1,418 research outputs found
Molecular random tilings as glasses
We have recently shown [Blunt et al., Science 322, 1077 (2008)] that
p-terphenyl-3,5,3',5'-tetracarboxylic acid adsorbed on graphite self-assembles
into a two-dimensional rhombus random tiling. This tiling is close to ideal,
displaying long range correlations punctuated by sparse localised tiling
defects. In this paper we explore the analogy between dynamic arrest in this
type of random tilings and that of structural glasses. We show that the
structural relaxation of these systems is via the propagation--reaction of
tiling defects, giving rise to dynamic heterogeneity. We study the scaling
properties of the dynamics, and discuss connections with kinetically
constrained models of glasses.Comment: 5 pages, 5 figure
Intelligent sampling for the measurement of structured surfaces
Uniform sampling in metrology has known drawbacks such as coherent spectral aliasing and a lack of efficiency in terms of measuring time and data storage. The requirement for intelligent sampling strategies has been outlined over recent years, particularly where the measurement of structured surfaces is concerned. Most of the present research on intelligent sampling has focused on dimensional metrology using coordinate-measuring machines with little reported on the area of surface metrology. In the research reported here, potential intelligent sampling strategies for surface topography measurement of structured surfaces are investigated by using numerical simulation and experimental verification. The methods include the jittered uniform method, low-discrepancy pattern sampling and several adaptive methods which originate from computer graphics, coordinate metrology and previous research by the authors. By combining the use of advanced reconstruction methods and feature-based characterization techniques, the measurement performance of the sampling methods is studied using case studies. The advantages, stability and feasibility of these techniques for practical measurements are discussed
Macroscopic Equations of Motion for Two Phase Flow in Porous Media
The established macroscopic equations of motion for two phase immiscible
displacement in porous media are known to be physically incomplete because they
do not contain the surface tension and surface areas governing capillary
phenomena. Therefore a more general system of macroscopic equations is derived
here which incorporates the spatiotemporal variation of interfacial energies.
These equations are based on the theory of mixtures in macroscopic continuum
mechanics. They include wetting phenomena through surface tensions instead of
the traditional use of capillary pressure functions. Relative permeabilities
can be identified in this approach which exhibit a complex dependence on the
state variables. A capillary pressure function can be identified in equilibrium
which shows the qualitative saturation dependence known from experiment. In
addition the new equations allow to describe the spatiotemporal changes of
residual saturations during immiscible displacement.Comment: 15 pages, Phys. Rev. E (1998), in prin
Visible-Light-Active Iodide-Doped BiOBr Coatings for Sustainable Infrastructure
The search for efficient materials for sustainable infrastructure is an urgent challenge toward potential negative emission technologies and the global environmental crisis. Pleasant, efficient sunlight-activated coatings for applications in self-cleaning windows are sought in the glass industry, particularly those produced from scalable technologies. The current work presents visible-light-active iodide-doped BiOBr thin films fabricated using aerosol-assisted chemical vapor deposition. The impact of dopant concentration on the structural, morphological, and optical properties was studied systematically. The photocatalytic properties of the parent materials and as-deposited doped films were evaluated using the smart ink test. An optimized material was identified as containing 2.7 atom % iodide dopant. Insight into the photocatalytic behavior of these coatings was gathered from photoluminescence and photoelectrochemical studies. The optimum photocatalytic performance could be explained from a balance between photon absorption, charge generation, carrier separation, and charge transport properties under 450 nm irradiation. This optimized iodide-doped BiOBr coating is an excellent candidate for the photodegradation of volatile organic pollutants, with potential applications in self-cleaning windows and other surfaces
Accurate exchange-correlation energies for the warm dense electron gas
Density matrix quantum Monte Carlo (DMQMC) is used to sample exact-on-average
-body density matrices for uniform electron gas systems of up to 10
matrix elements via a stochastic solution of the Bloch equation. The results of
these calculations resolve a current debate over the accuracy of the data used
to parametrize finite-temperature density functionals. Exchange-correlation
energies calculated using the real-space restricted path-integral formalism and
the -space configuration path-integral formalism disagree by up to
\% at certain reduced temperatures and densities . Our calculations confirm the accuracy of the configuration
path-integral Monte Carlo results available at high density and bridge the gap
to lower densities, providing trustworthy data in the regime typical of
planetary interiors and solids subject to laser irradiation. We demonstrate
that DMQMC can calculate free energies directly and present exact free energies
for and .Comment: Accepted version: added free energy data and restructured text. Now
includes supplementary materia
Continuum-scale characterization of solute transport based on pore-scale velocity distributions
We present a methodology to characterize a continuum-scale model of transport in porous media on the basis of pore-scale distributions of velocities computed in three-dimensional pore-space images. The methodology is tested against pore-scale simulations of flow and transport for a bead pack and a sandstone sample. We employ a double continuum approach to describe transport in mobile and immobile regions. Model parameters are characterized through inputs resulting from the micron-scale reconstruction of the pore space geometry and the related velocity field. We employ the outputs of pore-scale analysis to (i) quantify the proportion of mobile and immobile fluid regions, and (ii) assign the velocity distribution in an effective representation of the medium internal structure. Our results (1) show that this simple conceptual model reproduces the spatial profiles of solute concentration rendered by pore-scale simulation without resorting to model calibration, and (2) highlight the critical role of pore-scale velocities in the characterization of the model parameters
The sign problem and population dynamics in the full configuration interaction quantum Monte Carlo method
The recently proposed full configuration interaction quantum Monte Carlo
method allows access to essentially exact ground-state energies of systems of
interacting fermions substantially larger than previously tractable without
knowledge of the nodal structure of the ground-state wave function. We
investigate the nature of the sign problem in this method and how its severity
depends on the system studied. We explain how cancelation of the positive and
negative particles sampling the wave function ensures convergence to a
stochastic representation of the many-fermion ground state and accounts for the
characteristic population dynamics observed in simulations.Comment: 11 pages. 6 figure
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