Investigation of laser induced phosphorescence and fluorescence of acetone at low pressure for molecular tagging velocimetry in gas microflows

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Laser-induced fluorescence and phosphorescence properties of gaseous acetone in argon are measured and analyzed in a pressure ranging from 10(5) to 10(2) Pa, with the aim of analyzing by molecular tagging velocimetry gas microflows in rarefied regimes which requires operation at low pressure. Acetone is excited at a wavelength of 266 nm and immediately emits short lifetime fluorescence rapidly followed by long lifetime phosphorescence. At atmospheric pressure, the early phosphorescence intensity is more than 600 times lower than the fluorescence one. The phosphorescence signal is rapidly decreasing with time, closely following a power law. Both fluorescence and phosphorescence signals are decreasing with pressure. The systematic analysis of fluorescence and phosphorescence of acetone molecules shows that although the signal is dramatically reduced at low pressure, the on-chip integration technique and the optimization of the acquisition parameters provide an exploitable signal for molecular tagging velocimetry in rarefied microflows, in a Knudsen number range corresponding to the early slip flow regime

High efficiency coupling of free electrons to sub-λ3\lambda^3 modal volume, high-Q photonic cavities

We report on the design, realization and experimental investigation by spatially resolved monochromated electron energy loss spectroscopy (EELS) of high quality factor cavities with modal volumes smaller than $\lambda^3$, with $\lambda$ the free-space wavelength of light. The cavities are based on a slot defect in a 2D photonic crystal slab made up of silicon. They are optimized for high coupling of electrons accelerated to 100 kV, to quasi-Transverse Electrical modes polarized along the slot direction. We studied the cavities in two geometries. The first geometry, for which the cavities have been designed, corresponds to an electron beam travelling along the slot direction. The second consists in the electron beam travelling perpendicular to the slab. In both cases, a large series of modes is identified. The dielectric slot modes energies are measured to be in the 0.8- 0.85 eV range, as per design, and surrounded by two bands of dielectric and air modes of the photonic structure. The dielectric even slot modes, to which the cavity mode belongs, are highly coupled to the electrons with up to 3.2$\%$ probability of creating a slot photon per incident electron. Although the experimental spectral resolution (around 30 meV) alone does not allow to disentangle cavity photons from other slot photons, the remarkable agreement between the experiments and FDTD simulations permits us to deduce that amongst the photons created in the slot, around 30$\%$ are stored in the cavity mode. A systematic study of the energy and coupling strength as a function of the photonic band gap parameters permits to foresee increase of coupling strength by fine-tuning phase matching. Our work demonstrates free electron coupling to high quality factor cavities with low mode density, sub-$\lambda^3$ modal volume, making it an excellent candidate for applications such as quantum nano-optics with free electrons

Role of Diffusion on Molecular Tagging Velocimetry Technique for Rarefied Gas Flow Analysis

The Molecular Tagging Velocimetry (MTV) is a well-suited technique for velocity field measurement in rarefied gas flows. Typically, a line is tagged by a laser beam within the gas flow seeded with luminescent acetone molecules. Positions of the luminescent molecules are then observed at two successive times and the velocity field is deduced from the analysis of the tagged line displacement and deformation. However, this displacement is affected by molecular diffusion, and consequently, there is not a direct and simple relationship between the velocity field and the measured displacement of the initial tagged line. In this paper, the role of the tracer molecules diffusion is investigated using the Direct Simulation Monte Carlo (DSMC) method. The simulations demonstrate that the diffusion of these molecules closely depends on the flow regime, and becomes significant as the degree of rarefaction of the gas flow increases. A simple reconstruction algorithm based on the advection-diffusion equation has been developed to obtain the velocity profile form the displacement field. This reconstruction algorithm has been numerically tested on DSMC generated data. It is shown that the reconstruction becomes less effective as the degree of gas flow rarefaction increases. As expected, DSMC tests demonstrate that the reconstruction error is reduced as the binary diffusion coefficient decreases

On the use of a friction model in a Volume of Fluid solver for the simulation of dynamic contact lines

International audienceWe consider the implementation of a friction contact angle model in a Navier-Stokes VoF-CSF solver for the simulation of moving contact lines at the nano-scale. A liquid-liquid interface confined in a Couette flow generated by two solid walls moving at the same velocity in opposite directions is considered to discuss the relevance of the friction model. The simulations are compared with a reference case obtained using MD simulations by Qian et al. [46]. We show that the Navier Stokes simulations are able to reproduce the MD simulations for both the interface shape and the velocity field. The appropriate contact line friction is found to be grid convergent and of the same order as the friction measured in MD simulations. A detailed investigation of the interface shape has revealed an auto-similar linear profile in the center of the channel. Close to the wall the interface shape follows the classical Log evolution given by the Cox relation despite the wall confinement

Velocity measurements in low pressure confined gas flow by molecular tagging velocimetry

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DSMC simulations of MTV experiments on gas flows in micro-channels

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DSMC simulations of MTV experiments on gas flows in micro-channels

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Velocity measurements in low pressure confined gas flow by molecular tagging velocimetry

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Automated Multi-Dataset Analysis (AMDA): An on-line database and analysis tool for heliospheric and planetary plasma data

International audienceAccessing, visualizing and analyzing heterogeneous plasma datasets has always been a tedious task that hindered students and senior researchers as well. Offering user friendly and versatile tools to perform basic research tasks is therefore pivotal for data centres including the Centre de Données de la Physique des Plasmas (CDPP http://www.cdpp.eu/) which holds a large variety of plasma data from various Earth, planetary and heliophysics missions and observatories in plasma physics. This clearly helps gaining increased attention, relevant feedback, and enhanced science return on data. These are the key ideas that crystallized at CDPP more than 15 years ago and resulted in the lay-out of the concepts, and then development, of AMDA, the Automated Multi-Dataset Analysis software (http://amda.cdpp.eu/). This paper gives a description of the architecture of AMDA, describes its functionalities, presents some use cases taken from the literature or fruitful collaborations and shows how it offers unique capabilities for educational purposes