A place-focused model for social networks in cities

The focused organization theory of social ties proposes that the structure of human social networks can be arranged around extra-network foci, which can include shared physical spaces such as homes, workplaces, restaurants, and so on. Until now, this has been difficult to investigate on a large scale, but the huge volume of data available from online location-based social services now makes it possible to examine the friendships and mobility of many thousands of people, and to investigate the relationship between meetings at places and the structure of the social network. In this paper, we analyze a large dataset from Foursquare, the most popular online location-based social network. We examine the properties of city-based social networks, finding that they have common structural properties, and that the category of place where two people meet has very strong influence on the likelihood of their being friends. Inspired by these observations in combination with the focused organization theory, we then present a model to generate city-level social networks, and show that it produces networks with the structural properties seen in empirical data.Comment: 13 pages, 7 figures. IEEE/ASE SocialCom 201

Group colocation behavior in technological social networks.

We analyze two large datasets from technological networks with location and social data: user location records from an online location-based social networking service, and anonymized telecommunications data from a European cellphone operator, in order to investigate the differences between individual and group behavior with respect to physical location. We discover agreements between the two datasets: firstly, that individuals are more likely to meet with one friend at a place they have not visited before, but tend to meet at familiar locations when with a larger group. We also find that groups of individuals are more likely to meet at places that their other friends have visited, and that the type of a place strongly affects the propensity for groups to meet there. These differences between group and solo mobility has potential technological applications, for example, in venue recommendation in location-based social networks.This is the final manuscript published by PLOS One. It was originally published here: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0105816

Caractérisation du comportement au feu des matériaux de l'habitat : Influence de l'effet d'échelle.

The origin of home fires can be as diverse as poor wiring, a smoldering cigarette, poor maintenance of a fireplace... This manuscript deals with the flammability of various household materials exposed to a radiative source. Several parameters such as dimension, orientation and nature of materials were investigated in similar operating conditions. These studies led to the development of new experimental device enabling working at different scales: RAPACES (RAdiant PAnel Concentrator Experimental Setup). The results obtained on PMMA showed that geometric factors play a major role (i) on ignition (variation of thermal radiative properties) and (ii) on combustion dynamics through structural phenomena (destabilization, collapsing and polymer melting). This approach was also applied to complex materials (plywood, tapestry, carpet). A numerical model was used to validate some hypothesis regarding the fire behavior of PMMA. Finally, original configurations including material assemblies were performed and clearly evidenced the effect of fuel interactions on ignition and flame propagation.L’origine des incendies domestiques peut être aussi diverse qu’une mauvaise installation électrique, une cigarette mal éteinte, un mauvais entretien d’une cheminée… Ce mémoire traite de l’inflammabilité de différents matériaux de l’habitat face à une source radiative. Plusieurs paramètres tels que la dimension, l’orientation et la nature des matériaux ont été étudiés dans des conditions opératoires similaires. Ces travaux ont conduit au développement d’un nouveau dispositif expérimental permettant de travailler à différentes échelles : le RAPACES (RAdiant PAnel Concentrator Experimental Setup). Les résultats obtenus sur le PMMA ont permis de mettre en évidence que les facteurs géométriques jouent un rôle prépondérant; d’une part sur l’ignition (variation des propriétés thermo-radiatives) et d’autre part sur la dynamique de combustion à travers des phénomènes structuraux (déstabilisation structurelle, affaissement et écoulement du matériau). Cette démarche a également été appliquée à des matériaux plus complexes (contreplaqué, tapisserie, moquette). L’utilisation d’un modèle numérique a permis de valider certaines hypothèses concernant le comportement au feu du PMMA. Enfin, des configurations originales constituées d’assemblages de matériaux ont été testées et ont permis de mettre en évidence les interactions entre combustibles en termes d’ignition et de propagation de flamme

Immobilization of Metal Hexacyanoferrate Ion-Exchangers for the Synthesis of Metal Ion Sorbents—A Mini-Review

Metal hexacyanoferrates are very efficient sorbents for the recovery of alkali and base metal ions (including radionuclides such as Cs). Generally produced by the direct reaction of metal salts with potassium hexacyanoferrate (the precursors), they are characterized by ion-exchange and structural properties that make then particularly selective for Cs(I), Rb(I) and Tl(I) recovery (based on their hydrated ionic radius consistent with the size of the ion-exchanger cage), though they can bind also base metals. The major drawback of these materials is associated to their nanometer or micrometer size that makes them difficult to recover in large-size continuous systems. For this reason many techniques have been designed for immobilizing these ion-exchangers in suitable matrices that can be organic (mainly polymers and biopolymers) or inorganic (mineral supports), carbon-based matrices. This immobilization may proceed by in situ synthesis or by entrapment/encapsulation. This mini-review reports some examples of hybrid materials synthesized for the immobilization of metal hexacyanoferrate, the different conditionings of these composite materials and, briefly, the parameters to take into account for their optimal design and facilitated use

Les nouveaux entrants de l'information locale: un journalisme de rupture?

International audienc

Les nouveaux entrants de l'information locale: un journalisme de rupture?

International audienc

Fire behavior of innovative alginate foams

International audienceA new biosourced composite foam (AF, associating foamed alginate matrix and orange peel filler) is successfully tested for fire-retardant properties. This material having similar thermal insulating properties and density than fire-retardant polyurethane foam (FR-PUF, a commercial product) shows promising enhanced properties for flame retardancy, as assessed by different methods such as thermogravimetric analysis (TGA), pyrolysis combustion flow calorimetry (PCFC) and a newly designed apparatus called RAPACES for investigating large-scale samples. All these methods confirm the promising properties of this alternative material in terms of fire protection (pHRR, THR, EHC, time-to-ignition, flame duration or production of residue), especially for heat flux not exceeding 50 kW m−2. At higher heat flux (i.e., 75 kW m−2), flame retardant properties tend to decrease but maintain at a higher level than FR-PUF. The investigation of the effect of AF thickness shows that the critical thickness (CT) is close to 1.5–1.7 cm: heat diffusion and material combustion are limited to the CT layer that protects the underlying layers from combustion. A multiplicity of factors can explain this behavior, such as: (a) negligible heat conduction, (b) low heat of combustion, (c) charring formation, and (d) water release. Water being released from underlying layers, dilutes the gases emitted during the combustion of superficial layers and promotes the flame extinction

Effect of Bubbling on Ignition of PMMA Slab: Change in Thermo-Physical and Thermo-Radiative Properties

International audienceIn semi-transparent polymers, ignition is not only dependent on conductive thermal transfer into the material but also on in-depth absorption of the radiation. The aim of this work was to investigate the influence of bubbling on the thermo-physical and thermo-radiative properties of PMMA and how it may affect its ignition. PMMA plates of varying thickness were exposed to the heat flux of two radiative sources with different emission spectra. Exposure was stopped after different periods of time to study bubbling kinetics and bubble size distribution by optical microscopy. Front and back surface temperatures of samples were recorded during heat exposure. The results indicate that the bubble size distribution is closely related to the temperature gradient within the sample. Steep thermal gradients lead to small-sized bubbles underneath the exposed surface, while weak thermal gradients generate a wider size distribution with in-depth bubbling. All thermo-physical quantities k, ρ and Cp were shown to decrease with increasing bubbling degree. Likewise, it was highlighted that bubbling modifies the thermo-radiative properties of PMMA, especially in the near-infrared range. Transmittance decreases while absorbance increases with a bubbling degree. The increase in the absorption coefficient was attributed to multiple scattering by bubbles that expand the pathway of radiation into the materials. It was concluded that changes in both the thermo-physical and thermo-radiative properties with bubbling were likely to account for the delay in ignition observed when using the near-infrared heating source

Cumulative distribution function (CDF) of the distance from cellphone users' top locations to a group colocation venue.

<p>Top locations are the locations where a user has made or received a phone call the most in the past. ‘Closer’ refers to the distance to the closest of a user's top locations in a colocation event, ‘mean’ refers to the mean distance to the user's top locations, and ‘single’ is the function for all calls, not just colocations. Groups tend to congregate closer to their members' top locations than people tend to go in general in relation to their top locations.</p