263 research outputs found
New urban governance: a review of current themes and future priorities
This review paper explores some of the key concepts, trends and approaches in contemporary urban governance research. Based on a horizon scan of recent literature and a survey of local government officials it provides a big picture on the topic and identifies areas for future research. Bridging the gap between the scholarly research focus and the perceptions and requirements of city administrators represents a major challenge for the field. Furthermore, because global and comparative research on urban governance is confronted with an absence of systematically collected, comparable data, the paper argues that future efforts will require experimenting with methodologies that can generate new empirical insights
Thermodynamics and Phase Transitions of Electrolytes on Lattices with Different Discretization Parameters
Lattice models are crucial for studying thermodynamic properties in many
physical, biological and chemical systems. We investigate Lattice Restricted
Primitive Model (LRPM) of electrolytes with different discretization parameters
in order to understand thermodynamics and the nature of phase transitions in
the systems with charged particles. A discretization parameter is defined as a
number of lattice sites that can be occupied by each particle, and it allows to
study the transition from the discrete picture to the continuum-space
description. Explicit analytic and numerical calculations are performed using
lattice Debye-H\"{u}ckel approach, which takes into account the formation of
dipoles, the dipole-ion interactions and correct lattice Coulomb potentials.
The gas-liquid phase separation is found at low densities of charged particles
for different types of lattices. The increase in the discretization parameter
lowers the critical temperature and the critical density, in agreement with
Monte Carlo computer simulations results. In the limit of infinitely large
discretization our results approach the predictions from the continuum model of
electrolytes. However, for the very fine discretization, where each particle
can only occupy one lattice site, the gas-liquid phase transitions are
suppressed by order-disorder phase transformations.Comment: Submitted to Molecular Physic
Path integral Monte Carlo simulations for rigid rotors and their application to water
In this work the path integral formulation for rigid rotors, proposed by
M\"user and Berne [Phys. Rev. Lett. {\bf 77}, 2638 (1996)], is described in
detail. It is shown how this formulation can be used to perform Monte Carlo
simulations of water. Our numerical results show that whereas some properties
of water can be accurately reproduced using classical simulations with an
empirical potential which, implicitly, includes quantum effects, other
properties can only be described quantitatively when quantum effects are
explicitly incorporated. In particular, quantum effects are extremely relevant
when it comes to describing the equation of state of the ice phases at low
temperatures, the structure of the ices at low temperatures, and the heat
capacity of both liquid water and the ice phases. They also play a minor role
in the relative stability of the ice phases.Comment: to appear in Molecular Physics (2011
Networked urban governance: a socio-structural analysis of transport strategies in London and New York
This paper investigates urban governance empirically by applying social network analysis methods to data gathered through structured interviews in London and New York. We explore how decisions are made in complex institutional environments inhabited by various types of actors. Owing to the time-consuming data collection and treatment processes, the research zooms in on transport. The comparative approach enabled the detection of different structural features in the governance networks shaping transport strategies in both cities. The perceived relative power, influence, dependence and/or affinity between the actors involved is discussed based on network attributes. The evidence suggests that transport governance in London is more centralised (and, arguably, more technocratic and integrated), in the sense that a few prestigious entities are clearly more prominent. In New York the institutional environment is typified by many checks and balances (and, arguably, more democratic and fragmented), where central actors are less obvious
Analytical approximations for spatial stochastic gene expression in single cells and tissues
Gene expression occurs in an environment in which both stochastic and diffusive effects are significant. Spatial stochastic simulations are computationally expensive compared with their deterministic counterparts, and hence little is currently known of the significance of intrinsic noise in a spatial setting. Starting from the reactionâdiffusion master equation (RDME) describing stochastic reactionâdiffusion processes, we here derive expressions for the approximate steady-state mean concentrations which are explicit functions of the dimensionality of space, rate constants and diffusion coefficients. The expressions have a simple closed form when the system consists of one effective species. These formulae show that, even for spatially homogeneous systems, mean concentrations can depend on diffusion coefficients: this contradicts the predictions of deterministic reactionâdiffusion processes, thus highlighting the importance of intrinsic noise. We confirm our theory by comparison with stochastic simulations, using the RDME and Brownian dynamics, of two models of stochastic and spatial gene expression in single cells and tissues
Alterations in vascular function in primary aldosteronism - a cardiovascular magnetic resonance imaging study
Introduction: Excess aldosterone is associated with increased cardiovascular risk. Aldosterone has a permissive effect on vascular fibrosis. Cardiovascular magnetic resonance imaging (CMR) allows study of vascular function by measuring aortic distensibility. We compared aortic distensibility in primary aldosteronism (PA), essential hypertension (EH) and normal controls and explored the relationship between aortic distensibility and pulse wave velocity (PWV).<p></p>
Methods: We studied PA (n=14) and EH (n=33) subjects and age-matched healthy controls (n=17) with CMR, including measurement of aortic distensibility, and measured PWV using applanation tonometry. At recruitment, PA and EH patients had similar blood pressure and left ventricular mass.<p></p>
Results: Subjects with PA had significantly lower aortic distensibilty and higher PWV compared to EH and healthy controls. These changes were independent of other factors associated with reduced aortic distensibility, including aging. There was a significant relationship between increasing aortic stiffness and age in keeping with physical and vascular aging. As expected, aortic distensibility and PWV were closely correlated.<p></p>
Conclusion: These results demonstrate that PA patients display increased arterial stiffness compared to EH, independent of vascular aging. The implication is that aldosterone invokes functional impairment of arterial function. The long-term implications of arterial stiffening in aldosterone excess require further study.<p></p>
Population inversion of a NAHS mixture adsorbed into a cylindrical pore
A cylindrical nanopore immersed in a non-additive hard sphere binary fluid is
studied by means of integral equation theories and Monte Carlo simulations. It
is found that at low and intermediate values of the bulk total number density
the more concentrated bulk species is preferentially absorbed by the pore, as
expected. However, further increments of the bulk number density lead to an
abrupt population inversion in the confined fluid and an entropy driven
prewetting transition at the outside wall of the pore. These phenomena are a
function of the pore size, the non-additivity parameter, the bulk number
density, and particles relative number fraction. We discuss our results in
relation to the phase separation in the bulk.Comment: 7 pages, 8 Figure
Models for synthetic biology
Synthetic biological engineering is emerging from biology as a distinct discipline based on quantification. The technologies propelling synthetic biology are not new, nor is the concept of designing novel biological molecules. What is new is the emphasis on system behavior
Computing the Viscosity of Supercooled Liquids: Markov Network Model
The microscopic origin of glass transition, when liquid viscosity changes continuously by more than ten orders of magnitude, is challenging to explain from first principles. Here we describe the detailed derivation and implementation of a Markovian Network model to calculate the shear viscosity of deeply supercooled liquids based on numerical sampling of an atomistic energy landscape, which sheds some light on this transition. Shear stress relaxation is calculated from a master-equation description in which the system follows a transition-state pathway trajectory of hopping among local energy minima separated by activation barriers, which is in turn sampled by a metadynamics-based algorithm. Quantitative connection is established between the temperature variation of the calculated viscosity and the underlying potential energy and inherent stress landscape, showing a different landscape topography or âterrainâ is needed for low-temperature viscosity (of order 10[superscript 7] Pa·s) from that associated with high-temperature viscosity (10[superscript â5] Pa·s). Within this range our results clearly indicate the crossover from an essentially Arrhenius scaling behavior at high temperatures to a low-temperature behavior that is clearly super-Arrhenius (fragile) for a Kob-Andersen model of binary liquid. Experimentally the manifestation of this crossover in atomic dynamics continues to raise questions concerning its fundamental origin. In this context this work explicitly demonstrates that a temperature-dependent âterrainâ characterizing different parts of the same potential energy surface is sufficient to explain the signature behavior of vitrification, at the same time the notion of a temperature-dependent effective activation barrier is quantified.Corning IncorporatedBoston University. Center for Scientific Computing and VisualizationNational Science Foundation (U.S.) (grant DMR-1008104)National Science Foundation (U.S.) (grant DMR-0520020)United States. Air Force Office of Scientific Research (FA9550-08-1-0325
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