7,121 research outputs found
Sharp-Interface Limit of a Fluctuating Phase-Field Model
We present a derivation of the sharp-interface limit of a generic fluctuating
phase-field model for solidification. As a main result, we obtain a
sharp-interface projection which presents noise terms in both the diffusion
equation and in the moving boundary conditions. The presented procedure does
not rely on the fluctuation-dissipation theorem, and can therefore be applied
to account for both internal and external fluctuations in either variational or
non-variational phase-field formulations. In particular, it can be used to
introduce thermodynamical fluctuations in non-variational formulations of the
phase-field model, which permit to reach better computational efficiency and
provide more flexibility for describing some features of specific physical
situations. This opens the possibility of performing quantitative phase-field
simulations in crystal growth while accounting for the proper fluctuations of
the system.Comment: 21 pages, 1 figure, submitted to Phys. Rev.
Numerical study of the shape and integral parameters of a dendrite
We present a numerical study of sidebranching of a solidifying dendrite by
means of a phase--field model. Special attention is paid to the regions far
from the tip of the dendrite, where linear theories are no longer valid. Two
regions have been distinguished outside the linear region: a first one in which
sidebranching is in a competition process and a second one further down where
branches behave as independent of each other. The shape of the dendrite and
integral parameters characterizing the whole dendrite (contour length and area
of the dendrite) have been computed and related to the characteristic tip
radius for both surface tension and kinetic dominated dendrites. Conclusions
about the different behaviors observed and comparison with available
experiments and theoretical predictions are presented.Comment: 10 pages, 7 figures, Accepted for publication in Phys. Rev.
In situ surface coverage analysis of RuO<sub>2</sub>-catalysed HCl oxidation reveals the entropic origin of compensation in heterogeneous catalysis
In heterogeneous catalysis, rates with Arrhenius-like temperature dependence are ubiquitous. Compensation phenomena, which arise from the linear correlation between the apparent activation energy and the logarithm of the apparent pre-exponential factor, are also common. Here, we study the origin of compensation and find a similar dependence on the rate-limiting surface coverage term for each Arrhenius parameter. This result is derived from an experimental determination of the surface coverage of oxygen and chlorine species using temporal analysis of products and prompt gamma activation analysis during HCl oxidation to Cl2 on a RuO2 catalyst. It is also substantiated by theory. We find that compensation phenomena appear when the effect on the apparent activation energy caused by changes in surface coverage is balanced out by the entropic configuration contributions of the surface. This result sets a new paradigm in understanding the interplay of compensation effects with the kinetics of heterogeneously catalysed processes
Strong covalent bonding between two graphene layers
We show that two graphene layers stacked directly on top of each other (AA
stacking) form strong chemical bonds when the distance between planes is 0.156
nm. Simultaneously, C-C in-plane bonds are considerably weakened from partial
double-bond (0.141 nm) to single bond (0.154 nm). This polymorphic form of
graphene bilayer is meta-stable w.r.t. the one bound by van der Waals forces at
a larger separation (0.335 nm) with an activation energy of 0.16 eV/cell.
Similarly to the structure found in hexaprismane, C forms four single bonds in
a geometry mixing 90^{0} and 120^{0} angles. Intermediate separations between
layers can be stabilized under external anisotropic stresses showing a rich
electronic structure changing from semimetal at van der Waals distance, to
metal when compressed, to wide gap semiconductor at the meta-stable minimum.Comment: tar gzip latex 4 pages 4 figure
Ethylene epoxidation in microwave heated structured reactors
In the present work we show the microwave-induced heating of monolithic reactors containing a thin-layered catalyst that exhibits a strong and selective heating susceptibility under microwave irradiation. The combination of microwave radiation and structured reactors has been successfully applied for the intensification of the selective oxidation of ethylene to ethylene oxide (epoxidation) while operating at lower power consumptions and with higher energy efficiencies than in conventional heating conditions. The microwave radiation selectively heats the catalyst and the monolith walls while maintaining a relatively colder gas stream thereby creating a gas/solid temperature gradient of up to ~70 °C at a reaction temperature of 225 °C. Moreover, the influence of different parameters such as the distribution of the catalyst onto the structured monoliths or the temperature measurement techniques employed to determine the heating profiles (Optic Fibers and/or IR thermography) have been also thoroughly evaluated to justify the obtained catalytic results
General Non-equilibrium Theory of Colloid Dynamics
A non-equilibrium extension of Onsager's canonical theory of thermal
fluctuations is employed to derive a self-consistent theory for the description
of the statistical properties of the instantaneous local concentration profile
n(r,t) of a colloidal liquid in terms of the coupled time evolution equations
of its mean value n(r,t) and of the covariance {\sigma}(r,r';t) \equiv
of its fluctuations {\delta}n(r, t) = n(r, t) -
n(r, t). These two coarse-grained equations involve a local mobility function
b(r, t) which, in its turn, is written in terms of the memory function of the
two-time correlation function C(r, r' ; t, t') \equiv <{\delta}n(r,
t){\delta}n(r',t')>. For given effective interactions between colloidal
particles and applied external fields, the resulting self-consistent theory is
aimed at describing the evolution of a strongly correlated colloidal liquid
from an initial state with arbitrary mean and covariance n^0(r) and
{\sigma}^0(r,r') towards its equilibrium state characterized by the equilibrium
local concentration profile n^(eq)(r) and equilibrium covariance
{\sigma}^(eq)(r,r').
This theory also provides a general theoretical framework to describe
irreversible processes associated with dynamic arrest transitions, such as
aging, and the effects of spatial heterogeneities
Detailed abundances for M giants in two inner bulge fields from Infrared Spectroscopy
We report abundance analysis for 30 M giant stars in two inner Galactic bulge
fields at (l,b)=(0,-1.75) deg and at (l,b)=(1,-2.65) deg, based on R=25,000
infrared spectroscopy from 1.5-1.8um using NIRSPEC at the Keck II telescope. We
find iron abundances of =-0.16 +/- 0.03 dex with a 1-sigma dispersion
of 0.12 +/- 0.02 and =-0.21 +/- 0.02 dex, with a 1-sigma dispersion of
0.09+/- 0.016 for the (l,b)=(0,-1.75) and (l,b)=(1,-2.65) deg fields,
respectively. In agreement with all prior studies, we find enhanced [alpha/Fe]
of +0.3 dex. We confirm the lack of any major vertical abundance or composition
gradient in the innermost ~600 pc between Baade's window and 150 pc from the
Galactic plane. We also confirm that the known enhancement of alpha elements
observed between 500 and 1000 pc from the nucleus is also present over the
volume of the inner bulge and may therefore be presumed to be a general
characteristic of bulge/bar stars within 1 kpc of the Galactic Center.Comment: Accepted for publication in the Astrophysical Journal. 27 pages
manuscript format, 6 figure
Cytokine expression in synovial tissue of psoriatic arthritis and its relationship with lymphoid neogenesis, disease activity and erosive disease: a longitudinal study
Modelling and Simulation of the Radiant Field in an Annular Heterogeneous Photoreactor Using a Four-Flux Model
- …