Showing papers by "Laurent Perrin published in 2011"

Ignition and explosion of nanopowders: something new under the dust

Abstract: This work deals with the study of ignition and explosion characteristics of nanoparticles. It has been carried out on various powders: zinc, aluminum, carbon blacks... Specific behaviours have been highlighted during the first phase of this project (Nanosafe 2). For instance, it has been demonstrated that there mainly exists two combustion regimes that are either kinetically controlled, for small size particles, or diffusion controlled, for large size particles (generally with diameters greater than 1 or 2 µm). It has been found that as the particle size decreases, minimum ignition temperature and minimum ignition energy decrease (even lower than 1 mJ), indicating higher potential inflammation and explosion risks for metallic nanopowders. Moreover, the presence of agglomerates in the nanopowders could modify their reactivity. Thus, the explosion severity of Al powders tends to increase as the specific surface area decreases, before reaching a peak for 1 µm particle size. These results are essential for industries producing or handling nanopowders in order to propose/design new and proper prevention and protection means. Nevertheless, the validity of the classical characterization tools with regard to nanopowders should be discussed. For example, the experimental laminar flame velocity of Al dusts has been compared to a theoretical one, determined by Huang's model, which assumes that the propagation of the flame is run mainly by conduction. It has shown a good agreement. However, under certain conditions, the Al flame propagation is expected to be mainly conducted by radiation. Two hypotheses can then be made. On the one hand, it can be assumed that the 20 L sphere probably disturbs the flame propagation and thermal mechanisms by absorbing radiation (wall quenching effect). On the other hand, it has been observed, thanks to the use of a high speed camera that the preheating zone is smaller for some nanopowders than for micro-particles (figure below). It could notably be explained by the fact that the flame radiation is absorbed by the cloud of unburnt Al nanopowders. Several other factors may have an impact on the explosion severity. If these points are correctly addressed, it will be possible to get more reliable ignition and explosion characteristics.

Insights into Hox Protein Function from a Large Scale Combinatorial Analysis of Protein Domains

Abstract: Protein function is encoded within protein sequence and protein domains. However, how protein domains cooperate within a protein to modulate overall activity and how this impacts functional diversification at the molecular and organism levels remains largely unaddressed. Focusing on three domains of the central class Drosophila Hox transcription factor AbdominalA (AbdA), we used combinatorial domain mutations and most known AbdA developmental functions as biological readouts to investigate how protein domains collectively shape protein activity. The results uncover redundancy, interactivity, and multifunctionality of protein domains as salient features underlying overall AbdA protein activity, providing means to apprehend functional diversity and accounting for the robustness of Hox-controlled developmental programs. Importantly, the results highlight context-dependency in protein domain usage and interaction, allowing major modifications in domains to be tolerated without general functional loss. The non-pleoitropic effect of domain mutation suggests that protein modification may contribute more broadly to molecular changes underlying morphological diversification during evolution, so far thought to rely largely on modification in gene cis-regulatory sequences.

Tissue-specific cell sorting from Drosophila embryos: application to gene expression analysis.

Abstract: Comprehensive understanding of tissues and organs development requires a detailed description of tissues specific developmental programs. In particular, Gene Regulatory Networks need to be analyzed at the tissue level, requiring organ specific transcriptional landscapes to be established. Here, we describe an efficient and stringent strategy for cell purification of differentiating cells from Drosophila embryos by flow cytometry. This, combined to mRNA amplification, can be used for transcriptomic analysis of small, tissue-specific cell populations. We present an application to the Drosophila cardiac system, whose cell population represents 0.5 to 1% of total cells within the whole embryo. Based on widely available fluorescent reporter transgenes, this method should be applicable to a number of tissues and organs.

Ecoulements granulaires : approche rhéologique de l'agglomération des nanoparticules

Abstract: Le suivi au rheometre a poudres de la viscosite de differentes poudres nanometriques a permis de montrer que dans la thematique des ecoulements granulaires, le comportement des nanoparticules est particulier, de par leur facilite d'agglomeration. L'ecoulement de nanoparticules suit trois regimes : un newtonien, coulombien et un cinetique dans lequel en estimant la desagglomeration est minimale. Une approximation d'un taux maximal de desagglomeration peut etre infere de par les signatures des cisaillements rheologiques des nanopoudres.

Re-examination of safety parameters using kinetic theory of nano-granular flows

Abstract: The origin of the kinetic theory of granular flow was originally credited to Bagnold [1]. By using a very primitive expression of the particle collision frequency, he derived an expression for the repulsive pressure of the particles in uniform shear flows. His repulsive pressure was proportional to the square of the velocity gradient and the particle diameter and directly proportional to the particle density. This theory was later extended by Savage [2] and Gidaspow [3]. Such theories provide insight on the dependence of the viscosity, and various moduli (elastic, non elastic, viscous...) in terms of the granular temperature and the associated shear-rates. Until recently, such parameters were difficult to measure because of the lack of specifically designed equipment. This challenge was successfully taken up and resolved by P. Marchal of ENSIC who designed a new rheometer for powders (figure 1). This equipment can put in evidence the importance of the granular temperature on the elastic and viscous behaviors of the granular flows. Such rheological behavior is important in risk analysis for nanopowders, because as the nanopowder may be subjected to process shear rates and stresses, its structural and topological changes, in terms of the transformation of agglomerates into primary nanoparticles, have strong impacts on emission factors of nanosized particles that can be released in the environment or into a workplace from such dense-phase nanopowder processes. Such transformation can be analyzed by studying the nano-granular rheological signature of the system. Such risk assessment approach using these new fundamental rheological safety parameters is described in this paper.

Evolution du niveau d'agglomération de nanopoudres d'aluminium : une approche rhéologique

Abstract: Lorsque l'on passe de la dimension micrometrique a la dimension nanometrique, l'augmentation importante des energies de surface rend preponderants les phenomenes d'agglomeration. Ceci modifie l'apprehension des risques lies aux poudres. Pour parler de toxicite, de risques d'inflammation et d'explosion des nanoparticules, nous devons prendre en compte la presence d'agglomerats. L'effet de l'agglomeration sur la dispersibilite des nanopoudres a ete etudie a partir d'observations experimentales de nanopoudres agglomerees d'aluminium soumises au cisaillement fourni par un rheometre a poudres. Pour comprendre l'effet de la desagglomeration sur les poudres, nous avons deja estime la contrainte d'agglomeration mise en jeu a partir de la porosite du lit de poudres contenues dans la cellule du rheometre. Cette contrainte est globalement ici comprise entre 10 kPa et 1MPa. Les tests en rheologie ont montre qu'en comparaison avec une poudre d'aluminium micrometrique, les nanopoudres d'aluminium ont un comportement particulier vis-a-vis de la classification de Geldart, appartenant a la fois aux classes A et C, ceci etant du a la facilite des nanoparticules a s'agglomerer et a le rester.