Biomedical subjects

S Mossa

Publications and source records attributed to S Mossa.

10 recordsLinked to original sources

PubMed2007

Aging in a Laponite colloidal suspension: a Brownian dynamics simulation study.

The authors report Brownian dynamics simulation of the out-of-equilibrium dynamics (aging) in a colloidal suspension composed of rigid charged disks, one possible model for Laponite, a synthetic clay deeply investigated in the last few years by means of various experimental techniques. At variance with previous numerical investigations, mainly focusing on static structure and equilibrium dynamics, the authors explore the out-of-equilibrium aging dynamics. They analyze the wave vector and waiting time dependence of the dynamics, focusing on the single-particle and collective density fluctuations (intermediate scattering functions), the mean-squared displacement, and the rotational dynamics. Their findings confirm the complexity of the out-of-equilibrium dynamical behavior of this class of colloidal suspensions and suggest that an arrested disordered state driven by a repulsive Yukawa potential, i.e., a Wigner glass, can be observed in this model.

Journal Article↗

PubMed2005

The worldwide air transportation network: Anomalous centrality, community structure, and cities' global roles.

We analyze the global structure of the worldwide air transportation network, a critical infrastructure with an enormous impact on local, national, and international economies. We find that the worldwide air transportation network is a scale-free small-world network. In contrast to the prediction of scale-free network models, however, we find that the most connected cities are not necessarily the most central, resulting in anomalous values of the centrality. We demonstrate that these anomalies arise because of the multicommunity structure of the network. We identify the communities in the air transportation network and show that the community structure cannot be explained solely based on geographical constraints and that geopolitical considerations have to be taken into account. We identify each city's global role based on its pattern of intercommunity and intracommunity connections, which enables us to obtain scale-specific representations of the network.

Journal Article↗

PubMed2005

Breast density changes associated with hormone replacement therapy in postmenopausal women. Effects on the specificity and sensitivity of mammographic screening.

Postmenopausal HRT use is associated with an increase of mammographic density and reduction of sensitivity and specificity of mammography results and an increase of false-positive and false-negative outcomes. The increased density does not allow a good evaluation of the exam. Mammographic density is an independent risk factor for breast cancer, but the link between changes in breast density and difference in breast cancer risk, remain uncertain. On the other hand, today specific guidelines and protocols to optimize the screening of neoplastic breast pathology in HRT users do not exist and it is unknown if short-term suspension of therapy improves mammographic sensitivity. More information is required to define this important risk factor.

Breast Neoplasms↗

PubMed2005

Microsurgery versus laparoscopy in distal tubal obstruction hysterosalpingographically or laparoscopically investigated.

OBJECTIVE: To compare pregnancy rates after laparotomic microsurgical or laparoscopic distal tuboplasty. DESIGN: Two hundred and twenty-four women with infertility due to distal tubal occlusion were randomized to be treated with either laparotomy or laparoscopy from 1987 to 2001 at the Institute of Gynaecology and Obstetrics, University of Rome, "La Sapienza". RESULTS: The results were evaluated taking into account the type of surgical approach, the severity of tubal damage and of adhesions. After a 24-month follow-up period, the overall pregnancy rate obtained with microsurgery was 43.7%, of which 33.3% were term pregnancies, 5.0% abortions, and 5.0% ectopic pregnancies. After laparoscopy, the overall pregnancy rate was 41.6%, of which 29.1% were term pregnancies, 8.3% abortions and 3.9% ectopic pregnancies. No significant differences was observed between the two groups in terms of fertility rate (chi-square 0.016, p = 0.9003). CONCLUSIONS: Laparotomy plus microsurgery and laparoscopy were equally effective in restoring fertility in women with comparable tubal damage. The severity of the damage is a critical factor for the results.

Adult↗

PubMed2004

Ground-state clusters for short-range attractive and long-range repulsive potentials.

We report calculations of the ground-state energies and geometries for clusters of different sizes (up to 80 particles), where individual particles interact simultaneously via a short-ranged attractive potential, modeled with a generalization of the Lennard-Jones potential, and a long-ranged repulsive Yukawa potential. We show that for specific choices of the parameters of the repulsive potential, the ground-state energy per particle has a minimum at a finite cluster size. For these values of the parameters in the thermodynamic limit, at low temperatures and small packing fractions, where clustering is favored and cluster-cluster interactions can be neglected, thermodynamically stable cluster phases can be formed. The analysis of the ground-state geometries shows that the spherical shape is marginally stable. In the majority of the studied cases, we find that above a certain size, ground-state clusters preferentially grow almost in one dimension.

Models, Chemical↗

PubMed2004

Disentangling density and temperature effects in the viscous slowing down of glassforming liquids.

We present a consistent picture of the respective role of density (rho) and temperature (T) in the viscous slowing down of glassforming liquids and polymers. Specifically, based in part upon a new analysis of simulation and experimental data on liquid ortho-terphenyl, we conclude that a zeroth-order description of the approach to the glass transition (in the range of experimentally accessible pressures) should be formulated in terms of a temperature-driven super-Arrhenius activated behavior rather than a density-driven congestion or jamming phenomenon. The density plays a role at a quantitative level, but its effect on the viscosity and the alpha-relaxation time can be simply described via a single parameter, an effective interaction energy that is characteristic of the high-T liquid regime; as a result, rho does not affect the "fragility" of the glassforming system.

Journal Article↗

PubMed2004

Liquid stability in a model for ortho-terphenyl.

We report an extensive study of the phase diagram of a simple model for ortho-terphenyl, focusing on the limits of stability of the liquid state. Reported data extend previous studies of the same model to both lower and higher densities and to higher temperatures. We estimate the location of the homogeneous liquid-gas nucleation line and of the spinodal locus. Within the potential energy landscape formalism, we calculate the distributions of depth, number, and shape of the potential energy minima and show that the statistical properties of the landscape are consistent with a Gaussian distribution of minima over a wide range of volumes. We report the volume dependence of the parameters entering in the Gaussian distribution (amplitude, average energy, variance). We finally evaluate the locus where the configurational entropy vanishes, the so-called Kauzmann line, and discuss the relative location of the spinodal and Kauzmann loci.

Journal Article↗

PubMed2002

Dynamics and configurational entropy in the Lewis-Wahnström model for supercooled orthoterphenyl.

We study thermodynamic and dynamic properties of a rigid model of the fragile glass-forming liquid orthoterphenyl. This model, introduced by Lewis and Wahnström in 1993, collapses each phenyl ring to a single interaction site; the intermolecular site-site interactions are described by the Lennard-Jones potential whose parameters have been selected to reproduce some bulk properties of the orthoterphenyl molecule. A system of N=343 molecules is considered in a wide range of densities and temperatures, reaching simulation times up to 1 micros. Such long trajectories allow us to equilibrate the system at temperatures below the mode coupling temperature T(c) at which the diffusion constant reaches values of order 10(-10) cm(2)/s and thereby to sample in a significant way the potential energy landscape in the entire temperature range. Working within the inherent structures thermodynamic formalism, we present results for the temperature and density dependence of the number, depth and shape of the basins of the potential energy surface. We evaluate the total entropy of the system by thermodynamic integration from the ideal-noninteracting-gas state and the vibrational entropy approximating the basin free energy with the free energy of 6N-3 harmonic oscillators. We evaluate the configurational part of the entropy as a difference between these two contributions. We study the connection between thermodynamical and dynamical properties of the system. We confirm that the temperature dependence of the configurational entropy and of the diffusion constant, as well as the inverse of the characteristic structural relaxation time, are strongly connected in supercooled states; we demonstrate that this connection is well represented by the Adam-Gibbs relation, stating a linear relation between logD and the quantity 1/TS(c). This relation is found to hold both above and below the critical temperature T(c)-as previously found in the case of silica-supporting the hypothesis that a connection exists between the number of basins and the connectivity properties of the potential energy surface.

Journal Article↗

PubMed2001

Molecular dynamics simulation of the fragile glass former orthoterphenyl: a flexible molecule model. II. Collective dynamics.

We present a molecular dynamics study of the collective dynamics of a model for the fragile glass former orthoterphenyl. In this model, introduced by Mossa, Di Leonardo, Ruocco, and Sampoli [Phys. Rev. E 62, 612 (2000)], the intramolecular interaction among the three rigid phenyl rings is described by a set of force constants whose value has been fixed in order to obtain a realistic isolated molecule spectrum. The interaction between different molecules is described by a Lennard Jones site-site potential. We study the behavior of the coherent scattering functions F(t)(q,t), considering the density fluctuations of both molecular and phenyl-ring centers of mass; moreover we directly simulate the neutron scattering spectra taking into account both the contributions due to carbon and hydrogens atoms. We compare our results with the main predictions of the mode-coupling theory and with the available coherent neutron scattering experimental data.

Journal Article↗

PubMed2000

Molecular dynamics simulation of the fragile glass-former orthoterphenyl: A flexible molecule model

We present a realistic model of the fragile glass-former orthoterphenyl and the results of extensive molecular dynamics simulations in which we investigated its basic static and dynamic properties. In this model the internal molecular interactions between the three rigid phenyl rings are described by a set of force constants, including harmonic and anharmonic terms; the interactions among different molecules are described by Lennard-Jones site-site potentials. Self-diffusion properties are discussed in detail together with the temperature and momentum dependencies of the self-intermediate scattering function. The simulation data are compared with existing experimental results and with the main predictions of the mode-coupling theory.

Journal Article↗