Séverine Barbosa

Near-critical-angle scattering for the characterization of clouds of bubbles: particular effects

We report experimental investigations on the influence of various optical effects on the far-field scattering pattern produced by a cloud of optical bubbles near the critical scattering angle. Among the effects considered, there is the change of the relative refractive index of the bubbles (gas bubbles or some liquid-liquid droplets), the influence of intensity gradients induced by the laser beam intensity profile and by the spatial filtering of the collection optics, the coherent and multiple scattering effects occurring for densely packed bubbles, and the tilt angle of spheroidal optical bubbles. The results obtained herein are thought to be fundamental for the development of future works to model these effects and for the extension of the range of applicability of an inverse technique (referenced herein as the critical angle refractometry and sizing technique), which is used to determine the size distribution and composition of bubbly flows.

Influence of nanosecond repetitively pulsed discharges on the stability of a swirled propane/air burner representative of an aeronautical combustor

This paper reports on an experimental study of the influence of a nanosecond repetitively pulsed spark discharge on the stability domain of a propane/air flame. This flame is produced in a lean premixed swirled combustor representative of an aeronautical combustion chamber. The lean extinction limits of the flame produced without and with plasma are determined and compared. It appears that only a low mean discharge power is necessary to increase the flame stability domain. Lastly, the effects of several parameters (pulse repetition frequency, global flowrate, electrode location) are studied.

Physical-optics approximation of near-critical-angle scattering by spheroidal bubbles

A first-order approximation is derived for the near-critical-angle scattering of a large spheroidal bubble illuminated by a plane wave propagating along the bubble axis of symmetry. The intensity of the far-field scattering pattern is expressed as a function of the relative refractive index and the two radii of curvature of the spheroidal bubble at the critical impact point.

In-situ characterisation of the dynamics of a growing dust particle cloud in a direct-current argon glow discharge

The growth and the dynamics of a tungsten nanoparticle cloud were investigated in a direct-current low pressure argon glow discharge. Real-time analyses of the dust particle size and number concentration were performed in-situ by light extinction spectrometry, while spatial dynamics of the cloud was investigated with the laser light-sheet scattering method. Additional off-line electron microscopy and Raman spectroscopy measurements were also performed for comparison purpose. This experimental work reveals the existence of an agglomeration phase followed by the appearance of a new dust particle generation. While growing, the dust cloud is pushed towards the anode and the discharge edge. Afterwards, a new dust particle generation can grow in the space freed by the first generation of nanoparticles. The continuous growth, below the light extinction spectrometry scanning positions, explains the apparent dissimilarities observed between the in-line optical and the off-line electron microscopy analyses.

A review on Light Extinction Spectrometry as a diagnostic for dust particle characterization in dusty plasmas

In this article, a detailed description of the light extinction spectrometry diagnostics is given. It allows the direct in-situ measurement of the particle size distribution and absolute concentration of a dust cloud levitating in plasmas. Using a relatively simple and compact experimental setup , the dust cloud parameters can be recovered with a good accuracy making minimum assumptions on their physical properties. Special emphasises are given to the inversion of light extinction spectra and all the required particle shape, refractive index and the light extinction models. The parameter range and the limitations of the diagnostic are discussed. In addition, two examples of measurements in low-pressure gas discharges are presented: in a DC glow discharge in which nanopar-ticles are growing from the sputtering of a tungsten cathode, and in an Argon-Silane radio-frequency discharge.

On the size and morphological characterization of needle-shaped TiO2 nanoparticles in suspension

We report the work in progress to develop a non invasive and fast optical method allowing characterizing the concentration, aspect ratio and mean size of acicular, nanorod and nanotube shaped particles. This method is based on the analysis of the spectral extinction of nanoparticles illuminated by polarized-light and whose orientation can be controlled by means of electrostatic and/or hydronamical forces. T-Matrix calculations and preliminary experimental results illustrate some aspects and features of this method.

Inverse near-critical-angle scattering as a tool to characterize bubble clouds

Under real flow conditions, the critical-angle scattering of a spherical bubble is too noisy to obtain directly the bubble diameter and refractive index. To solve this problem and to limit the drawback of any counting technique, the critical angle refractometry and sizing (CARS) technique has been extended as a collective ensemble technique. As an inverse method it allows to get the size distribution and composition of a cloud of bubbles. In this paper we review the principle, the advantages and limits of this new optical particle characterization method.