Elastic instability in stratified core annular flow

We study experimentally the interfacial instability between a layer of dilute polymer solution and water flowing in a thin capillary. The use of microfluidic devices allows us to observe and quantify in great detail the features of the flow. At low velocities, the flow takes the form of a straight jet, while at high velocities, steady or advected wavy jets are produced. We demonstrate that the transition between these flow regimes is purely elastic -- it is caused by viscoelasticity of the polymer solution only. The linear stability analysis of the flow in the short-wave approximation captures quantitatively the flow diagram. Surprisingly, unstable flows are observed for strong velocities, whereas convected flows are observed for low velocities. We demonstrate that this instability can be used to measure rheological properties of dilute polymer solutions that are difficult to assess otherwise.Comment: 4 pages, 4 figure

Étude de la formation de fibres en microfluidique : compétition entre mise en forme et gélification de fluides complexes sous écoulement

Cette thèse est consacrée à l’étude en microfluidique de la fabrication de fibres. Les deux étapes critiques sont : - la mise en forme du matériau : nous avons étudié des instabilités qui peuvent se déclencher dans des coécoulements coeur/écorce faisant intervenir des fluides complexes (polymères, suspensions concentrées), celles-ci peuvent empêcher un contrôle de cette étape ; - le figeage de cette forme : nous avons étudié la gélification de l’alginate (un biopolymère formant un gel par l’ajout d’ions calcium) sous écoulement. Nous avons étudié des phénomènes de diffusion-réaction sous écoulement pour comprendre les points de fonctionnement de nos dispositifs. Une fois ces étapes contrôlées, nous nous sommes intéressés à la fabrication des fibres d’alginates fortement chargées en cellules pour l’ingénierie tissulaire.Abstrac

Étude de la formation de fibres en microfluidique : compétition entre mise en forme et gélification de fluides complexes sous écoulement

Cette thèse est consacrée à l’étude en microfluidique de la fabrication de fibres. Les deux étapes critiques sont : - la mise en forme du matériau : nous avons étudié des instabilités qui peuvent se déclencher dans des coécoulements coeur/écorce faisant intervenir des fluides complexes (polymères, suspensions concentrées), celles-ci peuvent empêcher un contrôle de cette étape ; - le figeage de cette forme : nous avons étudié la gélification de l’alginate (un biopolymère formant un gel par l’ajout d’ions calcium) sous écoulement. Nous avons étudié des phénomènes de diffusion-réaction sous écoulement pour comprendre les points de fonctionnement de nos dispositifs. Une fois ces étapes contrôlées, nous nous sommes intéressés à la fabrication des fibres d’alginates fortement chargées en cellules pour l’ingénierie tissulaire.Abstrac

Thermally Enhanced Electro-osmosis to Control Foam Stability

Liquid foam is a dense dispersion of liquid bubbles in a surfactant solution. Because of its large surface area, it is an out-of-equilibrium material that evolves with space and time because of coarsening, coalescence, and liquid drainage. In many applications, it is required to control the lifetime of a foam by limiting the drainage or triggering the collapse at a specific location or a given time. We show here that applying an external electric field at the edge of the foam induces some liquid flows. Depending on the flow magnitude, it controls either gravity driven drainage, the foam stability, or the foam collapse at a specific location. Thus, applying an electric field to a liquid foam can control its stability. The experimental results are quantitatively described by a simple model taking into account first the electro-osmotic transport in such a deformable medium and second the Marangoni flows induced by heterogeneous heating due to Joule effect. More specifically, we show for the first time that electro-osmosis can be strongly enhanced due to thermal gradients generated by the applied electric field

Liquid Water : When Hyperpolarizability Fluctuations Boost and Reshape the Second Harmonic Scattering Intensities

Second Harmonic Scattering (SHS) is a method of choice to investigate the molecular structure of liquids. While a clear interpretation of SHS intensity exists for diluted solutions of dyes, the scattering due to solvents remains difficult to interpret quantitatively. Here, we report a quantum mechanics/molecular mechanics (QM/MM) approach to model the polarization-resolved SHS intensity of liquid water, quantifying different contributions to the signal. We point out that the molecular hyperpolarizability fluctuations and correlations cannot be neglected. The intermolecular orientational and hyperpolarizability correlations up to the third solvation layer strongly increase the scattering intensities, and modulate the polarization-resolved oscillation that is predicted here by QM/MM without fitting parameters. Our approach can be generalized to other pure liquids to provide a quantitative interpretation of SHS intensities in terms of short-range molecular ordering

First hyperpolarizability of water in the bulk phase: long-range electrostatic effects included via the second hyperpolarizability

The molecular first hyperpolarizability β contributes to second-order optical non-linear signals collected from molecular liquids. For the Second Harmonic Generation (SHG) response, the first hyperpolarizability β (2ω,ω,ω) often depends on the molecular electrostatic environment. This is especially true for water, due to its large second hyperpolarizability γ(2ω,ω,ω,0). In this study we compute the electronic γ(2ω,ω,ω,0) and β (2ω,ω,ω) for water molecules in their bulk phase using QM/MM calculations. The average value of γ(2ω,ω,ω,0) is smaller than the one for the gaz phase, and its standard deviation among the molecules is relatively small. In addition, we demonstrate that the average second hyperpolarizability ⟨γ(2ω,ω,ω,0)⟩ can be used to describe the electrostatic effects of the neighborhood on the β (2ω,ω,ω) ; but only the more distant neighbors can be accounted this way. This study paves the way towards QM/MM calculations of the first hyperpolarizability of molecules in complex environments, in which long-range electrostatic effects can be crucial, for example nearby charged interfaces

Osteoid Osteoma with a Multicentric Nidus: Interstitial Laser Ablation under MRI Guidance

Osteoid osteoma (OO) is a common benign tumor of the bone and is typically treated by thermal ablation with computed tomography (CT) guidance. Only a few cases of multicentric OO have been described. We here report the case of an 11-year-old boy with multicentric OO of the right femur treated with laser ablation under open high-field MRI guidance. The steps of the interventional MRI procedure are described, discussing the benefits and disadvantages of MRI versus CT guidance especially with regard to younger patients

Electrokinetic transport in liquid foams

International audienceInvestigating electrokinetic transport in a liquid foam is at the confluence of two well developed research areas. On one hand, the study of electrokinetic flows (i.e. surface-driven flows generated close to a charged interface) is fairly well understood in regards the solid/liquid interface. On the other hand, the flow of liquid in a 3D de-formable network, i.e a foam, under a volume force such as gravity has been thoroughly studied over the past decade. The overlapping zone of these two frameworks is of great interest for both communities as it gives rise to challenging new questions such as: what is the importance of the nature of the charged interface, created by mobile and soluble surfactants in the case of foam, on electrokinetic transport? How does a foam behave when submitted to a surface-driven flow? Can we compensate a volume-driven flow, i.e. gravity, by a surface-driven flow, i.e. electroosmosis? In this review, we will explore these questions on three different scales: a surfactant laden interface, a foam film and a macroscopic foam

Nonlinear Optical Signature of Nanostructural Transition in Ionic Liquids

International audienceThe homologous series of the ionic liquids 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C n C 1 im][NTf 2 ] (n=2,4,6,8 and 10) was studied by polarization-resolved second harmonic scattering (SHS). The analysis of the polarization plots clearly demonstrates that the SHS response is dominated by an octupolar contribution. Orientational correlations are observed on the nanometer scale in all the investigated ionic liquids, even for the shortest alkyl chain length. A radial distribution of the nonlinear optical sources is evidenced. A sharp transition from short to longer range correlations occurs between ionic liquids with n=4 and 6, signature of the dominating role taken by the alkyl chains. The SHS technique therefore provides structural observation in liquids at scales wellbelow the diffraction limit and these findings open new interesting perspectives in the domain