Non-uniformities in polymer/liquid crystal mixtures

We theoretically model the nucleation of nematic droplets during phase ordering in mixtures of a flexible polymer and a low-molecular-weight liquid crystal. By appealing to classical nucleation theory (CNT), we calculate the energy barrier to nucleation and the size of a critical nucleus. We study the influence of a metastable intermediate phase on the nucleation of the nematic. Below a triple point in the phase diagram, there are two distinct mechanisms for the formation of a nematic nucleus: 1) direct nucleation from the isotropic phase and 2) nucleation via a precursor metastable isotropic phase. We calculate the crossover concentration as a function of temperature, delineating the regions of the phase diagram in which each mechanism prevails. In the latter case, the presence of a hidden metastable isotropic-isotropic binodal may either promote or delay the nucleation of a nematic phase. Received 9 August 2002 RID="a" ID="a"e-mail: matuyama@chem.mie-u.ac.jp     

气泡成核过程的格子Boltzmann方法模拟

利用精确差分格子Boltzmann模型探讨水在特定温度下的亚稳态及不稳定平衡态, 获得等温相变过程中形成气泡和液滴的条件, 模型预测结果与理论解符合良好. 在该等温模型的基础上耦合能量方程, 通过调节流体-壁面相互作用力获得不同的气泡与固壁间接触角, 从而建立了一种新的描述气液相变的格子Boltzmann理论模型. 利用该新模型模拟不同流体-壁面相互作用力下凹坑气泡成核过程, 再现了气泡成核过程中的三阶段特性; 探讨了接触角、曲率半径及气泡体积随气泡成核过程的变化关系, 获得了与文献结果定性符合的曲率-气泡体积关系曲线. 关键词： 格子Boltzmann方法 气泡成核过程 气液相变 接触角     

Nonequilibrium collective phenomena in the onset of pitting corrosion

Nonequilibrium collective phenomena and effects of nonlinear pattern formation play the crucial role in the onset of pitting corrosion on stainless steels. Such materials are naturally protected by the oxide layer covering their surface. The onset of pitting corrosion involves the development of microscopic metastable pits, each persisting for about a second. As proposed in our publications and confirmed in subsequent experiments, sudden transition to active corrosion results from chain reproduction of metastable pits on the stainless steel surface leading to an autocatalytic explosion.     

Auto-oscillation instabilities of systems with internal flux: Why the simmering kettle sings

A study is performed of a thermodynamical-nonequilibrium stationary system with flux in the space of internal degrees of freedom. The flux characteristics in certain cases are argued to play the role of new hydrodynamical modes. The retardation effects in the flux-mode-collective-modes interaction are shown to be able to lead to instabilities in the system. A particular model example is the sound wave in a metastable medium with homogeneous nucleation.     

Evidence for spinodal singularities in high-dimensional nearest-neighbor ising models

Conventional theories of nucleation predict that the metastable state has an average lifetime which monotonically decreases as the system is quenched further from the condensation point. However, theories based on the coarsegrained Ginzburg-Landau free energy functional seem to indicate that for systems above six dimensions there is a sharp spinodal dividing the metastable and unstable regimes where the lifetime of the metastable state diverges. Monte Carlo simulations are used to investigate this discrepency. Both nucleation rates and bulk susceptibility measurements seem to support the prediction of the Ginzburg-Landau theories.     

Investigation of etch pits on KDP crystals with porous sol–gel coatings

We have investigated etch-pit formation on potassium dihydrogen phosphate (KDP) crystals with porous anti-reflective coatings. Etch pits develop beneath the sol–gel coatings after exposure to ambient humidity. The etch pits are homogeneously distributed with a density and an average size governed by the relative humidity and the coating thickness. Furthermore, the etch pits are self-similar in shape and possess facets corresponding to low-energy planes of KDP. Results from optical microscopy, light scattering, and atomic force microscopy support the following model for etch-pit formation in this system. Water adsorbed from the environment into the porous sol–gel coating contacts the crystal surface, causing etch-pit nucleation at high undersaturation. The plume of KDP rising from an etch pit slowly diffuses laterally, reducing the undersaturation and shutting off nucleation in the surrounding area. Because surface kinetics are rapid compared to mass transport through the sol–gel, etch pits continually undergo equilibration to maintain a low-energy geometry and generate an average spacing. Growth continues until the reservoir of water in the sol–gel is saturated with KDP. Coarsening is only observed in high relative humidity environments. Received: 13 Juni 2001 / Accepted: 30 July 2001 / Published online: 30 October 2001     

金属熔体中气泡形核的理论分析

引入界面接触角，考虑表面张力对气泡形貌的影响，以熔体中均质形核、夹杂物的平表面上异质形核和圆锥形凹坑内异质形核三种典型模型对气泡形核机理进行理论研究.研究发现，三种形核模型下具有相等的微米量级的气泡临界形核半径，并随气压的增大而减小.结果表明，用以制备藕状规则多孔金属的Gasar工艺中能够形成的最小气孔的直径为微米量级(0.1—1.0 MPa气压).在圆锥形凹坑内异质形核时存在最佳圆锥顶角(对应最小气泡体积)，其值与气压无关，只随接触角的增大而增大.在接触角处于90°—180°范围内，最佳圆锥顶角下圆锥形 关键词： 气泡 形核 多孔金属 Gasar     

Nucleation, Growth, and Scaling in Slow Combustion

We study the nucleation and growth of flame fronts in slow combustion. This is modeled by a set of reaction-diffusion equations for the temperature field, coupled to a background of reactants and augmented by a term describing random temperature fluctuations for ignition. We establish connections between this model and the classical theories of nucleation and growth of droplets from a metastable phase. Our results are in good agreement with theoretical predictions.     

Stochastic theory of metastable states and nucleation

We study on a model the role of fluctuations size in the nucleation of a first order phase transition. A bifurcation point exists between metastable and stable equilibrium solutions for a critical value of fluctuations size.     

Phase transitions in condensed media at a finite rate of formation of a metastable state

The influence of the finite rate of formation of a metastable state on the kinetics of the first-order phase transition is analyzed. The conditions determined by the thermodynamic parameters and the cooling rate of the system under consideration are derived. Under these conditions, the formation of the metastable state can be treated either as an instantaneous process, when nucleation occurs at the end of the cooling stage, or as a slow process, when intensive nucleation of a new phase proceeds within the cooling stage. An equation describing the time and temperature that correspond to intensive nucleation of new-phase particles is obtained. The nucleation stage of the new phase takes place in the immediate vicinity of the temperature determined from this equation. All the other parameters, which determine the kinetics of the initial and transient stages of the phase transition, are calculated with respect to this temperature. As an example, all the relationships for a weak solid solution are presented.     

Design principles for broad-spectrum protein-crystal nucleants with nanoscale pits

Growing high-quality crystals is a bottleneck in the determination of protein structures by x-ray diffraction. Experiments find that materials with a disordered pitted surface seed the growth of protein crystals. Here we report computer simulations of rapid crystal nucleation in nanoscale pits. Nucleation is rapid, as the crystal forms in pits that have filled with liquid via capillary condensation. Surprisingly, we find that pits whose surfaces are rough are better than pits with crystalline surfaces; the roughness prevents the growing crystal from trying to conform to the pit surface and becoming strained.     

Localised surface segregation and martensite nucleation in noble metal based ternary alloys

A recent model connecting martensite nucleation with localised surface segregation phenomena is extended to dilute ternary alloys based on -CuZn and -AuCd. Experimental data about composition induced variations of the critical temperature for the onset of martensitic transformation are organised in a coherent fashion by the atomistic Valence Electron Localisation Degree variation model. An analysis of the surface segregation properties of the original and effective alloys involved in micro nucleation events of martensite is performed by a model relying upon charge transfer effects. It is concluded that martensite nucleation may be interpreted in terms of preferential, localised surface segregation events.Preliminary results of the research were presented at the Discussion Meeting on CuZnAl Martensite Shape Memory Alloys, 19–21 June 1984, Leuven, Belgium     

Simulations of spinodal nucleation in systems with elastic interactions

Systems with long-range interactions quenched into a metastable state near the pseudospinodal exhibit nucleation that is qualitatively different from classical nucleation near the coexistence curve. We observe nucleation droplets in Langevin simulations of a two-dimensional model of martensitic transformations and determine that the structure of the nucleating droplet differs from the stable martensite structure. Our results, together with experimental measurements of the phonon dispersion curve, allow us to predict the nature of the droplet. The results have implications for nucleation in many solid-solid transitions and the structure of the final state.     

Deep metastable eutectic nanometer-scale particles in the MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> system

Laboratory vapor phase condensation experiments systematically yield amorphous, homogeneous, nanoparticles with unique deep metastable eutectic compositions. They formed during the nucleation stage in rapidly cooling vapor systems. These nanoparticles evidence the complexity of the nucleation stage. Similar complex behavior may occur during the nucleation stage in quenched-melt laboratory experiments. Because of the bulk size of the quenched system many of such deep metastable eutectic nanodomains will anneal and adjust to local equilibrium but some will persist metastably depending on the time–temperature regime and melt/glass transformation.     

Properties of metastable ising models evolving under the Swendsen-Wang dynamics

The behavior of the metastable nearest neighbor Ising model governed by Swendsen-Wang dynamics (SW) is investigated ind=2. The results are compared to those obtained in standard Metropolis dynamics. Both the SW and Metropolis systems are observed to decay from the metastable state via the formation of nucleating droplets. Nucleation rates are measured and found to agree with those predicted by classical nucleation theory. The growth rates of the droplets are observed to differ between the two dynamics. In addition, the dynamic critical exponentz is measured in a mean-field (Curie-Weiss) metastable Ising model at the spinodal. It is found that for SW dynamics,z=2. Since this is the same value as that obtained in the Metropolis case, this result shows that SW does not change the dynamical universality class at the spinodal.     

The Gibbs-Thomson effect in the evolution of particulate assemblages in a metastable liquid

The influence of the phase transition temperature shift on surfaces of spherical particles (the Gibbs-Thomson effect) evolving in a metastable liquid on the evolution of particulate assemblages is studied. The integro-differential model of kinetic and balance equations describing the phase transition process at the intermediate stage is analytically solved. It is shown that the temperature shift substantially influences the particle-size distribution function and the desupercooling/desupersaturation dynamics in a metastable liquid. Namely, the metastability degree reduces slowly and the distribution function is higher and shifted to smaller particle radii with allowance for the Gibbs-Thomson effect. The main conclusion is that this effect must be taken into account when considering the nucleation and growth of small particles at the intermediate stage of phase transition processes in a metastable liquid.     

Fluctuation-dissipation theorem for metastable systems

We show that an appropriately defined fluctuation-dissipation theorem, connecting generalized susceptibilities and time correlation functions, is valid for times shorter than the nucleation time of the metastable state of Markovian systems satisfying detailed balance. This is done by assuming that such systems can be described by a superposition of the ground and first excited states of the master equation. We corroborate our results numerically for the metastable states of a two-dimensional Ising model.     

Specifics of nucleation in superheated water and supersaturated vapor

This work is dedicated to the experimental studying of the nucleation kinetics in superheated water and supersaturated water vapor. A percolation model for the liquid water structure that explains a number of anomalous thermophysical properties of water and water vapor in the metastable region is proposed.     

Effect of an electric field on the limits of liquid stability

When electric fields are applied to metastable liquids, the energy barrier for nucleation of the vapor phase will increase. Reported observations of field-induced nucleation should not be used to infer stability limits of bulk liquids.