Local mean-field study of capillary condensation in silica aerogels

We apply local mean-field (i.e. density functional) theory to a lattice model of a fluid in contact with a dilute, disordered gel network. The gel structure is described by a diffusion-limited cluster aggregation model. We focus on the influence of porosity on both the hysteretic and the equilibrium behavior of the fluid as one varies the chemical potential at low temperature. We show that the shape of the hysteresis loop changes from smooth to rectangular as the porosity increases and that this change is associated to disorder-induced out-of-equilibrium phase transitions that differ on adsorption and on desorption. Our results provide insight in the behavior of $^4$He in silica aerogels.Comment: 19 figure

Constraining Dynamical Dark Energy Models through the Abundance of High-Redshift Supermassive Black Holes

We compute the number density of massive Black Holes (BHs) at the centre of galaxies at z=6 in different Dynamical Dark Energy (DDE) cosmologies, and compare it with existing observational lower limits, to derive constraints on the evolution of the Dark Energy equation of state parameter w. Our approach only assumes the canonical scenario for structure formation from the collapse of overdense regions of the Dark Matter dominated primordial density field on progressively larger scales; the Black Hole accretion and merging rate have been maximized in the computation so as to obtain robust constraints on w and on its look-back time derivative w_a. Our results provide independent constraints complementary to those obtained by combining Supernovae, Cosmic Microwave Background and Baryonic Acoustic Oscillations; while the latter concern combinations of w_0 and w_a leaving the time evolution of the state parameter w_a highly unconstrained, the BH abundance mainly provide upper limits on w_a, only weakly depending on w_0. Combined with the existing constraints, our results significantly restrict the allowed region in DDE parameter space, ruling out DDE models not providing cosmic time and fast growth factor large enough to allow for the building up of the observed abundance of BHs; in particular, models with -1.2 \leq w_0 \leq -1 and positive redshift evolution w_a > 0.8 - completely consistent with previous constraints - are strongly disfavoured by our independent constraints from BH abundance. Such range of parameters corresponds to "Quintom" DDE models, with w crossing -1 starting from larger values.Comment: 19 pages, 6 figures, accepted to MNRA

Comparative analysis of simulation and experiment for the Hall-type thruster BPT-4000

Since 2004, the Plasma Physics Laboratory (PPL/PI/UnB) has been developing permanent magnet Hall thrusters due to their high efficiency. However, permanent magnets have limited use due to their maximum working temperature (the point at which they begin to demagnetize). Therefore, strict control of the temperature of the magnets must be carried out. In the present work we perform a numerical thermal simulation of a Hall thruster taken from the literature (e.g., the BPT-4000) and compare the obtained simulated thruster temperatures with experimental values taken in the laboratory. This work aims to verify the reliability of our simulation strategy in order to simulate other Hall-type thrusters in the future. The obtained simulation data was consistent with experimental results, demonstrating that the used simulation tool is reliable. In the future we will use this strategy for the simulation of improved plasma Hall thrusters developed at PPL aiming at improving its engineering model in order to prepare for future space qualification tests

Mídia e política no Brasil: textos e agenda de pesquisa Midia and politics in Brazil: texts and research agenda

Um especialista em estudos de comunicação e um cientista político apresentam conjuntamente um panorama da pesquisa sobre as relações entre os meios de comunicação e os processos políticos no Brasil. Uma agenda de pesquisa é proposta e um elenco de textos nessa área é apresentado.<br>A specialist in communication studies and a political scientist present together a panorama of research on the relations between communication midia and political processes in Brazil A research agenda is proposed and a list of texts in this area is presented