Un potentiel adaptatif simple pour la modélisation par Dynamique Moléculaire de la séparation de phase dans un verre de silice binaire

National audienc

Investigation of the local environment of Eu3+ in a silicophosphate glass using site-selective spectroscopy and Molecular Dynamics simulations

Silicophosphate glasses (SiO2-P2O5) doped with Eu3+ ions were synthesized by the sol-gel process. Optical properties of these glasses were investigated by means of emission spectra and lifetime measurements. The Fluorescence Line Narrowing (FLN) technique was also used to explore the local structure around the Eu3+ ions in this host and to understand the role of phosphate as a codopant. As it is the case for aluminum, the ability of phosphate to avoid the rare earth clustering was investigated, and the role of this codopant in modifying the local order around the rare earth ion was evidenced. The analysis of the FLN spectra and lifetime measurements is consistent with this interpretation. Molecular Dynamics simulations were performed to evaluate and confirm these structural features. Two classes of europium sites were distinguished in agreement with the experimental characterization

Fluctuations in active membranes

Active contributions to fluctuations are a direct consequence of metabolic energy consumption in living cells. Such metabolic processes continuously create active forces, which deform the membrane to control motility, proliferation as well as homeostasis. Membrane fluctuations contain therefore valuable information on the nature of active forces, but classical analysis of membrane fluctuations has been primarily centered on purely thermal driving. This chapter provides an overview of relevant experimental and theoretical approaches to measure, analyze and model active membrane fluctuations. In the focus of the discussion remains the intrinsic problem that the sole fluctuation analysis may not be sufficient to separate active from thermal contributions, since the presence of activity may modify membrane mechanical properties themselves. By combining independent measurements of spontaneous fluctuations and mechanical response, it is possible to directly quantify time and energy-scales of the active contributions, allowing for a refinement of current theoretical descriptions of active membranes.Comment: 38 pages, 9 figures, book chapte

Robust gap repair in the contractile ring ensures timely completion of cytokinesis

Cytokinesis in animal cells requires the constriction of an actomyosin contractile ring, whose architecture and mechanism remain poorly understood. We use laser microsurgery to explore the biophysical properties of constricting rings in Caenorhabditis elegans embryos. Laser cutting causes rings to snap open. However, instead of disintegrating, ring topology recovers and constriction proceeds. In response to severing, a finite gap forms and is repaired by recruitment of new material in an actin polymerization-dependent manner. An open ring is able to constrict, and rings repair from successive cuts. After gap repair, an increase in constriction velocity allows cytokinesis to complete at the same time as controls. Our analysis demonstrates that tension in the ring increases while net cortical tension at the site of ingression decreases throughout constriction and suggests that cytokinesis is accomplished by contractile modules that assemble and contract autonomously, enabling local repair of the actomyosin network. Consequently, cytokinesis is a highly robust process impervious to discontinuities in contractile ring structure.European Research Council grant: (640553); Fundo Europeu de Desenvolvimento Regional (FEDER) funds: (Operational Competitiveness Program - COMPETE); Fundação para a Ciência e a Tecnologia grant:(NORTE-07-0124-FEDER-000003); Fundação Luso-Americana para o Desenvolvimento (Life Science 2020); Louis-Jeantet Young Investigator Award; European Social Fund (Programa Operacional Temático Potencial Type 4.2); Programa Operacional Regional do Norte (Quadro de Referência Estratégico Nacional - FEDER).info:eu-repo/semantics/publishedVersio

I. Obtention et texture d’oxydes cuivriques divisés

Différentes techniques d’études des solides (microscopie électronique, rayons X, diffusion des rayons X aux petits angles, spectroscopie IR) ont été mises à profit pour examiner les propriétés texturales de plusieurs oxydes cuivriques CuO issus de la décomposition d’un même hydroxyde de cuivre Cu(OH)2. Leur composition exacte a été déterminée. Nous avons montré que la vapeur d’eau joue un rôle essentiel dans le développement de la texture des oxydes pendant leur préparation, et dans l’évolution de leur porosité, au cours du temps. Enfin, il est notable que deux échantillons d’oxyde cuivrique de composition voisine et de morphologie identique, se distinguent pourtant par des propriétés superficielles très différentes

Molecular Dynamics Simulations of Rare-Earth-Doped Nanoparticles in Silica Matrix: Drawing of a Preform to a Fiber

International audienc

Modeling of Rare Earth-doped Silicate Glasses: Codoping Effects on the Luminescent Sites Structures and Formation of Dielectrics Nanoparticles

International audienceRare-earth (RE) containing glasses are known to be good candidate materials for devices such as optical fiber amplifiers, upconversion lasers and glass lasers. In such materials, strong changes in the luminescent properties can be induced by changes in the distribution and local environment of the RE ions. The investigation of the local structure around the RE ions in glasses is therefore a key issue to interpret their luminescent properties in terms of local structure. Our group is for a long time involved in the study of these properties by means of an original approach which combines photolumines-cence measurements and Molecular Dynamics (MD) modeling [1]. We use the Fluorescence Line Narrowing (FLN) as a site-selective technique to determine the different types of site occupied by the Eu 3+ ions. Structural information on these sites is deduced from FLN spectra and from the structures modeled by MD simulations. Using this methodology, we especially study the effective role of a codoping in the enhancement of the fluorescence of RE ions, which is commonly attributed to the ability of certain cations to avoid the RE clustering. The presented work focuses on the comparison between Al 3+ and P 5+ codoping of a RE-doped silica glass. Surprisingly, both cations do not act to disperse significantly the clustered RE ions but strongly modify the local structure of the luminescent ions. Another unexpected result is the striking difference between Al 3+ and P 5+ in the way they modify the first and the second coordination shell of RE 3+ [2][3]. Mg-codoping of a RE-doped silica glass is another route to tailor the spectroscopic features of optical fibers. Since it was experimentally evidenced that the RE ions can be embedded in nanoparticles (NP) formed in situ in silica through phase separation, we have developed an adaptive and transferable MD potential to model and track the formation of Mg-rich amorphous NP in a Si-rich matrix [4]. We present here our results on the dependence between the RE environment and the size of the containing NP. [1] S. Snapshot of a modeled structure: Er 3+-doped nanoparticles of Mg-rich phase in a silica matrix