Academic literature on the topic 'Fluorescence de Poincaré'

The efficiency of MEMS-based time-periodic micro chaotic mixers is experimentally and theoretically investigated in this study. A time-periodic flow perturbation was realized using digitally controlled solenoid valves to activate a source and sink alternately, acting together as a pair, with different driving frequencies. Working fluids with and without fluorescent dye were used in the micromixing experiments. The spatio-temporal variation of the mixing concentration during the mixing process was characterized at different Strouhal numbers, ranging from 0.03 to 0.74, using fluorescence microscopy. A simple kinematical model for the micromixer was used to demonstrate the presence of chaotic mixing. Specific stretching rate, Lyapunov exponent, and local bifurcation and Poincaré section analyses were used to identify the emergence of chaos. Two different numerical methods were employed to verify that the maximum Lyapunov exponent was positive in the proposed micromixer model. A simplified analytical analysis of the effect of Strouhal number is presented. Kolmogorov–Arnold–Mose (KAM) curves, which are mixing barriers, were also found in Poincaré sections. From a comparative study of the experimental results and theoretical analysis, a finite-time Lyapunov exponent (FTLE) was shown to be a more practical mixing index than the classical Lyapunov exponent because the time spent in mixing is the main concern in practical applications, such as bio-medical diagnosis. In addition, the FTLE takes into account both fluid stretching in terms of the stretching rate and fluid folding in terms of curvature.

In a recent study, Sotiropoulos et al. (2001) studied numerically the chaotic particle paths in the interior of stationary vortex-breakdown bubbles that form in a closed cylindrical container with a rotating lid. Here we report the first experimental verification of these numerical findings along with new insights into the dynamics of vortex-breakdown bubbles. We visualize the Lagrangian transport within the bubbles using planar laser-induced fluorescence (LIF) and show that even though the flow fields are steady – from the Eulerian standpoint – the spatial distribution of the dye tracer varies continuously, and in a seemingly random manner, over very long observation intervals. This finding is consistent with the arbitrarily long šil'nikov transients of upstream-originating orbits documented numerically by Sotiropoulos et al. (2001). Sequences of instantaneous LIF images also show that the steady bubbles exchange fluid with the outer flow via random bursting events during which blobs of dye exit the bubble through the spiral-in saddle. We construct experimental Poincaré maps by time-averaging a sufficiently long sequence of instantaneous LIF images. Ergodic theory concepts (Mezić & Sotiropoulos 2002) can be used to formally show that the level sets of the resulting time-averaged light intensity field reveal the invariant sets (unmixed islands) of the flow. The experimental Poincaré maps are in good agreement with the numerical computations. We apply this method to visualize the dynamics in the interior of the vortex-breakdown bubble that forms in the wake of the first bubble for governing parameters in the steady, two-bubble regime. In striking contrast with the asymmetric image obtained for the first bubble, the time-averaged light intensity field for the second bubble is remarkably axisymmetric. Numerical computations confirm this finding and further reveal that the apparent axisymmetry of this bubble is due to the fact that orbits in its interior exhibit quasi-periodic dynamics. We argue that this stark contrast in dynamics should be attributed to differences in the swirl-to-axial velocity ratio in the vicinity of each bubble. By studying the bifurcations of a simple dynamical system, with manifold topology resembling that of a vortex-breakdown bubble, we show that sufficiently high swirl intensities can stabilize the chaotic orbits, leading to quasi-periodic dynamics.

Even though the first theoretical example of chaotic advection was a three-dimensional flow (Hénon 1966), the number of theoretical studies addressing chaos and mixing in three-dimensional flows is small. One problem is that an experimentally tractable three-dimensional system that allows detailed experimental and computational investigation had not been available. A prototypical, bounded, three-dimensional, moderate-Reynolds-number flow is presented; this system lends itself to detailed experimental observation and allows high-precision computational inspection of geometrical and dynamical effects. The flow structure, captured by means of cuts with a laser sheet (experimental Poincaré section), is visualized via continuously injected fluorescent dye streams, and reveals detailed chaotic structures and chains of high-period islands. Numerical experiments are performed and compared with particle image velocimetry (PIV) and flow visualization results. Predictions of existing theories for chaotic advection in three-dimensional volume-preserving flows are tested. The ratio of two frequencies of particle motion – the frequency of motion around the vertical axis and the frequency of recirculation in the plane containing the axis – is identified as the crucial parameter. Using this parameter, the number of islands in the chain can be predicted. The same parameter – using as a base-case the integrable motion – allows the identification of operating conditions where small perturbations lead to nearly complete mixing.

Les molécules hydrocarbures aromatiques polycycliques (HAP) sont à l'heure actuelle considérées comme probablement responsables des bandes d'émission infrarouge non identifiées du milieu interstellaire (MIS). La dynamique de refroidissement des molécules HAP est essentielle pour estimer leur photo-stabilité, leur durée de vie et les distributions de taille dans le MIS. Au cours des dernières années, les expériences s'appuyant sur le stockage électrostatique d'ions moléculaires ou d'agrégats sont devenus des outils puissants pour étudier leur refroidissement dans une large gamme de temps allant de la microseconde à quelques secondes. En général, l'étude des courbes de déclin associées aux processus de dissociation dans le cas des cations ou bien de détachement d'électrons dans le cas des anions fournit des informations sur l'évolution de l'énergie interne des ions stockés. Dans ce travail de thèse, le refroidissement de cations d'anthracène a été étudié dans un anneau de stockage électrostatique compact, le Mini-Ring, jusqu'à 8 ms. Les courbes de déclin spontané provenant de la dissociation par émission de fragment C2H2 ou H neutres montrent trois régions distinctives. Ces trois régions indiquent différents régimes de refroidissement en fonction du temps de stockage, la dissociation domine pour les temps inférieurs à 1 ms, l'effet de l'émission radiative entre alors en compétition avec la dissociation puis domine au-delà de 3 ms
The polycyclic aromatic hydrocarbon (PAH) molecules have been considered as possible carrier of the unidentified infrared emission bands from the interstellar medium (ISM) for about thirty years. The cooling dynamics of the PAH molecules which is essential to estimate their photostability and therefore their lifetime and size distributions in the ISM, has attracted numerous theoretical and experimental studies. In recent years, electrostatic storage devices (ESD) became powerful tool to investigate the cooling regime of molecules and clusters in a large time range from microseconds to seconds. Generally speaking, the decay of the emitted neutral yields due to dissociation of molecular cations or electron detachment of anions in such experiments carries information on the internal energy of the stored molecular ions. In this thesis work, the cooling regimes of anthracene cations are studied by following the time evolution of the internal energy distribution (IED) of the stored anthracene cations. A spontaneous neutral yield curve obtained from the stored molecular ions as a function of the storage time shows three distinguishable regions. The three regions indicate different cooling regimes at corresponding storage time range, i.e., the dissociation mechanism of the molecule dominates at storage time t < 1 ms, quenching of the dissociation by radiative cooling processes occurs during 1 < t < 3 ms and radiative cooling governs at t > 3 ms

L'étude des Hydrocarbures Aromatiques Polycycliques (PAH) a connu un intérêt croissant depuis une trentaine d'années, en raison notamment de leur possible présence dans le milieu interstellaire qui expliquerait l'observation des bandes d'émission IR encore non attribuées. Dans ce travail de thèse, la dynamique de refroidissement du cation naphtalène C10H8+, la plus petite molécule de la famille des PAH, est étudiée dans un anneau de stockage électrostatique, le Mini-Ring. La distribution en énergie interne des ions stockés est sondée en induisant la photo dissociation d'une fraction des ions par une excitation laser à la longueur d'onde 532 nm. La dynamique de refroidissement des cations photo-excités est observée en mesurant en fonction du temps le nombre de neutres émis par dissociation. La courbe de déclin du signal de neutres est directement reliée à la distribution en énergie interne des ions à l'instant de l'excitation laser. Cette distribution en énergie interne peut alors être déterminée à différents temps de stockage en analysant les courbes de déclin à l'aide d'un programme numérique développé pendant la thèse. L'évolution temporelle de la distribution met en évidence un processus de refroidissement rapide caractérisé par un taux de refroidissement compris entre 70 et 90 s−1 pour des énergies internes de 5.9 et 6.8 eV. Ce refroidissement rapide ne peut être expliqué par l'émission de photons infrarouges. Il est attribué à la "fluorescence de Poincaré" caractérisée par un processus de conversion interne inverse suivie de l'émission d'un photon dans le domaine du visible. Cette fluorescence de Poincaré ou fluorescence récurrente a été prédite il y a plus de 20 ans mais n'a jamais été mesurée directement à ce jour. Les taux de refroidissement mesurés lors de ce travail de thèse apportent une évidence indirecte de ce processus
The study of Polycyclic Aromatic Hydrocarbons (PAH) has been of increasing interest during the last thirty years: their possible presence in the interstellar medium is commonly invoked to explain the observation of still unassigned IR emission bands. In this thesis, the cooling dynamics of the naphthalene cations C10H8 +, the smallest molecule of the PAH family, is studied in an electrostatic storage ring, the Mini-Ring. Particularly, we consider the two main cooling processes for naphthalene cation, the dissociation and photon emission. Naphthalene molecules are ionized in an electron cyclotron resonance source (ECR), accelerated to 12 keV and then injected and stored in the Mini-Ring for several milliseconds. The internal energy distribution of the stored ions is probed by laser induced dissociation using an excitation wavelength at 532 nm. The cooling dynamics of the photo-excited cations is observed by measuring the number of emitted neutrals as a function of time. The decay curve of the neutral signal is directly related to the internal energy distribution of the ions at the excitation time. This internal energy distribution can then be determined at various storage times by analyzing the decay curves using a code developed during this thesis. The time evolution of the internal energy distribution shows a fast cooling process characterized by a cooling rate increasing from 70 to 90 s−1 for internal energies from 5.9 to 6.8 eV. This fast cooling process can’t be explained by infrared photons emission. It is attributed to the "Poincaré fluorescence " which involves an inverse internal conversion process followed by the emission of a visible photon. This fluorescence from thermally excited electron or recurrent fluorescence was predicted more than 20 years ago, but has never been measured directly up to now. The measured cooling rates in this thesis provide indirect evidence of this process