequently observed in common life, a jet of a viscous liquid impacting on a horizontal surface does not remain straight but instead buckles and folds periodically. We report experiments with planar (ribbonlike) jets of silicone oil impacting the free surface of the same liquid and describe the way in which jet folds incorporate air. It is shown that air ingestion proceeds through different modes, each of them acting as a source of monodisperse bubbles and featuring a threshold in jet height. These sources result from the breakup of remarkable cuspidal structures, produced by the recession of air domains within liquid folds

Implementing both short- and long-working-distance optical trappings into a commercial microscope

10 pagesInternational audienceOptical tweezers are now a widespread tool based on three-dimensional trapping by a tightly focused single laser beam. This configuration only works with large numerical aperture and short-working-distance (SWD) objectives, restricting optical manipulation to the high magnification end of the microscope nosepiece. Certain applications of optical trapping demand long-working distances (LWDs) at moderate magnification, imposing a more complex two-beam trapping configuration. In this article, we describe a complete setup that incorporates both SWD and LWD optical trapping functionalities into a single Axiovert 200M Zeiss microscope. We evaluate the performance of the setup in both trapping modes with latex particles, either fluorescent or not, of different sizes, in the 1-20 μm range. We provide practical information allowing for optimal configuration of the two-beam geometry, in relation with longitudinal and lateral stabilities of the trap

Adhesion of Latex Spheres to Giant Phospholipid Vesicles: Statics and Dynamics

We studied the sequence of phenomena which occur when a solid microsphere is brought in contact with an isolated giant lipid vesicle. We used Latex beads, a few microns in diameter, which were manipulated individually by means of a long-working-distance optical trap. The evolution of the bead/vesicle system was characterized in time, from ~ 1 ms to ~ 100 s. In this time range, we identified different steps, namely adhesion, ingestion, expulsion and recapture. In the adhesion step the sphere moves quickly in direction to the vesicle interior and the surface of the particle becomes wetted by lipids. We propose a simple model, based on the counter-balance between adhesion and stretching of the lipid lamella, which explains the experimental equilibrium configuration. The bead/vesicle configuration after the adhesion step pertains to partial or complete wetting, depending on the initial vesicle state. Partial wetting can be followed by a second step, which we named “particle ingestion”, and which leads to complete (or nearly complete) wetting of the particle surface. Ingestion is characterized by a further penetration of the particle across the vesicle contour, in concomitance with a decrease of the vesicle size. The phenomenon is attributed to the occurrence of a dynamically stabilized pore across the membrane, which allows part of the water initially inside the vesicle to flow out. Ingestion can be followed by a back and forth movement (expulsion and re-capture) of the particle. In the ultimate configuration, the solid surface is totally wetted by lipids, however with a finite contact angle between the membrane and the solid surface

Écoulement et resuspension visqueuse d'une pâte granulaire modèle en cellule de Couette à large entrefer

Le sable mouillé, certaines boues ou le béton frais sont constitués de grains dans un fluide visqueux. Les grains sont assez gros pour ne pas être browniens, et en grande concentration, ce qui procure au matériau les propriétés mécaniques d’une pâte (on parle de « pâte granulaire »). Décrire les façons dont ce genre de matériau peut couler est un enjeu important, du point de vue académique et pour de nombreuses applications. L’écoulement implique un cisaillement, dont l’effet est de disperser les grains, concurremment à la gravité qui a tendance à les entasser, et par là bloquer l’écoulement. On appelle « resuspension visqueuse » la compétition entre les deux effets. Les nombreux travaux déjà consacrés à ces questions, concernent en général des suspensions adaptées en densité, c'est à dire dans la limite où l’effet de la gravité est négligeable (geff ≈ 0). Nous présentons une étude comparative des cas sans et avec gravité, sur la base d’expériences réalisées avec une pâte granulaire modèle dans un écoulement de Couette à large entrefer. Les données sont ensuite analysées dans le cadre du modèle d’équilibre de contraintes normales de Morris et Boulay. Les expériences sont réalisées avec une pâte modèle transparente et mettent en œuvre des méthodes optiques et d’analyse d’images innovantes, qui nous permettent de mesurer les vitesses azimuthales des particules et les fluctuations de position angulaire, ainsi que le champ de concentration des particules dans l’entrefer. Pour le système isodense, l’accord théorie-expérience est excellent, pour un choix convenable des formes des fonctions viscométriques. En revanche, la même théorie ne rend compte que qualitativement du cas général geff ≠ 0. Les figures calculées reproduisent qualitativement la localisation de l’écoulement observée dans les expériences, mais un désaccord subsiste dans la description des zones de très fortes concentrations

Optical levitation and long-working-distance trapping: From spherical up to high aspect ratio ellipsoidal particles

Radiation pressure forces from a moderately focused vertical laser beam are used to levitate transparent particles, a few micrometers in size. Having recalled basic results about levitation of spheres, and applications to long-working distance trapping, we turn to ellipsoid-shaped particles. Experiments are carried out with polystyrene particles, inside a glass chamber filled with water. The particles are lifted up to contact with the chamber top surface. We examine particle equilibrium in such conditions and show that the system ''bifurcates'' between static on-axis equilibrium with short ellipsoids, to sustained oscillations with longer ones. A similar Hopf bifurcation is found using a simple ray-optics model of the laser-ellipsoid interaction, providing a qualitative account of the observed oscillations

Laminar plume formation by high pressure CO2

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Size segregation and particle velocity fluctuations in settling concentrated suspensions

16 pagesInternational audienceWe investigate the sedimentation of concentrated suspensions at low Reynolds numbers to study collective particle effects on local particle velocity fluctuations and size segregation effects. Experiments are carried out with polymethylmetacrylate (PMMA) spheres of two different mean diameters (190 and 25 μm) suspended in a hydrophobic index-matched fluid. Spatial repartitions of both small and large spheres and velocity fluctuations of particles are measured using fluorescently labelled PMMA spheres and a particle image velocimetry method. We also report measurements of the interstitial fluid pressure during settling. Experiments show that size segregation effects can occur during the sedimentation of concentrated suspensions of either quasi-monodisperse or bidisperse spheres. Size segregation is correlated to the organi-sation of the sedimentation velocity field into vortexlike structures of finite size. A loss of size segregation together with a significant decrease of the fluid pressure gradient in the bulk suspension is observed when the size of vortex-like structures gets on the order of the container size. However, the emergence of channels through the settling zone prevents a complete loss of size segregation in very concentrated suspensions