A microfluidic distribution system for an array of hollow microneedles

9 pagesInternational audienceWe report a microfluidic device able to control the ejection of fluid through a matrix of out-of-plane microneedles. The device comprises a matrix of open dispensing units connected to needles and filled by a common filling system. A deformable membrane (e.g. in PDMS) is brought into contact with the dispensing units. Pressure exerted on the deformable membrane closes (and thus individualizes) each dispensing unit and provokes the ejection of the dispensing unit content through the outlets. Sufficient pressure over the deformable membrane ensures that all dispensing units deliver a fixed volume (their content) irrespective of the hydrodynamic pressure outside the dispensing unit outlet. The size of the ensemble matrix of dispensing units, the number of liquid reservoirs, as well as the material can vary depending on the considered application of the device or on the conditions of use. In the present paper, the liquid reservoirs are geometrically identical. The geometrical parameters of the device are optimized to avoid as much dead volume as possible, as it was to handle plasmid DNA solutions which are very expensive. The conception, the fabrication and the experimental results are described in this paper. Our prototype is conceived to inject in a uniform way 10 µl of drug through 100 microneedles distributed over 1 cm2

Foam Drainage Control Using Thermocapillary Stress in a Two-Dimensional Microchamber

International audienceWe investigate the drainage of a 2D microfoam in a vertical Hele-Shaw cell, and show that the Marangoni stress at the air-water interface generated by a constant temperature gradient applied in situ can be tuned to control the drainage. The temperature gradient is applied in such a way that thermocapillarity and gravity have an antagonist e ect. We characterize the drainage over time by measuring the liquid volume fraction in the cell and find that thermocapillarity can overcome the e ect of gravity, e ectively draining the foam towards the top of the cell, or exactly compensate it, maintaining the liquid fraction at its initial value over at least 60 s. We quantify these results by solving the mass balance in the cell, and provide insight on the interplay between gravity, thermocapillarity and capillary pressure governing the drainage dynamics

Droplets in Microchannels: Dynamical Properties of the Lubrication Film

International audienceWe study the motion of droplets in a confined, micrometric geometry, by focusing on the lubrication film between droplet and wall. When capillary forces dominate, the lubrication film thickness evolves non linearly with the capillary number due to viscous dissipation between meniscus and wall. However, this film may become thin enough (tens of nanometers) that intermolecular forces come into play and affect classical scalings. Our experiments yield highly resolved topographies of the shape of the interface and allow us to bring new insights into droplet dynamics in microfluidics. We report the novel characterization of two dynamical regimes as the capillary number increases: (i) at low capillary numbers, the film thickness is constant and set by the disjoinging pressure, while (ii) above a critical capillary number, the interface behavior is well described by a viscous scenario. At a high surfactant concentration, structural effects lead to the formation of patterns on the interface , which can be used to trace the interface velocity that yield direct confirmation of boundary condition in viscous regime. The dynamics of a droplet confined between solid walls and pushed by a surrounding liquid is an old problem, however recent theories are still being developed to describe unexplored regimes and experimental characterizations are still lacking to shed light on these novel developments. A complete understanding of the droplet velocity calls for accurate knowledge of the dissipation mechanisms involved, particularly in the lubrication film. Our understanding of the lubrication properties of menisci travelling in confined geometries has been steadily refined since the pioneering work of Taylor & Saffman [1]. Notably, the influence of the lubrication film left along the wall by the moving meniscus was first taken into account by Bretherton, who investigated the motion of an inviscid bubble in a cylindrical tube [2]. Far from the meniscus, this dynamical film reaches a uniform thickness h ∞ , related to the bubble velocity through the capillary number Ca = µ f U d /γ, where U d is the bubble velocity , µ f the viscosity of the continuous phase, and γ the surface tension. When the capillary pressure dominates over the viscous stress, i.e. in the regime where Ca 1, the thickness of the film follows h Breth = 1.34 r Ca 2/3 , where r is the radius of the capillary tube. Besides bubbles , the case of viscous droplets remains however largely unexplored. A recent theoretical advance in the field by Hodges et al. [3] shows by numerical calculations of the whole flow pattern that significant corrections in the thickness of lubrication films can arise at very low Ca. Furthermore, the regime of the Bretherton theory is only valid for lubrication films thicker than the molecular sizes or than the range of interfacial interactions. The typical velocities and lengthscales involved in droplet-based microfluidics would lead to lubrication films h ∞ ∼

Spanwise dispersion optimizes the efficiency of dense microfluidic trap arrays

Microfluidic Trap Arrays (MTAs) have proved efficient tools for several applications requiring working at the single cell level like cancer understanding and treatment or immune synapse research. Unfortunately, it generally appears that many traps stay empty, even after a long time of injection which can drastically reduce the number of samples available for post-treatment. It has been shown that these unfilled traps were due to the symmetrical nature of the flow around the traps, with a break in symmetry improving capture efficiency. In this work, we use a numerical approach to show that it is possible to generate optimal geometries that significantly improve capture efficiency. This efficiency is associated with an increase in the lateral dispersion of the objects; we show that adding disorder to the layout of the traps is the most optimal solution and may stay very efficient independently of the trap array size. These numerical results are corroborated by experiments, validating our approach.Comment: 23 pages, supplementary information include

Vorticity statistics in the two-dimensional enstrophy cascade

We report the first extensive experimental observation of the two-dimensional enstrophy cascade, along with the determination of the high order vorticity statistics. The energy spectra we obtain are remarkably close to the Kraichnan Batchelor expectation. The distributions of the vorticity increments, in the inertial range, deviate only little from gaussianity and the corresponding structure functions exponents are indistinguishable from zero. It is thus shown that there is no sizeable small scale intermittency in the enstrophy cascade, in agreement with recent theoretical analyses.Comment: 5 pages, 7 Figure

Effect of micro-confinement on diphasic systems: modifications, benefits ?

International audienc

DISPERSION DE TRACEURS PASSIFS DANS DES EXPERIENCES DE TURBULENCE BIDIMENSIONNELLE

NOUS PRESENTONS DES RESULTATS EXPERIMENTAUX CONCERNANT LA DISPERSION DE TRACEURS PASSIFS DANS DES EXPERIENCES DE TURBULENCE BIDIMENSIONNELLE ENTRETENUE. LES EXPERIENCES SONT REALISEES POUR DES ECOULEMENTS FORCES ELECTROMAGNETIQUEMENT DANS DE MINCES COUCHES DE FLUIDES STRATIFIEES DE FACON STABLE. LES CHAMPS DE VITESSE SONT ANALYSES A L'AIDE D'UNE TECHNIQUE DE VELOCIMETRIE. DANS UNE PREMIERE APPROCHE, NOUS FAISONS UNE ANALYSE STATISTIQUE DE LA DISPERSION D'UNE TACHE DE FLUORESCEINE, SUCCESSIVEMENT DANS LA CASCADE DIRECTE D'ENSTROPHIE ET DANS LA CASCADE INVERSE D'ENERGIE, CONFORMEMENT AUX CONJECTURES FORMULEES PAR KRAICHNAN EN 1967 A PROPOS DU MECANISME DE DOUBLE CASCADE EN TURBULENCE BIDIMENSIONNELLE. DANS LA CASCADE D'ENSTROPHIE, NOUS MONTRONS QUE LE SPECTRE DES FLUCTUATIONS DE CONCENTRATION SUIT LA LOI DE BATCHELOR EN K - 1, ET QUE LES SOLUTIONS ANALYTIQUES FORMULEES PAR CHERTKOV ET AL. SONT EN TRES BON ACCORD AVEC NOS EXPERIENCES : AILES EXPONENTIELLES POUR LES DISTRIBUTIONS DE FLUCTUATIONS AINSI QUE DE SES INCREMENTS, LOI LOGARITHMIQUE POUR LA FONCTION DE STRUCTURE D'ORDRE 2. DANS LA CASCADE INVERSE, NOUS MONTRONS QUE LA STATISTIQUE D'ORDRE 2 EST BIEN DECRITE PAR LE FORMALISME DE CORRSIN-OBUKHOV. L'ANALYSE DES MOMENTS D'ORDRES PLUS ELEVES REVELE EN REVANCHE UNE SATURATION DES EXPOSANTS DE FONCTION DE STRUCTURE VERS UNE VALEUR CONSTANTE. UNE SECONDE APPROCHE EST UNE APPROCHE LAGRANGIENNE. ELLE CONSISTE A SUIVRE DES PAIRES DE PARTICULES LE LONG DE LEUR TRAJECTOIRES, DETERMINEES EN INTEGRANT NUMERIQUEMENT LES CHAMPS DE VITESSE EXPERIMENTAUX. CETTE ETUDE A ETE REALISEE DANS LA CASCADE INVERSE D'ENERGIE. LA LOI HYPERDIFFUSIVE DE RICHARDSON R 2 T 3 EST OBSERVEE ET UN COMPORTEMENT FORTEMENT NON-GAUSSIEN EST OBTENU POUR LES DISTRIBUTIONS DES SEPARATIONS DES PAIRES. LE PROCESSUS EST TEMPORELLEMENT AUTO-SIMILAIRE ET DE FORTES CORRELATIONS TEMPORELLES SONT PRESENTES.PARIS-BIUSJ-Thèses (751052125) / SudocCentre Technique Livre Ens. Sup. (774682301) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Migration of a bubble towards higher surface tension under the effect of thermocapillary stresses

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Temperature-induced migration of a bubble in a soft microcavity

International audienceWe perform studies of pancake-like shaped bubbles submitted to a temperature gradient in a micrometric height Hele-Shaw cell. We show that under the experimental conditions, usually found in microfluidic devices, the temperature-induced dilation of the cavity overcomes the thermocapillary convection due to surface tension variation, effectively driving the bubble toward the cold side of the cavity. The bubble velocity is experimentally characterized as a function of the bubble radius, the temperature gradient, and the initial Hele-Shaw cell thickness. We propose a theoretical prediction of the bubble velocity, based on the analytical resolution of the hydrodynamical problem. The equations set closure is ensured by the pressure value near the bubble and by the dissipation in the moving meniscus

Migration of a bubble towards higher surface tension under the effect of thermocapillary stresses

International audienc