647 research outputs found
Unified derivation of phase-field models for alloy solidification from a grand-potential functional
In the literature, two quite different phase-field formulations for the
problem of alloy solidification can be found. In the first, the material in the
diffuse interfaces is assumed to be in an intermediate state between solid and
liquid, with a unique local composition. In the second, the interface is seen
as a mixture of two phases that each retain their macroscopic properties, and a
separate concentration field for each phase is introduced. It is shown here
that both types of models can be obtained by the standard variational procedure
if a grand-potential functional is used as a starting point instead of a
free-energy functional. The dynamical variable is then the chemical potential
instead of the composition. In this framework, a complete analogy with
phase-field models for the solidification of a pure substance can be
established. This analogy is then exploited to formulate quantitative
phase-field models for alloys with arbitrary phase diagrams. The precision of
the method is illustrated by numerical simulations with varying interface
thickness.Comment: 36 pages, 1 figur
Dynamics of gas bubble growth in a supersaturated solution with Sievert's solubility law
This paper presents a theoretical description of diffusion growth of a gas
bubble after its nucleation in supersaturated liquid solution. We study systems
where gas molecules completely dissociate in the solvent into two parts, thus
making Sievert's solubility law valid. We show that the difference between
Henry's and Sievert's laws for chemical equilibrium conditions causes the
difference in bubble growth dynamics. Assuming that diffusion flux is steady we
obtain a differential equation on bubble radius. Bubble dynamics equation is
solved analytically for the case of homogeneous nucleation of a bubble, which
takes place at a significant pressure drop. We also obtain conditions of
diffusion flux steadiness. The fulfillment of these conditions is studied for
the case of nucleation of water vapor bubbles in magmatic melts.Comment: 22 pages, 3 figure
Molecular Dynamics Simulation of Solvent-Polymer Interdiffusion. I. Fickian diffusion
The interdiffusion of a solvent into a polymer melt has been studied using
large scale molecular dynamics and Monte Carlo simulation techniques. The
solvent concentration profile and weight gain by the polymer have been measured
as a function of time. The weight gain is found to scale as t^{1/2}, which is
expected for Fickian type of diffusion. The concentration profiles are fit very
well assuming Fick's second law with a constant diffusivity. The diffusivity
found from fitting Fick's second law is found to be independent of time and
equal to the self diffusion constant in the dilute solvent limit. We separately
calculated the diffusivity as a function of concentration using the Darken
equation and found that the diffusivity is essentially constant for the
concentration range relevant for interdiffusion.Comment: 17 pages and 7 figure
Direct Observation of Martensitic Phase-Transformation Dynamics in Iron by 4D Single-Pulse Electron Microscopy
The in situ martensitic phase transformation of iron, a complex solid-state transition involving collective atomic displacement and interface movement, is studied in real time by means of four-dimensional (4D) electron microscopy. The iron nanofilm specimen is heated at a maximum rate of ā¼10^(11) K/s by a single heating pulse, and the evolution of the phase transformation from body-centered cubic to face-centered cubic crystal structure is followed by means of single-pulse, selected-area diffraction and real-space imaging. Two distinct components are revealed in the evolution of the crystal structure. The first, on the nanosecond time scale, is a direct martensitic transformation, which proceeds in regions heated into the temperature range of stability of the fcc phase, 1185ā1667 K. The second, on the microsecond time scale, represents an indirect process for the hottest central zone of laser heating, where the temperature is initially above 1667 K and cooling is the rate-determining step. The mechanism of the direct transformation involves two steps, that of (barrier-crossing) nucleation on the reported nanosecond time scale, followed by a rapid grain growth typically in ā¼100 ps for 10 nm crystallites
Extended Smoothed Boundary Method for Solving Partial Differential Equations with General Boundary Conditions on Complex Boundaries
In this article, we describe an approach for solving partial differential
equations with general boundary conditions imposed on arbitrarily shaped
boundaries. A continuous function, the domain parameter, is used to modify the
original differential equations such that the equations are solved in the
region where a domain parameter takes a specified value while boundary
conditions are imposed on the region where the value of the domain parameter
varies smoothly across a short distance. The mathematical derivations are
straightforward and generically applicable to a wide variety of partial
differential equations. To demonstrate the general applicability of the
approach, we provide four examples herein: (1) the diffusion equation with both
Neumann and Dirichlet boundary conditions; (2) the diffusion equation with both
surface diffusion and reaction; (3) the mechanical equilibrium equation; and
(4) the equation for phase transformation with the presence of additional
boundaries. The solutions for several of these cases are validated against
corresponding analytical and semi-analytical solutions. The potential of the
approach is demonstrated with five applications: surface-reaction-diffusion
kinetics with a complex geometry, Kirkendall-effect-induced deformation,
thermal stress in a complex geometry, phase transformations affected by
substrate surfaces, and a self-propelled droplet.Comment: This document is the revised version of arXiv:0912.1288v
Statics and Dynamics of Colloid-Polymer Mixtures Near Their Critical Point of Phase Separation: A Computer Simulation Study of a Continuous AO Model
We propose a new coarse-grained model for the description of liquid-vapor
phase separation of colloid-polymer mixtures. The hard-sphere repulsion between
colloids and between colloids and polymers, which is used in the well-known
Asakura-Oosawa (AO) model, is replaced by Weeks-Chandler-Anderson potentials.
Similarly, a soft potential of height comparable to thermal energy is used for
the polymer-polymer interaction, rather than treating polymers as ideal gas
particles. It is shown by grand-canonical Monte Carlo simulations that this
model leads to a coexistence curve that almost coincides with that of the AO
model and the Ising critical behavior of static quantities is reproduced. Then
the main advantage of the model is exploited - its suitability for Molecular
Dynamics simulations - to study the dynamics of mean square displacements of
the particles, transport coefficients such as the self-diffusion and
interdiffusion coefficients, and dynamic structure factors. While the
self-diffusion of polymers increases slightly when the critical point is
approached, the self-diffusion of colloids decreases and at criticality the
colloid self-diffusion coefficient is about a factor of 10 smaller than that of
the polymers. Critical slowing down of interdiffusion is observed, which is
qualitatively similar to symmetric binary Lennard-Jones mixtures, for which no
dynamic asymmetry of self-diffusion coefficients occurs.Comment: 42 pages, 17 figures, submitted to J. Chem. Phy
Glycerol monolaurate prevents mucosal SIV transmission
Although there has been great progress in treating human immunodeficiency virus 1 (HIV-1) infection1, preventing transmission has thus far proven an elusive goal. Indeed, recent trials of a candidate vaccine and microbicide have been disappointing, both for want of efficacy and concerns about increased rates of transmission2ā4. Nonetheless, studies of vaginal transmission in the simian immunodeficiency virus (SIV)ārhesus macaque (Macacca mulatta) model point to opportunities at the earliest stages of infection in which a vaccine or microbicide might be protective, by limiting the expansion of infected founder populations at the portal of entry5,6. Here we show in this SIVāmacaque model, that an outside-in endocervical mucosal signalling system, involving MIP-3Ī± (also known as CCL20), plasmacytoid dendritic cells and CCR5+ cell-attracting chemokines produced by these cells, in combination with the innate immune and inflammatory responses to infection in both cervix and vagina, recruits CD4+ T cells to fuel this obligate expansion. We then show that glycerol monolaurateāa widely used antimicrobial compound7with inhibitory activity against the production of MIP-3Ī± and other proinflammatory cytokines8ācan inhibit mucosal signalling and the innate and inflammatory response to HIV-1 and SIV in vitro, and in vivo it can protect rhesus macaques from acute infection despite repeated intra-vaginal exposure to high doses of SIV. This new approach, plausibly linked to interfering with innate host responses that recruit the target cells necessary to establish systemic infection, opens a promising new avenue for the development of effective interventions to blockHIV-1 mucosal transmission
Return of the Great Spaghetti Monster : Learnings from a Twelve-Year Adventure in Web Software Development
The widespread adoption of the World Wide Web has fundamentally changed the landscape of software development. Only ten years ago, very few developers would write software for the Web, let alone consider using JavaScript or other web technologies for writing any serious software applications. In this paper, we reflect upon a twelve-year adventure in web development that began with the development of the Lively Kernel system at Sun Microsystems Labs in 2006. Back then, we also published some papers that identified important challenges in web-based software development based on established software engineering principles. We will revisit our earlier findings and compare the state of the art in web development today to our earlier learnings, followed by some reflections and suggestions for the road forward.Peer reviewe
C. elegans VANG-1 Modulates Life Span via Insulin/IGF-1-Like Signaling
The planar cell polarity (PCP) pathway is highly conserved from Drosophila to humans and a PCP-like pathway has recently been described in the nematode Caenorhabditis elegans. The developmental function of this pathway is to coordinate the orientation of cells or structures within the plane of an epithelium or to organize cell-cell intercalation required for correct morphogenesis. Here, we describe a novel role of VANG-1, the only C. elegans ortholog of the conserved PCP component Strabismus/Van Gogh. We show that two alleles of vang-1 and depletion of the protein by RNAi cause an increase of mean life span up to 40%. Consistent with the longevity phenotype vang-1 animals also show enhanced resistance to thermal- and oxidative stress and decreased lipofuscin accumulation. In addition, vang-1 mutants show defects like reduced brood size, decreased ovulation rate and prolonged reproductive span, which are also related to gerontogenes. The germline, but not the intestine or neurons, seems to be the primary site of vang-1 function. Life span extension in vang-1 mutants depends on the insulin/IGF-1-like receptor DAF-2 and DAF-16/FoxO transcription factor. RNAi against the phase II detoxification transcription factor SKN-1/Nrf2 also reduced vang-1 life span that might be explained by gradual inhibition of insulin/IGF-1-like signaling in vang-1. This is the first time that a key player of the PCP pathway is shown to be involved in the insulin/IGF-1-like signaling dependent modulation of life span in C. elegans
Glycerol monolaurate prevents mucosal SIV transmission
Although there has been great progress in treating human immunodeficiency virus 1 (HIV-1) infection1, preventing transmission has thus far proven an elusive goal. Indeed, recent trials of a candidate vaccine and microbicide have been disappointing, both for want of efficacy and concerns about increased rates of transmission2ā4. Nonetheless, studies of vaginal transmission in the simian immunodeficiency virus (SIV)ārhesus macaque (Macacca mulatta) model point to opportunities at the earliest stages of infection in which a vaccine or microbicide might be protective, by limiting the expansion of infected founder populations at the portal of entry5,6. Here we show in this SIVāmacaque model, that an outside-in endocervical mucosal signalling system, involving MIP-3Ī± (also known as CCL20), plasmacytoid dendritic cells and CCR5+ cell-attracting chemokines produced by these cells, in combination with the innate immune and inflammatory responses to infection in both cervix and vagina, recruits CD4+ T cells to fuel this obligate expansion. We then show that glycerol monolaurateāa widely used antimicrobial compound7with inhibitory activity against the production of MIP-3Ī± and other proinflammatory cytokines8ācan inhibit mucosal signalling and the innate and inflammatory response to HIV-1 and SIV in vitro, and in vivo it can protect rhesus macaques from acute infection despite repeated intra-vaginal exposure to high doses of SIV. This new approach, plausibly linked to interfering with innate host responses that recruit the target cells necessary to establish systemic infection, opens a promising new avenue for the development of effective interventions to blockHIV-1 mucosal transmission
- ā¦