Thermocapillary flows and interface deformations produced by localized laser heating in confined environment

The deformation of a fluid-fluid interface due to the thermocapillary stress induced by a continuous Gaussian laser wave is investigated analytically. We show that the direction of deformation of the liquid interface strongly depends on the viscosities and the thicknesses of the involved liquid layers. We first investigate the case of an interface separating two different liquid layers while a second part is dedicated to a thin film squeezed by two external layers of same liquid. These results are predictive for applications fields where localized thermocapillary stresses are used to produce flows or to deform interfaces in presence of confinement, such as optofluidics

Influence of wettability on liquid water transport in gas diffusion layer of proton exchange membrane fuel cells (PEMFC)

Water management is a key factor that limits PEFC's performance. We show how insights into this problem can be gained from pore-scale simulations of water invasion in a model fibrous medium. We explore the influence of contact angle on the water invasion pattern and water saturation at breakthrough and show that a dramatic change in the invasion pattern, from fractal to compact, occurs as the system changes from hydrophobic to hydrophilic. Then, we explore the case of a system of mixed wettability, i.e. containing both hydrophilic and hydrophobic pores. The saturation at breakthrough is studied as a function of the fraction of hydrophilic pores. The results are discussed in relation with the water management problem, the optimal design of a GDL and the fuel cell performance degradation mechanisms. We outline how the study could be extended to 3D systems, notably from binarised images of GDLs obtained by X ray microtomography

Optohydrodynamics of soft fluid interfaces : Optical and viscous nonlinear effects

Recent experimental developments showed that the use of the radiation pressure, induced by a continuous laser wave, to control fluid-fluid interface deformations at the microscale, represents a very promising alternative to electric or magnetic actuation. In this article, we solve numerically the dynamics and steady state of the fluid interface under the effects of buoyancy, capillarity, optical radiation pressure and viscous stress. A precise quantitative validation is shown by comparison with experimental data. New results due to the nonlinear dependence of the optical pressure on the angle of incidence are presented, showing different morphologies of the deformed interface going from needle-like to finger-like shapes, depending on the refractive index contrast. In the transient regime, we show that the viscosity ratio influences the time taken for the deformation to reach steady state

Eddies and interface deformations induced by optical streaming

We study flows and interface deformations produced by the scattering of a laser beam propagating through non-absorbing turbid fluids. Light scattering produces a force density resulting from the transfer of linear momentum from the laser to the scatterers. The flow induced in the direction of the beam propagation, called 'optical streaming', is also able to deform the interface separating the two liquid phases and to produce wide humps. The viscous flow taking place in these two liquid layers is solved analytically, in one of the two liquid layers with a stream function formulation, as well as numerically in both fluids using a boundary integral element method. Quantitative comparisons are shown between the numerical and analytical flow patterns. Moreover, we present predictive simulations regarding the effects of the geometry, of the scattering strength and of the viscosities, on both the flow pattern and the deformation of the interface. Finally, theoretical arguments are put forth to explain the robustness of the emergence of secondary flows in a two-layer fluid system

Optical flow focusing: Light-induced destabilization of stable liquid threads

International audienceConfinement of flowing liquid threads by solid walls makes them stable with respect to the Rayleigh–Plateau instability. We demonstrate here that light can break this stability, by forcing locally the deformation of the liquid interface through thermally-induced Marangoni stresses. Depending upon the confining conditions and fluid properties, this optocapillary deformation either pinches or inflates the thread, which may in both cases lead to its localized fragmentation into droplets. In the pinching regime, the laser beam behaves as a wall-free constriction that flow fo-cuses the thread, leading to successive regimes of single and multiple periodicity. Light-driven local Marangoni stresses may prove an elegant contactless alternative to control reversibly the thread-to-droplet transition for digital microfluidics

Cerebellar stroke complicating coronary catheterization: a case report

Cerebrovascular events are rare but devastating events that can complicate any coronary intervention. In the vast majority of cases, they involve major cerebral arteries. We report the case of a 56-year-old woman admitted for unstable angina associated with severe left systolic dysfunction. She developed moderate cerebellar stroke while undergoing percutaneous coronary intervention, with a national institutes of health stroke scale score of 5. Immediate systemic thrombolysis was performed, but her neurological status deteriorated. A large hemorrhagic transformation was then diagnosed, and she died despite surgical intervention. Periprocedural strokes are marred with high morbidity and mortality, therefore preventionis key, as many risk factors can be controlled or mitigated. Our patient presented many of these factors; they can be procedure-related (transfemoral approach, anticoagulation) or patient-related (age, diabetes mellitus, uncontrolled hypertension, diffuse atherosclerosis)

Simulation numérique de l'hydrodynamique d'interfaces liquide-liquide contrôlées par laser

Le contrôle de la déformation sans contact des interfaces liquides à l'échelle micro-métrique est un enjeu majeur pour toute une série d'applications en micro-fluidique. Une technique originale récemment développée au Laboratoire CPMOH (Bordeaux), consiste à employer la pression de radiation d'une onde laser continue, pour déformer des interfaces liquides à l'échelle du micron. Cela conduit pour de faibles intensité, à des formes en cloche indépendantes du sens de propagation du faisceau puis pour des intensités plus élevées, à des formes surprenantes de tétines quand le faisceau se propage du milieu le plus réfringeant vers le moins réfringeant, ou de jet de micro-gouttes pour une direction de propagation inverse. Afin de mieux comprendre la physique de ces écoulements et en maîtriser les applications, nous avons developpé un outil numérique basé sur la méthode des élements de frontière couplant à la fois l'hydrodynamique et l'électromagnétisme. Les résultats numériques obtenus avec cette méthode sont comparés aux résultats expérimentaux en régime de déformation linéaire et non-linéaire et ceci pour les deux sens de propagation

Simulation de l'opto-hydrodynamique des interfaces liquides

The aim of this work is to study the coupling between the propagation of an optical wave and the hydrodynamics of a liquid-liquid interface, giving birth to a new discipline : the optohydrodynamics. Applications in this field are numerous including for instance the optical measurement of liquid physical properties or contactless manipulation of objects at micrometric scale. Our objective is to understand surface and volume effects of an intense optical wave on a system composed of two immiscible liquids. Momentum and mass conservation equations along with a stress jump condition at the interface representing the equilibrium between viscous, capillary, gravitational forces and optical radiation pressure are solved numerically. this is performed in axisymmetric coordinates using a Boundary Integral Element Method (BIEM). The resolution is validated for both equilibrium and dynamics of the interface by comparing numerical results to experimental data and analytical predictions obtained from theoretical models in the case of small deformation amplitude when interface extension is very large compared to the beam waist. The influence of the direction of propagation of the laser on the interface deformation is analyzed and a comparison is performed with experimental results. Volume effects of the optical wave are studied showing permanent flows within the two phases interplaying with the shape of the interface. Finite size and finite volume effects are finally studied. In particular, effects of the optical radiation pressure on the deformation of a liquid drop are studied in both cases of a stretched or compressed sessile drop.Ce travail a pour objectif l'étude du couplage entre la propagation d'une onde optique et l'hydrodynamique d'une interface liquide-liquide, donnant naissance à une discipline nouvelle : l'opto-hydrodynamique. Les applications envisageables dans ce domaine sont nombreuses comme, par exemple, la mesure optique des propriétés physiques des fluides ou encore la manipulation sans contact d'objets à l'échelle micrométrique. Notre étude vise à comprendre les effets en surface et en volume d'une onde lumineuse intense sur un système à deux liquides immiscibles. Les simulations numériques, basées sur une méthode intégrale et d'éléments de frontière (BIEM), consistent à résoudre les équation de Stokes et de conservation de la masse en axisymétrique avec une condition de saut de contraintes à l'interface traduisant l'équilibre entre forces visqueuses, capillaires, gravitationnelle et pression de radiation optique. Le code de calcul est validé à l'aide de comparaisons avec des résultats expérimentaux ou avec des prédictions issues de modèles analytiques en régime de faible déformation pour l'équilibre ainsi que pour la dynamique de l'interface lorsque celle-ci est de grande extension par rapport à la taille du faisceau. Une analyse est menée sur l'effet du sens de propagation du faisceau laser sur la déformation de l'interface. Une comparaison avec des données expérimentales est également menée. Les effets en volume de l'onde optique sont étudiés mettant en évidence l'existence d'écoulements permanents au sein des phases qui interagissent avec la forme de l'interface. On étudie pour finir l'effet de taille et de volume finis avec en particulier les effets de la pression de radiation optique sur la déformation d'une goutte liquide dans les deux cas d'étirement et de compression de la goutte

Deformation and shapping of optically trapped microdroplets: an ab-initio numerical study

We numerically study the deformation of optically trapped microdroplets with the optical radiation pressure using a house-made code based on the boundary elements method. Particular attention is paid to coupling between the electromagnetic waves propagation within the droplets and the resulting droplets mor­phologies

Étirement et Compression de gouttes liquides diélectriques par pression de radiation optique

Colloque avec actes et comité de lecture. Internationale.International audienceLorsqu un faisceau laser impacte une goutte fluide immergée dans un autre, celle-ci est comprimée ou étirée, selon le rapport des indices de réfraction. Si la goutte est étirée, elle prend une forme quasi-conique stable. Lorsqu elle est comprimée, elle prend une forme toroïdale. Ces différents cas sont étudiés numériquement par méthode des éléments de frontières. Ils étendent l approche éléctro-hydrodynamique de déformation de gouttes au domaine optique