6,526 research outputs found
Pattern formation during diffusion limited transformations in solids
We develop a description of diffusion limited growth in solid-solid
transformations, which are strongly influenced by elastic effects. Density
differences and structural transformations provoke stresses at interfaces,
which affect the phase equilibrium conditions. We formulate equations for the
interface kinetics similar to dendritic growth and study the growth of a stable
phase from a metastable solid in both a channel geometry and in free space. We
perform sharp interface calculations based on Green's function methods and
phase field simulations, supplemented by analytical investigations. For pure
dilatational transformations we find a single growing finger with symmetry
breaking at higher driving forces, whereas for shear transformations the
emergence of twin structures can be favorable. We predict the steady state
shapes and propagation velocities, which can be higher than in conventional
dendritic growth.Comment: submitted to Philosophical Magazin
Counterion density profiles at charged flexible membranes
Counterion distributions at charged soft membranes are studied using
perturbative analytical and simulation methods in both weak coupling
(mean-field or Poisson-Boltzmann) and strong coupling limits. The softer the
membrane, the more smeared out the counterion density profile becomes and
counterions pentrate through the mean-membrane surface location, in agreement
with anomalous scattering results. Membrane-charge repulsion leads to a
short-scale roughening of the membrane.Comment: 4 pages, 4 figure
Recommended from our members
Crack kinking at the tip of a mode I crack in an orthotropic solid.
The competition between crack penetration and crack kinking is addressed for a mode I macroscopic crack in an orthotropic elastic solid. Cohesive zones of finite peak strength and finite toughness are placed directly ahead of and orthogonal to the plane of the parent crack. The cohesive zone ahead of the crack tip is tensile in nature and leads to crack penetration, whereas the inclined zones slide without opening under a combined shear and normal traction, and give crack kinking. Thereby, the competition between continued crack growth by penetration ahead of the crack tip versus kinking is determined as a function of the relative strength and relative toughness of the cohesive zones. This competition is plotted in the form of a failure mechanism map, with the role of material orthotropy emphasized. Synergistic toughening is observed, whereby the parent crack tip is shielded by the activation of both the tensile and shear (kinking) cohesive zones, and the macroscopic toughness is elevated. The study is used to assess the degree to which various classes of composite have the tendency to undergo kinking
One-dimensional metallic behavior of the stripe phase in LaSrCuO
Using an exact diagonalization method within the dynamical mean-field theory
we study stripe phases in the two-dimensional Hubbard model. We find a
crossover at doping from diagonal stripes to vertical
site-centered stripes with populated domain walls, stable in a broad range of
doping, . The calculated chemical potential shift and the doping dependence of the magnetic incommensurability are in
quantitative agreement with the experimental results for doped
LaSrCuO. The electronic structure shows one-dimensional
metallic behavior along the domain walls, and explains the suppression of
spectral weight along the Brillouin zone diagonal.Comment: 4 pages, 4 figure
Radiative transfer in very optically thick circumstellar disks
In this paper we present two efficient implementations of the diffusion
approximation to be employed in Monte Carlo computations of radiative transfer
in dusty media of massive circumstellar disks. The aim is to improve the
accuracy of the computed temperature structure and to decrease the computation
time. The accuracy, efficiency and applicability of the methods in various
corners of parameter space are investigated. The effects of using these methods
on the vertical structure of the circumstellar disk as obtained from
hydrostatic equilibrium computations are also addressed. Two methods are
presented. First, an energy diffusion approximation is used to improve the
accuracy of the temperature structure in highly obscured regions of the disk,
where photon counts are low. Second, a modified random walk approximation is
employed to decrease the computation time. This modified random walk ensures
that the photons that end up in the high-density regions can quickly escape to
the lower density regions, while the energy deposited by these photons in the
disk is still computed accurately. A new radiative transfer code, MCMax, is
presented in which both these diffusion approximations are implemented. These
can be used simultaneously to increase both computational speed and decrease
statistical noise. We conclude that the diffusion approximations allow for fast
and accurate computations of the temperature structure, vertical disk structure
and observables of very optically thick circumstellar disks.Comment: Accepted for publication in A&
Fracture of bio-cemented sands
Bio-chemical reactions enable the production of biomimetic materials such as
sandstones. In the present study, microbiologically-induced calcium carbonate
precipitation (MICP) is used to manufacture laboratory-scale specimens for
fracture toughness measurement. The mode I and mixed-mode fracture toughnesses
are measured as a function of cementation, and are correlated with strength,
permeability and porosity. A micromechanical model is developed to predict the
dependence of mode I fracture toughness upon the degree of cementation. In
addition, the role of the crack tip -stress in dictating kink angle and
toughness is determined for mixed mode loading. At a sufficiently low degree of
cementation, the zone of microcracking in the vicinity of the crack tip is
sufficiently large for a crack tip -field to cease to exist and for crack
kinking theory to not apply. The interplay between cementation and fracture
properties of sedimentary rocks is explained; this understanding underpins a
wide range of rock fracture phenomena including hydraulic fracture
Biochemical adaptations in early starvation: observations on sex difference.
Twenty-two students, eleven male and eleven female, volunteered to fast for 3 d. Twenty-four hour urine collections were obtained over the period of study, and total nitrogen, urea, and 3-hydroxybutyrate (3-OH-butyrate) were determined in each
Эволюция информационных технологий и их роль в развитии экономики и общества
In Russia there is a problem of formation of information economy is one of the most pressing problems of our time. The information economy is needed for further stable development of the country and the subsequent introduction of Russia into the global economy
- …