Shear instability of an axisymmetric air-water coaxial jet

We study the destabilization of a round liquid jet by a fast annular gas stream. We measure the frequency of the shear instability waves for several geometries and air/water velocities. We then carry out a linear stability analysis, and show that there are three competing mechanisms for the destabilization: a convective instability, an absolute instability driven by surface tension, and an absolute instability driven by confinement. We compare the predictions of this analysis with experimental results, and propose scaling laws for wave frequency in each regime. We finally introduce criteria to predict the boundaries between these three regimes

Flapping instability of a liquid jet

International audienceIn air assisted atomization, small droplets arise from the stripping of a liquid jet (or a film) by a fast gas stream (Lasheras & Hopfinger 2000, Eggers & Villermaux 2008). Yet, the incoming liquid jet is seemingly never fully atomized by the stripping process alone. Instead, the remaining jet experiences a flapping instability, similar to the instability observed on liquid sheet configurations: the resulting large scale structures break into large liquid lumps some distance downstream the injection. Little is known on the underlying mechanism of this instability and on the characteristics of the large drops it produces, though these large drops probably control flame extent in combustion devices. We suggest in the present study that this instability could be triggered by non-axisymmetric Kelvin-Helmholtz modes. Indeed, in coaxial injector configuration, non-axisymmetric modes of the KH instability can be observed. First, we study the dependence of KH modes upon two control parameters, namely the liquid and gas velocities, and discuss the symmetry of these modes. Secondly, we investigate a possible link between non-symmetric modes of KH instability and the large scale instability. Finally, amplitude of the large scale oscillation is measured as a function of gas and liquid velocity

Correction of cell-induced optical aberrations in a fluorescence fluctuation microscope

We describe the effect of optical aberrations on fluorescence fluctuations microscopy (FFM), when focusing through a single living cell. FFM measurements are performed in an aqueous fluorescent solution and prove to be a highly sensitive tool to assess the optical aberrations introduced by the cell. We demonstrate an adaptive optics (AO) system to remove the aberration-related bias in the FFM measurements. Our data show that AO is not only useful when imaging deep in tissues but also when performing FFM measurements through a single cellular layer. This work paves the way for the application of FFM to complex three-dimensional multicellular samples

Multi-confocal Fluorescence Correlation Spectroscopy : experimental demonstration and potential applications for living cell measurements

We report, for the first time, a multi-confocal Fluorescence Correlation Spectroscopy (mFCS) technique which allows parallel measurements at different locations, by combining a Spatial Light Modulator (SLM), with an Electron Multiplying-CCD camera (EM-CCD). The SLM is used to produce a series of laser spots, while the pixels of the EM-CCD play the roles of virtual pinholes. The phase map addressed to the SLM is calculated by using the spherical wave approximation and makes it possible to produce several diffraction limited laser spots, either aligned or spread over the field of view. To attain fast enough imaging rates, the camera has been used in different acquisition modes, the fastest of which leads to a time resolution of 100 $\mu$s. We qualified the experimental set-up by using solutions of sulforhodamine G in glycerol and demonstrated that the observation volumes are similar to that of a standard confocal set-up. To demonstrate that our mFCS method is suitable for intracellular studies, experiments have been conducted on two stable cell lines: mouse embryonic fibroblasts expressing eGFP-actin and H1299 cells expressing the heat shock factor fusion protein HSF1-eGFP. In the first case we could recover, by analyzing the auto-correlation curves, the diffusion constant of G-actin within the cytoplasm, although we were also sensitive to the complex network of interactions with F-actin. Concerning HSF1, we could clearly observe the modifications of the number of molecules and of the HSF1 dynamics during heat shock

A Study of the Internal Two-Phase Flow in Gas-Centered Swirl Coaxial Injectors

International audienceAn effective atomization of liquid is of importance in the performance of combustion engines. For liquid hydrocarbon rocket engines with a staged combustion cycle for high-power application, the Gas-Centered Swirl Coaxial (GCSC) injector is widely employed. Gaseous oxidizer at high velocity enters directly through the center of the injector and is surrounded by a swirled liquid film injected along the periphery of the injection element. The swirled liquid film is stripped and fragmented into drops by the high velocity gas stream. The understanding of the atomization characteristics of the injector should be improved for the design of more reliable and efficient injectors dedicated to liquid rocket engines. In order to effectively evaluate atomization performances, it is essential to precisely predict liquid film dynamics inside the injector. The liquid film thickness and length are a function of the injector recess length, and they affect the atomized drop size. Internal flow visualization with a LIF (Laser Induced Fluorescence) method was conducted to investigate the overall form and the interface corrugation of the liquid flow at various swirl strength conditions. The swirl strength is varied by changing the inlet angle of tangential entry holes. The experimental results show clearly that the intact liquid length increases with increasing the swirl strength at the same dynamic pressure ratio. We also measured the frequency of the surface perturbations with a spectral method. We find that this frequency increases steadily with gas velocity, and appears to be independent of the initial swirl number

Automatic laser alignment for multifocal microscopy using a LCOS-SLM and a 32x32 pixel CMOS SPAD array

International audienceAlignment of a laser to a point source detector for confocal microscopy can be a time-consuming task. The problem is further exacerbated when multiple laser excitation spots are used in conjunction with a multiple pixel single photon detector; in addition to X, Y and Z positioning, pixels in a 2D array detector can also be misaligned in roll, pitch and yaw with respect to each other, causing magnification, rotation and focus variation across the array. We present a technique for automated multiple point laser alignment to overcome these issues using closed-loop feedback between a laser illuminated computer controlled Liquid Crystal on Silicon Spatial Light Modulator (LCOS-SLM) acting as the excitation source and a 32 32 pixel CMOS Single Photon Avalanche Diode (SPAD) array as the multiple pixel detection element. The alignment procedure is discussed and simulated to prove its feasibility before being implemented and tested in a practical optical system. We show that it is possible to align each independent laser point in a sub-second time scale, significantly simplifying and speeding up experimental set-up times. The approach provides a solution to the difficulties associated with multiple point confocal laser alignment to multiple point detector arrays, paving the way for further advances in applications such as Fluorescence Correlation Spectroscopy (FCS) and Fluorescence Lifetime Imaging Microscopy (FLIM)

Instabilité de flapping : origine et effets sur la structure et le spray d'un jet atomisé

Jet or sheet atomized by a fast coaxial gas jet is currently used in industry, like aeronautical propulsion (turbofan) or spatial propulsion (cryotechnic rocket engine). Many physical processes allows liquid coherent structure fragmentation into drops. Stripping, which appears downstream near injector, has been largely studied (Marmottant et Villermaux 2004, Hong & al 2004), mecanisms has been correctly described.However, the origin of large scale - or 'flapping' instabilities - intervening further downstream, instabilities that are causing the production of large drops, remains poorly understood. This is particularly true for cylindrical jets which, unlike the case of sheets, have been the subject of very few studies. We are therefore committed to understand the origin of the "flapping", to analyze its relationship with interfacial shear instabilities, and to quantify its impact on the structure of the jet as well as on the drops produced. For this, experiments were carried out in water/air on wide set of parameters, both in terms of phasic speed than the dimensions of the gas gap and liquid diameter. Special care were made to the internal flow control.For all the geometries, we showed that the length of the liquid cone is driven by the large scale displacements and not by the stripping process. Furthermore, the length of brokenness jet presents a decline marked with the gas speed, then remains constant beyond a critical gas speed. A model was proposed for this asymptotic behavior in which the break-up length is driven by the report of the liquid injection speed to a capillary speed built on the liquid diameter.Measurement of the frequency of large scale displacement technology has been implemented from images acquired by shadowgraphy proved operational over the gas velocity range considered. This frequency, which varies not spatially, present two behaviors: a first where it increases with the speed of the gas, and a second where it remains independent of the gas speed. This second scheme is not mentioned in the literature. For the original plan, the link between flapping and shear instability has been demonstrated based on analyses of stability. The associated Strouhal number is controlled by the shear gas side. The dependence of the frequency of heartbeat to the thickness of vorticity gas side is thus established when shear instability is driven by an inviscide mechanism. For the second scheme, the opportunistic nature of the flapping has been demonstrated using forcing experience: the flapping amplifies liquid structures of wavelength greater than those associated with shear instability. A Strouhal number built on liquid jet diameter and the speed of the liquid jet at break distance has been proposed. Finally, the ratio of the diameter of the liquid jet at the wavelength of the shear instability seems relevant to define the border between these two regimes.Sizes drops produced on the symmetry axis were measured using an optical probe. It appears that granulometric distribution is evolving strongly with speed gas, and it is multi-modal, reflecting the presence of several mechanisms of brokenness. The average size of the drops decreases overall as UG - 2, in the limit of strong numbers of aerodynamic Weber. This medium size is also very sensitive to geometry: it decreases when the thickness of the gas increases until it reaches a floor value, and it grows with the liquid diameter. Finally, by forcing large amplitude lateral displacement, the average radial distribution of sizes of drops has been made much more homogeneous, and the average size of the drops on the axis has been reduced by a factor of 2. These results therefore open opportunities in terms of control of atomization.L’atomisation d’un jet ou d’une nappe liquide assistée par un courant gazeux rapide est couramment utilisée dans l’industrie ainsi qu’en propulsion aéronautique (turboréacteur) et spatiale (moteur-fusée cryotechnique). Plusieurs processus permettent la fragmentation de la structure cohérente liquide en gouttes. L’épluchage, qui intervient à courte distance en aval de l’injection, a été assez largement étudié (Marmottant et Villermaux 2004, Hong et al 2004) et les mécanismes sont assez bien décrits. En revanche, l’origine des instabilités large échelle – ou « flapping » - intervenant plus loin en aval, instabilités qui sont à l’origine de la production de large gouttes, reste mal comprise. Ceci est particulièrement vrai pour des jets cylindriques qui, contrairement au cas de nappes, ont fait l’objet de très peu d’études. Nous nous sommes donc attachés à comprendre l’origine du « flapping », à analyser ses liens avec les instabilités interfaciales de cisaillement, et à quantifier son impact sur la structure du jet ainsi que sur les gouttes produites. Pour cela, des expériences ont été menées en eau/air sur de larges plages de paramètres, aussi bien en termes de vitesses phasiques que des dimensions des veines gaz et liquide. Un soin particulier a été apporté au contrôle des écoulements internes.Pour l’ensemble des géométries, nous avons montré que la longueur du dard liquide est pilotée par le battement large échelle et non par le processus d’épluchage. Par ailleurs, la longueur de brisure présente une décroissance marquée avec la vitesse gaz, puis reste constante au delà d’une vitesse gaz critique. Un modèle a été proposé pour ce comportement asymptotique dans lequel la longueur de brisure est pilotée par le rapport de la vitesse liquide d’injection à une vitesse capillaire construite sur le diamètre liquide.La technique de mesure de la fréquence du battement large échelle mise en œuvre à partir d’images acquises par ombroscopie s’est avérée opérationnelle sur toute la plage de vitesses gaz considérées. Cette fréquence, qui ne varie pas spatialement, présente deux comportements : un premier où elle augmente avec la vitesse gaz, et un second où elle reste indépendante de la vitesse gaz. Ce second régime n’est pas mentionné dans la littérature. Pour le premier régime, le lien entre flapping et instabilité de cisaillement a été démontré en s’appuyant notamment sur des analyses de stabilité. Le nombre de Strouhal associé est piloté par le cisaillement côté gaz. La dépendance de la fréquence de battement à l’épaisseur de vorticité côté gaz est ainsi établie lorsque l’instabilité de cisaillement est pilotée par un mécanisme inviscide. Pour le second régime, le caractère opportuniste du flapping a été démontré l’aide d’une expérience de forçage : le flapping amplifie des structures liquides de longueur d’onde plus grande que celle associée à l’instabilité de cisaillement. Un nombre de Strouhal construit sur le diamètre liquide du jet et la vitesse du jet liquide à la distance de brisure a été proposé. Enfin, le rapport du diamètre du jet liquide à la longueur d’onde de l’instabilité de cisaillement semble pertinent pour définir la frontière entre ces deux régimes.Les tailles des gouttes produites sur l’axe de symétrie ont été mesurées à l’aide d’une sonde optique. Il apparaît que la distribution granulométrique évolue fortement avec la vitesse gaz, et qu’elle est multi-modale, ce qui traduit la présence de plusieurs mécanismes de brisure. La taille moyenne des gouttes décroit globalement comme UG-2, dans la limite de forts nombres de Weber aérodynamique. Cette taille moyenne s’avère aussi très sensible à la géométrie : elle diminue lorsque l’épaisseur gaz augmente jusqu’à atteindre une valeur plancher, et elle croît avec le diamètre liquide

Flapping instability of a liquid jet

L’atomisation d’un jet ou d’une nappe liquide assistée par un courant gazeux rapide est couramment utilisée dans l’industrie ainsi qu’en propulsion aéronautique (turboréacteur) et spatiale (moteur-fusée cryotechnique). Plusieurs processus permettent la fragmentation de la structure cohérente liquide en gouttes. L’épluchage, qui intervient à courte distance en aval de l’injection, a été assez largement étudié (Marmottant et Villermaux 2004, Hong et al 2004) et les mécanismes sont assez bien décrits. En revanche, l’origine des instabilités large échelle – ou « flapping » - intervenant plus loin en aval, instabilités qui sont à l’origine de la production de large gouttes, reste mal comprise. Ceci est particulièrement vrai pour des jets cylindriques qui, contrairement au cas de nappes, ont fait l’objet de très peu d’études. Nous nous sommes donc attachés à comprendre l’origine du « flapping », à analyser ses liens avec les instabilités interfaciales de cisaillement, et à quantifier son impact sur la structure du jet ainsi que sur les gouttes produites. Pour cela, des expériences ont été menées en eau/air sur de larges plages de paramètres, aussi bien en termes de vitesses phasiques que des dimensions des veines gaz et liquide. Un soin particulier a été apporté au contrôle des écoulements internes.Pour l’ensemble des géométries, nous avons montré que la longueur du dard liquide est pilotée par le battement large échelle et non par le processus d’épluchage. Par ailleurs, la longueur de brisure présente une décroissance marquée avec la vitesse gaz, puis reste constante au delà d’une vitesse gaz critique. Un modèle a été proposé pour ce comportement asymptotique dans lequel la longueur de brisure est pilotée par le rapport de la vitesse liquide d’injection à une vitesse capillaire construite sur le diamètre liquide.La technique de mesure de la fréquence du battement large échelle mise en œuvre à partir d’images acquises par ombroscopie s’est avérée opérationnelle sur toute la plage de vitesses gaz considérées. Cette fréquence, qui ne varie pas spatialement, présente deux comportements : un premier où elle augmente avec la vitesse gaz, et un second où elle reste indépendante de la vitesse gaz. Ce second régime n’est pas mentionné dans la littérature. Pour le premier régime, le lien entre flapping et instabilité de cisaillement a été démontré en s’appuyant notamment sur des analyses de stabilité. Le nombre de Strouhal associé est piloté par le cisaillement côté gaz. La dépendance de la fréquence de battement à l’épaisseur de vorticité côté gaz est ainsi établie lorsque l’instabilité de cisaillement est pilotée par un mécanisme inviscide. Pour le second régime, le caractère opportuniste du flapping a été démontré l’aide d’une expérience de forçage : le flapping amplifie des structures liquides de longueur d’onde plus grande que celle associée à l’instabilité de cisaillement. Un nombre de Strouhal construit sur le diamètre liquide du jet et la vitesse du jet liquide à la distance de brisure a été proposé. Enfin, le rapport du diamètre du jet liquide à la longueur d’onde de l’instabilité de cisaillement semble pertinent pour définir la frontière entre ces deux régimes.Les tailles des gouttes produites sur l’axe de symétrie ont été mesurées à l’aide d’une sonde optique. Il apparaît que la distribution granulométrique évolue fortement avec la vitesse gaz, et qu’elle est multi-modale, ce qui traduit la présence de plusieurs mécanismes de brisure. La taille moyenne des gouttes décroit globalement comme UG-2, dans la limite de forts nombres de Weber aérodynamique. Cette taille moyenne s’avère aussi très sensible à la géométrie : elle diminue lorsque l’épaisseur gaz augmente jusqu’à atteindre une valeur plancher, et elle croît avec le diamètre liquide.Jet or sheet atomized by a fast coaxial gas jet is currently used in industry, like aeronautical propulsion (turbofan) or spatial propulsion (cryotechnic rocket engine). Many physical processes allows liquid coherent structure fragmentation into drops. Stripping, which appears downstream near injector, has been largely studied (Marmottant et Villermaux 2004, Hong & al 2004), mecanisms has been correctly described.However, the origin of large scale - or 'flapping' instabilities - intervening further downstream, instabilities that are causing the production of large drops, remains poorly understood. This is particularly true for cylindrical jets which, unlike the case of sheets, have been the subject of very few studies. We are therefore committed to understand the origin of the "flapping", to analyze its relationship with interfacial shear instabilities, and to quantify its impact on the structure of the jet as well as on the drops produced. For this, experiments were carried out in water/air on wide set of parameters, both in terms of phasic speed than the dimensions of the gas gap and liquid diameter. Special care were made to the internal flow control.For all the geometries, we showed that the length of the liquid cone is driven by the large scale displacements and not by the stripping process. Furthermore, the length of brokenness jet presents a decline marked with the gas speed, then remains constant beyond a critical gas speed. A model was proposed for this asymptotic behavior in which the break-up length is driven by the report of the liquid injection speed to a capillary speed built on the liquid diameter.Measurement of the frequency of large scale displacement technology has been implemented from images acquired by shadowgraphy proved operational over the gas velocity range considered. This frequency, which varies not spatially, present two behaviors: a first where it increases with the speed of the gas, and a second where it remains independent of the gas speed. This second scheme is not mentioned in the literature. For the original plan, the link between flapping and shear instability has been demonstrated based on analyses of stability. The associated Strouhal number is controlled by the shear gas side. The dependence of the frequency of heartbeat to the thickness of vorticity gas side is thus established when shear instability is driven by an inviscide mechanism. For the second scheme, the opportunistic nature of the flapping has been demonstrated using forcing experience: the flapping amplifies liquid structures of wavelength greater than those associated with shear instability. A Strouhal number built on liquid jet diameter and the speed of the liquid jet at break distance has been proposed. Finally, the ratio of the diameter of the liquid jet at the wavelength of the shear instability seems relevant to define the border between these two regimes.Sizes drops produced on the symmetry axis were measured using an optical probe. It appears that granulometric distribution is evolving strongly with speed gas, and it is multi-modal, reflecting the presence of several mechanisms of brokenness. The average size of the drops decreases overall as UG - 2, in the limit of strong numbers of aerodynamic Weber. This medium size is also very sensitive to geometry: it decreases when the thickness of the gas increases until it reaches a floor value, and it grows with the liquid diameter. Finally, by forcing large amplitude lateral displacement, the average radial distribution of sizes of drops has been made much more homogeneous, and the average size of the drops on the axis has been reduced by a factor of 2. These results therefore open opportunities in terms of control of atomization