Séminaires doctorants

Exposés des doctorants en première année de thèse.


21/06/2018 Nice Valrose
Publication : 21/06/2018
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10h30 Enzo d'Ambrosio

Doctorant avec F. Blanc et E. Lemaire

 Resuspension induit par cisaillement de particules sphériques dans un écoulement de Couette : Lien avec les contraintes normales particulaires

 

Les suspensions non-Browniennes se retrouvent dans de nombreux domaines : industriels (peinture, béton frais,…), alimentaire ou naturel (lave, boue, avalanches sous-marine,…). La complexité des interactions qui ont lieu au sein de la suspension, entre le fluide et les particules mais également inter-particulaire, induisent des phénomènes observables comme la migration de particule solide lors de l’écoulement de la suspension.

La resuspension correspond à un flux de particules solides, plus denses que le fluide suspendant, dirigé vers le haut qui vient contrebalancer la sédimentation des particules solides due à la gravité. Le paramètre qui contrôle ce phénomène est le nombre de Shield qui est le rapport entre les forces visqueuses et le poids des particules qui s’exerce sur le bas de la couche de particule. Cette resuspension est une migration particulière induite par cisaillement et peut être interprétée par différents modèles. Le premier modèle, développé par Acrivos, Morris et Fan en 1993, relie la resuspension à une diffusion induit par cisaillement. Un second modèle est le Suspension Balance
Model, développé par Morris et Boulay en 1999. Selon ce modèle, le flux de migration est proportionnel à la divergence du tenseur de contrainte particulaire dont les composants dépendent du taux de cisaillement et de la fraction volumique.

L’objectif de ma thèse est de mesurer précisément les profils de concentration pendant la resuspension, dans le but de vérifier la validité des modèles cités ci dessus. Pour cela, une suspension de particule lourde est cisaillée dans un écoulement de Couette qui est éclairé par un laser plan verticale. La suspension est rendu transparente en accordant de manière précise les indices de réfraction des phases liquide et solide. Avec un traitement d’image approprié, le profil expérimental de concentration en est déduit pour différents nombres de Shield puis comparé à des prédictions fournies par le SBM et le modèle de diffusion.

Pour la suite de ma thèse, je prévois de mesurer directement les contraintes normales particulaires responsables de cette migration de manière complètement indépendante aux mesures de concentration pour ensuite pouvoir comparer les deux résultats. Par ailleurs, alors que pour le moment on se concentre sur l’étude du régime stationnaire, je prévois également d’étudier le régime transitoire du phénomène.

10h45 Vittorio Di Pietro

Doctorant avec A. Jullien et S. Residori  

 Liquid Crystal application for temporal measurements of ultrafast pulses


A new collaboration between Fastlite and INPHYNI aims to develop new Liquid Crystal (LC) devices for manipulation of ultrafast pulses. In particular the large tunability of the birefringence of the LC cells can be addressed using external electrical eld to develop new electro-optical instruments for temporal measurements.

Nematic thick cells (up to 200µm) are used in order to increase as much as possible the birefringence excursion. Apply positive voltage steps to dynamically scan the birefringence states are also crucial to speed up the process, even if a threshold voltage corresponding to a LC phase transition, has to be passed to ensure that LC molecules reorient correctly (F rederisckz transition, FT) and no optical losses appear. So we have performed an experimental study of the swich-on dynamics and disorder-induced losses during FT in cells of dierent thickness [1]. Therefore, a work map can be pictured as a function of thickness and voltage range in order to maximize the birefringence excursion and at the same time minimize the optical losses.

In order to extend the laser wavelength range towards mid-Infrared, we have discovered that an IR femtosecond oscillator undergoes strong spatial self-phase modulation in a LC cell, due to partial laser absorption in a yet optimized ITO coating (LC cell electrode) [2]. Self-induced spatial shaping generates a multiple-ring pattern, whose analysis enable to study the induced thermal eect. Depending on the laser average intensity and polarization state, it's  possible to modify the thermal gradient up to the molecules isotropic phase transition. This eect is characterized by the connement of the thermal gradient to the laser spot and long-term stability, therefore, numerous novel applications can be contemplated.

Références
[1] V. di Pietro, and co-autors , "Dynamical optical response of nematic liquid crystal cells through electrically driven Fréedericksz transition : inuence of the nematic layer thickness," Opt. Express 26, 10716-10728 (2018)
[2] V. di Pietro, and co-autors , "Thermally-induced nonlinear spatial shaping of infrared femtosecond pulses in nematic liquid crystals," Submitted in Optics Letters.

11h00 Marius Glaudesius

Doctorant avec G. Labeyrie et R. Kaiser  

 Radiation pressure based instabilities in the multiple-scattering limit

 

By tuning the confinement forces of a magneto-optical trap in particular ways it is possible for the trapped atoms to enter a self-oscillatory regime. The mechanism describing this cold atom regime is not yet understood, but, nevertheless, by studying it one has a potential of contributing to a better understanding of phenomena that are being dealt within a broad range of fields of physics, including plasma physics and astrophysics. In this presentation I will briefly summarize my research surrounding the self-oscillatory regime, the results I was able to produce so far, as well as future perspectives of the research.

11h15 Angela Guttilla

Doctorante avec M. Benabdesselam  

 Study of the response of optical fiber or preform silica based doped after x-ray irradiation changing the conditions of irradiation and the temperature

 

The main theme of the thesis is based on the knowledge about the basic mechanisms leading to the degradation throught the radiation-induced attenuation (RIA) of silica-based materials in several mixed enviroments combining temperature, radiations and hydrogen (R-H-T) constrains as those encountered by ANDRA.

11h30 Xin Hua

Doctorant avec O. Alibart et S. Tanzilli 

 Quantum cryptography at Université Côte d’Azur

 

Project Quantum Cryptography at Université Côte d’Azur will exploit an original all-optical synchronization, which will generate, manipulate, distribute and detect quantum information bits in form of single photons at a very high rate (~MHz). These Qbits will be used to implement quantum cryptography protocols between three sites of university: Valrose-Nice, Plain du Var-Nice and Sophia. The challenge and originality of the project lie in its operation that  exploits a technique of all-optical synchronization of quantum communication networks by means of an optical clock. The challenge is to develop a network infrastructure exploiting this solution and exhibit a full-scale demonstration of secure data exchange between several points.

In terms of experimental techniques, a lot of knowledges need to learn, just like quantum photonics, classical information fusing electronics and optics, the material science about the design and manufacture of integrated optical circuits at nano-scale and micro-metrics, fiber-based telecommunication systems, and information science.

For achieving the project, we will collaborate with some other academic and industrial partners which could propose us expertises in all sub-fields of classical and quantum information technologies: computer science (Inria and I3S), transfer of time-frequency references (OCA-Artemis), optical fiber deployment (Orange, Côte d'Azur division), design / development of optical networks (Sophia Consultant and Accenture) and single photon detection (IDQuantique SA).

11h45 Antonin Siciak

Doctorant avec J. P. Rivet et R. Kaiser 

 Exploring stellar evolution and quantum emission processes in stars

 

I will present the aim of my PhD. I will explain how we are opening up intensity interferometry as a viable technique for modern astrophysics, and particularly how we want to verify the existence of natural lasers such as the Eta Carinae stellar system.