Aging dynamics in a colloidal glass of Laponite

The aging dynamics of colloidal suspensions of Laponite, a synthetic clay, is investigated using dynamic light stattering (DLS) and viscometry after a quench into the glassy phase. DLS allows to follow the diffusion of Laponite particles and reveals that there are two modes of relaxation. The fast mode corresponds to a rapid diffusion of particles within "cages" formed by the neighboring particles. The slow mode corresponds to escape from the cages: its average relaxation time increases exponentially fast with the age of the glass. In addition, the slow mode has a broad distribution of relaxation times, its distribution becoming larger as the system ages. Measuring the concomitant increase of viscosity as the system ages, we can relate the slowing down of the particle dynamics to the viscosity.Comment: 9 pages, 8 Postscript figures, submitted to Phys. Rev.

On the Refractive Index of Ageing Dispersions of Laponite

Aqueous dispersion of Laponite at low ionic concentration is of interest since it undergoes structural evolution with respect to time, which is usually termed as ageing. In this work we study the refractive index behavior as a function of ageing time, concentration and temperature. We observed that the extended Lorenz-Lorentz equation fitted the refractive index dependence on concentration and temperature very well. The refractive index did not show any dependence on ageing time. However, the dependence of refractive index on concentration showed a marked change as the system underwent transition from an isotropic to a biphasic state. The slope of the refractive index-density data is remarkably close to that of water at all Laponite concentrations. In the context of transport phenomena, optical measurements such as interferometry can exploit the water-like behavior of Laponite dispersions.Comment: 13 pages, 3 figures, to appear in Applied Clay Scienc

Nouveau modèle pour le positionnement des senseurs avec contraintes de localisation

RÉSUMÉ Les réseaux de capteurs sans fil continuent de constituer sans doute un développement technologique majeur. Le problème de la planification s'inscrit dans un objectif global d'amélioration des performances. Le problème de planification doit permettre d’optimiser l'emplacement des capteurs relativement à des critères afin d’obtenir une certaine qualité de service par exemple en terme de couverture et de connectivité. L’objectif de ce mémoire est de proposer une stratégie de planification des réseaux de capteurs sans fil. Cette stratégie va permettre de garantir une couverture et une connectivité accrue. L’approche commence par proposer un formalisme combinatoire du problème d’optimisation avec des contraintes qui permettent d’exprimer les objectifs de localisation, de couverture et de connectivité. L'espace de recherche est discret et le choix parmi les sites potentiels est directement lié aux caractéristiques de l'environnement du déploiement. Ainsi, on suppose qu’on connait les données concernant les coordonnées des positions probables pour mobiles et des sites potentiels, la portée du signal émis, la sensibilité de réception aussi bien des mobiles que des capteurs à installer, le nombre minimal des signaux pour la localisation et le nombre maximal de sauts permis pour le routage entre capteurs. Une heuristique initiale de type glouton et une autre basée sur la recherche avec tabous permettront d’approcher la solution optimale. Cette solution va être comparée à une borne inférieure définie à partir d’une relaxation de certaines contraintes du modèle. Les simulations réalisées ont permis de démonter la validité de l’approche de planification. Cependant certaines limitations surgissent surtout pour la modélisation de la propagation radio. En effet, nous proposons une amélioration qui se traduit par l’intégration des mesures ou des estimations pour le niveau d’interférence des signaux propagés aussi bien des mobiles que des capteurs. À notre avis, cette approche va essentiellement garantir une localisation exacte des mobiles ainsi qu’une meilleure connectivité des noeuds du réseau de capteurs sans fil.----------ABSTRACT Wireless sensor networks continue to be without doubts a major research area. The objective of the wireless sensors network planning problem is to locate the sensors while respecting a set of performance constraints. In this work we consider coverage and connectivity constraints. Moreover we impose that each mobile station be located by the sensors (i.e. location constraints). The purpose of this work is to propose a planning strategy of wireless sensor networks. It will ensure full coverage and increased connectivity. The approach begins by proposing a formalism of combinatorial optimization model with coverage, location and connectivity constraints. The search space is discrete and the choice of potential sites is directly related to the environment of deployment characteristics. Thus, we assume known the coordinates of positions for mobile stations and potential sites, the characteristics of radio propagation, the receiver sensitivity for mobiles as well as for sensors to be installed, the minimum number of signals for the location, the maximum hops allowed for routing between sensors. An initial greedy heuristic is proposed as well as search metaheuristic. Solutions found are compared to a lower bound obtained by a relaxed version of the model. The simulations show the validity of the planning approach. However, some limitations arise especially for the modeling of radio propagation. Indeed, we propose an improvement which is reflected in the integration of measures or estimates for the level of interference signals. From our perspective, this approach will essentially guarantee an exact location of mobiles and a better connectivity of the nodes of wireless sensor network

Coupling between aging and convective motion in a colloidal glass of Laponite

We study thermal convection in a colloidal glass of Laponite in formation. Low concentration preparation are submitted to destabilizing vertical temperature gradient, and present a gradual transition from a turbulent convective state to a steady conductive state as their viscosity increases. The time spent under convection is found to depend strongly on sample concentration, decreasing exponentially with mass fraction of colloidal particles. Moreover, at fixed concentration, it also depends slightly on the pattern selected by the Rayleigh B\'{e}nard instability: more rolls maintain the convection state longer. This behavior can be interpreted with recent theoretical approaches of soft glassy material rheology.Comment: Eur. Phys. J. B 55, 101-107 (2007) The original publication is available at http://www.springerlink.co

Un algorithme constructif efficace pour le problème de coloration de graphe

RÉSUMÉ Le problème de coloration de graphe consiste à assigner à chaque sommet une couleur de sorte que deux sommets adjacents n’aient pas la même couleur tout en utilisant le nombre minimal de couleur. C’est l’un des problèmes les plus étudiés en optimisation combinatoire en raison de ses multiples applications (la planification des horaires, l’allocation des ressources, etc.) et de la complexité de sa résolution. De nombreuses méthodes de résolutions ont été proposées pour résoudre le problème de coloration de graphe. Elles peuvent être réparties en trois catégories : les méthodes exactes dont le temps de calcul croît exponentiellement avec le nombre de sommets du graphe, les méthodes constructives qui donnent rapidement une approximation de la solution optimale du problème, et les métaheuristiques qui fournissent de meilleurs résultats mais au prix d’algorithmes plus complexes et plus gourmands en temps de calcul. Dans le présent mémoire, nous avons conçu un algorithme constructif d’ordre polynomial qui colore le graphe, une couleur à la fois, en priorisant les voisins des voisins des nœuds déjà colorés. Les résultats des tests effectués sur les graphes classiques du banc d’essai démontrent l’efficacité de l’algorithme proposé qui, pour un temps raisonnable, donne des résultats similaires que TABUCOL, la métaheuristique la plus connue.----------ABSTRACT The Vertex Coloring Problem (VCP) requires to assign a color to each vertex in such a way that colors on adjacent vertices are different and the number of colors used is minimized. Due to its numerous practical applications (scheduling, resource allocation, etc.) and computational complexity, the VCP is one of the most studied problems in combinatorial optimization. Several methods have been proposed to solve the VCP. They can be classified in three families: exact approaches whose running time increases exponentially with the size of the graph, greedy algorithms which approximate the optimal solution in a short time and metaheuristic methods which are the best performing algorithms, but are more complex and time consuming. In this work, we design a polynomial incremental algorithm which colors the graph, one class at a time by favouring the neighbours of neighbours of already colored vertices. Computational results on the set of DIMACS benchmark instances demonstrate the efficiency of the proposed algorithm which gives the same results as the popular metaheuristic TABUCOL in reasonable running time

Les indicateurs d’alerte précoce dans le système financier marocain

Financial crises can cross borders and have negative repercussions on the real economy even more in countries where the financial system is very undeveloped. Therefore, considering the developed techniques is important to guarantee the resilience and the stability of the financial system as a whole. Several countries keep their efforts in terms of implementing a preventive systemic risk policy to reduce costs in the event of a financial crisis. Consequently, the development of an early warning system is essential to predict the onset of financial crises and avoid their repercussions on financial systems. It is in this context that we will focus on the development of an early warning model based on the review of existing literature on the subject. To do this, we first proceed to the development of a binary crisis index BSFI (Kibricioglu 2002) which is developed by three financial variables namely: customer deposits, loans to the private sector, and the currency liabilities. Then, we estimate the binary logit model, in order to predict the scenarios of the probabilities of financial crises that can affect our financial system based on financial and monetary variables.   keywords:  early warning system, financial system, financial crisis, logit model. JEL Classification : G19 ; D53 ; G01 ; C25. Paper type: Empirical Research.Les crises financières peuvent traverser les frontières et induire des répercussions négatives sur l’économie réelle même dans les pays où le système financier n’est pas très développé. De ce fait, il est judicieux de prendre en considération des techniques développées afin de garantir la résilience et la stabilité du système financier dans son ensemble, plusieurs pays poursuivent leurs efforts pour la mise en place d’une politique préventive des risques systémiques afin de réduire les coûts en cas de survenance d’une crise financière. Dès lors, le développement d’un système d’alerte précoce est primordial pour prédire l’apparition des crises financières et d’éviter leurs répercussions sur les systèmes financiers. C’est dans ce cadre que nous allons nous intéresser au développement d’un modèle d’alerte précoce en nous appuyant sur la revue de littérature existante en la matière. Pour ce faire, nous procédons en premier temps, à l’élaboration d’un indice de crise binaire BSFI (Kibricioglu 2002) qui est élaboré par trois variables financières à savoir : les dépôts de la clientèle, les crédits au secteur privé, et le passif en devise. Ensuite, nous estimons le modèle logit binaire, afin de prédire les scénarios des probabilités des crises financières qui peuvent affecter notre système financier en s’appuyant sur des variables financières et monétaires.   Mots clés : Indicateur d’alerte ; système financier ; crise financière ; modèle logit. Classification JEL : G19 ; D53 ; G01 ; C25. Type de l’article : Recherche empirique

Irreversible Aging Dynamics and Generic Phase Behavior of Aqueous Suspensions of Laponite

In this work we study the aging behavior of aqueous suspension of Laponite having 2.8 weight % concentration using rheological tools. At various salt concentration all the samples demonstrate orientational order when observed using crossed polarizers. In rheological experiments we observe inherent irreversibility in the aging dynamics which forces the system not to rejuvenate to the same state in the shear melting experiment carried out at a later date since preparation. The extensive rheological experiments carried out as a function of time elapsed since preparation demonstrate the self similar trend in the aging behavior irrespective of the concentration of salt. We observe that the exploration of the low energy states as a function of aging time is only kinetically affected by the presence of salt. We estimate that the energy barrier to attain the low energy states decreases linearly with increase in the concentration of salt. The observed superposition of all the elapsed time and the salt concentration dependent data suggests that the aging that occurs in low salt concentration systems over a very long period is qualitatively similar to the aging behavior observed in systems with high salt concentration over a shorter period.Comment: 27 pages, 8 figures. Langmuir, in pres

The interplay between screening properties and colloid anisotropy: towards a reliable pair potential for disc-like charged particles

The electrostatic potential of a highly charged disc (clay platelet) in an electrolyte is investigated in detail. The corresponding non-linear Poisson-Boltzmann (PB) equation is solved numerically, and we show that the far-field behaviour (relevant for colloidal interactions in dilute suspensions) is exactly that obtained within linearized PB theory, with the surface boundary condition of a uniform potential. The latter linear problem is solved by a new semi-analytical procedure and both the potential amplitude (quantified by an effective charge) and potential anisotropy coincide closely within PB and linearized PB, provided the disc bare charge is high enough. This anisotropy remains at all scales; it is encoded in a function that may vary over several orders of magnitude depending on the azimuthal angle under which the disc is seen. The results allow to construct a pair potential for discs interaction, that is strongly orientation dependent.Comment: 13 pages, 19 figure

Linear chaos for the Quick-Thinking-Driver model

The final publication is available at Springer via http://dx.doi.org/10.1007/s00233-015-9704-6In recent years, the topic of car-following has experimented an increased importance in traffic engineering and safety research. This has become a very interesting topic because of the development of driverless cars (Google driverless cars, http://en.wikipedia.org/wiki/Google_driverless_car).Driving models which describe the interaction between adjacent vehicles in the same lane have a big interest in simulation modeling, such as the Quick-Thinking-Driver model. A non-linear version of it can be given using the logistic map, and then chaos appears. We show that an infinite-dimensional version of the linear model presents a chaotic behaviour using the same approach as for studying chaos of death models of cell growth.The authors were supported by a grant from the FPU program of MEC and MEC Project MTM2013-47093-P.Conejero, JA.; Murillo Arcila, M.; Seoane-Sepúlveda, JB. (2016). 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Structure of laponite-styrene precursor dispersions for production of advanced polymer-clay nanocomposites

One method for production of polymer-clay nanocomposites involves dispersal of surface-modified clay in a polymerisable monomeric solvent, followed by fast in situ polymerisation. In order to tailor the properties of the final material we aim to control the dispersion state of the clay in the precursor solvent. Here, we study dispersions of surface-modified Laponite, a synthetic clay, in styrene via large-scale Monte-Carlo simulations and experimentally, using small angle X-ray and static light scattering. By tuning the effective interaction between simulated laponite particles we are able to reproduce the experimental scattering intensity patterns for this system, with good accuracy over a wide range of length scales. However, this agreement could only be obtained by introducing a permanent electrostatic dipole moment into the plane of each Laponite particle, which we explain in terms of the distribution of substituted metal atoms within each Laponite particle. This suggests that Laponite dispersions, and perhaps other clay suspensions, should display some of the structural characteristics of dipolar fluids. Our simulated structures show aggregation regimes ranging from networks of long chains to dense clusters of Laponite particles, and we also obtain some intriguing ‘globular’ clusters, similar to capsids. We see no indication of any ‘house-of-cards’ structures. The simulation that most closely matches experimental results indicates that gel-like networks are obtained in Laponite dispersions, which however appear optically clear and non-sedimenting over extended periods of time. This suggests it could be difficult to obtain truly isotropic equilibrium dispersion as a starting point for synthesis of advanced polymer-clay nanocomposites with controlled structures