349 research outputs found
Free-energy landscape of nucleation with an intermediate metastable phase studied using capillarity approximation
Capillarity approximation is used to study the free-energy landscape of
nucleation when an intermediate metastable phase exists. The critical nucleus
that corresponds to the saddle point of the free-energy landscape as well as
the whole free-energy landscape can be studied using this capillarity
approximation, and various scenarios of nucleation and growth can be
elucidated. In this study we consider a model in which a stable solid phase
nucleates within a metastable vapor phase when an intermediate metastable
liquid phase exists. We predict that a composite critical nucleus that consists
of a solid core and a liquid wetting layer as well as pure liquid and pure
solid critical nuclei can exist depending not only on the supersaturation of
the liquid phase relative to that of the vapor phase but also on the wetting
behavior of the liquid surrounding the solid. The existence of liquid critical
nucleus indicates that the phase transformation from metastable vapor to stable
solid occurs via the intermediate metastable liquid phase, which is quite
similar to the scenario of nucleation observed in proteins and colloidal
systems. By studying the minimum-free-energy path on the free-energy landscape,
we can study the evolution of the composition of solid and liquid within nuclei
not limited to the critical nucleus.Comment: 9 pages, 8 figures, Journal of chemical physics to be publishe
Steady-state nucleation rate and flux of composite nucleus at saddle point
The steady-state nucleation rate and flux of composite nucleus at the saddle
point is studied by extending the theory of binary nucleation. The
Fokker-Planck equation that describes the nucleation flux is derived using the
Master equation for the growth of the composite nucleus, which consists of the
core of the final stable phase surrounded by a wetting layer of the
intermediate metastable phase nucleated from a metastable parent phase recently
evaluated by the author [J. Chem. Phys. {\bf 134}, 164508 (2011)]. The
Fokker-Planck equation is similar to that used in the theory of binary
nucleation, but the non-diagonal elements exist in the reaction rate matrix.
First, the general solution for the steady-state nucleation rate and the
direction of nucleation flux is derived. Next, this information is then used to
study the nucleation of composite nucleus at the saddle point. The dependence
of steady-state nucleation rate as well as the direction of nucleation flux on
the reaction rate in addition to the free-energy surface is studied using a
model free-energy surface. The direction of nucleation current deviates from
the steepest-descent direction of the free-energy surface. The results show the
importance of two reaction rate constants: one from the metastable environment
to the intermediate metastable phase and the other from the metastable
intermediate phase to the stable new phase. On the other hand, the gradient of
the potential or the Kramers crossover function (the commitment or
splitting probability) is relatively insensitive to reaction rates or
free-energy surface.Comment: 12 pages, 6 figures, to be published in Journal of Chemical Physic
Direct numerical simulation of homogeneous nucleation and growth in a phase-field model using cell dynamics method
Homogeneous nucleation and growth in a simplest two-dimensional phase field
model is numerically studied using the cell dynamics method. Whole process from
nucleation to growth is simulated and is shown to follow closely the
Kolmogorov-Johnson-Mehl-Avrami (KJMA) scenario of phase transformation.
Specifically the time evolution of the volume fraction of new stable phase is
found to follow closely the KJMA formula. By fitting the KJMA formula directly
to the simulation data, not only the Avrami exponent but the magnitude of
nucleation rate and, in particular, of incubation time are quantitatively
studied. The modified Avrami plot is also used to verify the derived KJMA
parameters. It is found that the Avrami exponent is close to the ideal
theoretical value m=3. The temperature dependence of nucleation rate follows
the activation-type behavior expected from the classical nucleation theory. On
the other hand, the temperature dependence of incubation time does not follow
the exponential activation-type behavior. Rather the incubation time is
inversely proportional to the temperature predicted from the theory of
Shneidman and Weinberg [J. Non-Cryst. Solids {\bf 160}, 89 (1993)]. A need to
restrict thermal noise in simulation to deduce correct Avrami exponent is also
discussed.Comment: 9 pages, 8 figures, Journal of Chemical Physics to be publishe
Exergy analysis:Tthe effect of relative humidity, air temperature and effective clothing insulation on thermal comfort
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Capillary pressure of van der Waals liquid nanodrops
The dependence of the surface tension on a nanodrop radius is important for
the new-phase formation process. It is demonstrated that the famous Tolman
formula is not unique and the size-dependence of the surface tension can
distinct for different systems. The analysis is based on a relationship between
the surface tension and disjoining pressure in nanodrops. It is shown that the
van der Waals interactions do not affect the new-phase formation thermodynamics
since the effect of the disjoining pressure and size-dependent component of the
surface tension cancel each other.Comment: The paper is dedicated to the 80th anniversary of A.I. Rusano
Nucleation and Bulk Crystallization in Binary Phase Field Theory
We present a phase field theory for binary crystal nucleation. In the
one-component limit, quantitative agreement is achieved with computer
simulations (Lennard-Jones system) and experiments (ice-water system) using
model parameters evaluated from the free energy and thickness of the interface.
The critical undercoolings predicted for Cu-Ni alloys accord with the
measurements, and indicate homogeneous nucleation. The Kolmogorov exponents
deduced for dendritic solidification and for "soft-impingement" of particles
via diffusion fields are consistent with experiment.Comment: 4 pages, 4 figures, accepted to PR
The Ras Target AF-6 is a Substrate of the Fam Deubiquitinating Enzyme
The Ras target AF-6 has been shown to serve as one of the peripheral components of cellācell adhesions, and is thought to participate in cellācell adhesion regulation downstream of Ras. We here purified an AF-6-interacting protein with a molecular mass of ā¼220 kD (p220) to investigate the function of AF-6 at cellācell adhesions. The peptide sequences of p220 were identical to the amino acid sequences of mouse Fam. Fam is homologous to a deubiquitinating enzyme in Drosophila, the product of the fat facets gene. Recent genetic analyses indicate that the deubiquitinating activity of the fat facets product plays a critical role in controlling the cell fate. We found that Fam accumulated at the cellācell contact sites of MDCKII cells, but not at free ends of plasma membranes. Fam was partially colocalized with AF-6 and interacted with AF-6 in vivo and in vitro. We also showed that AF-6 was ubiquitinated in intact cells, and that Fam prevented the ubiquitination of AF-6
Use of the q-Gaussian mutation in evolutionary algorithms
Copyright @ Springer-Verlag 2010.This paper proposes the use of the q-Gaussian mutation with self-adaptation of the shape of the mutation distribution in evolutionary algorithms. The shape of the q-Gaussian mutation distribution is controlled by a real parameter q. In the proposed method, the real parameter q of the q-Gaussian mutation is encoded in the chromosome of individuals and hence is allowed to evolve during the evolutionary process. In order to test the new mutation operator, evolution strategy and evolutionary programming algorithms with self-adapted q-Gaussian mutation generated from anisotropic and isotropic distributions are presented. The theoretical analysis of the q-Gaussian mutation is also provided. In the experimental study, the q-Gaussian mutation is compared to Gaussian and Cauchy mutations in the optimization of a set of test functions. Experimental results show the efficiency of the proposed method of self-adapting the mutation distribution in evolutionary algorithms.This work was supported in part by FAPESP and CNPq in Brazil and in part by the Engineering and Physical Sciences Research Council (EPSRC) of the UK under Grant EP/E060722/1 and Grant EP/E060722/2
Surface Structure of Liquid Metals and the Effect of Capillary Waves: X-ray Studies on Liquid Indium
We report x-ray reflectivity (XR) and small angle off-specular diffuse
scattering (DS) measurements from the surface of liquid Indium close to its
melting point of C. From the XR measurements we extract the surface
structure factor convolved with fluctuations in the height of the liquid
surface. We present a model to describe DS that takes into account the surface
structure factor, thermally excited capillary waves and the experimental
resolution. The experimentally determined DS follows this model with no
adjustable parameters, allowing the surface structure factor to be deconvolved
from the thermally excited height fluctuations. The resulting local electron
density profile displays exponentially decaying surface induced layering
similar to that previously reported for Ga and Hg. We compare the details of
the local electron density profiles of liquid In, which is a nearly free
electron metal, and liquid Ga, which is considerably more covalent and shows
directional bonding in the melt. The oscillatory density profiles have
comparable amplitudes in both metals, but surface layering decays over a length
scale of \AA for In and \AA for Ga. Upon controlled
exposure to oxygen, no oxide monolayer is formed on the liquid In surface,
unlike the passivating film formed on liquid Gallium.Comment: 9 pages, 5 figures; submitted to Phys. Rev.
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