Fusion of the extended modified liquid drop model for nucleation and dynamical nucleation theory

We present a new phenomenological approach to nucleation, based on the combination of the "extended modified liquid drop" model and dynamical nucleation theory. The new model proposes a new cluster definition, which properly includes the effect of fluctuations, and it is consistent both thermodynamically and kinetically. The model is able to predict successfully the free energy of formation of the critical nucleus, using only macroscopic thermodynamic properties. It also accounts for the spinodal and provides excellent agreement with the result of recent simulations.     

Modelling chemical kinetics of small clusters after nanosecond laser ablation

Nanoclusters of various materials have recently been obtained by laser ablation. Strong evaporation of a condensed phase caused by laser irradiation is well known to generate an overcooled vapour. Further expansion thereof increases the oversaturation degree and facilitates homogeneous nucleation and cluster growth. To investigate homogeneous nucleation at very high expansion rates attained at nanosecond laser ablation, kinetic equations are applied describing all the possible gas-phase chemical reactions of dissociation and coalescing between small clusters. Additional cooling due to thermal emission by clusters is taken into account. Twenty smallest carbon molecules are considered. The model is applied to nanosecond laser ablation of graphite in vacuum. The resulted vapour molecular composition is characterised by dominating molecules C₃ and C₅ and an exponential drop of heavier clusters concentrations with their mass. The growth of heavier clusters is controlled by the balance between liberating the latent heat of their formation and the energy losses by expansion and thermal emission.     

Nonmonotonic behavior of the concentration in the kinetics of dissolution of fullerenes

A cluster model for the dissolution of C₆₀ fullerenes in a nonpolar solvent has been proposed. This model provides the explanation of a maximum experimentally observed in the time dependence of the solution concentration during dissolution. The model is based on the kinetic equations of nucleation theory and involves a balance between the flux of fullerene molecules from the solid phase and the sedimentation of large clusters from the solution. The formation of clusters is described using the drop model. Analysis of the numerical solutions of the equations reveals four qualitatively different dissolution regimes depending on the relation between the model parameters.     

Treatment of droplike clusters by means of the classical phase integral in nucleation theory

The translation inconsistency in the theory of nucleation is discussed in historical perspective. A theory is then developed, beginning with the classical phase integral, which not only allows all approximations to be well defined, but also leads to the most natural droplike model for the cluster. The theory makes it possible to apply, in a consistent manner, the thermodynamics of curved surfaces or, alternatively, moleculardynamic numerical computation schemes to the evaluation of the partition function of the cluster. If the cluster is treated as a macroscopic drop (having the free energy of a macroscopic drop), the result for the distribution of clusters differs in only a minor way from that prescribed by the conventional theory of nucleation. It is concluded that for liquid nuclei the conventional theory is consistent, but that a replacement factor may be necessary for solid nuclei. In general, however, the major problems confronting the theory involve the precise evaluation of the work of cluster formation.     

Kinetic pathways for the isotropic-nematic phase transition in a system of colloidal hard rods: a simulation study

We study the kinetic pathways for the isotropic-to-nematic transition in a fluid of colloidal hard rods. In order to follow the formation of the nematic phase, we develop a new cluster criterion that distinguishes nematic clusters from the isotropic phase. Applying this criterion in Monte Carlo simulations, we find spinodal decomposition as well as nucleation and growth depending on the supersaturation. We determine the height of the nucleation barrier and we study the shape and structure of the cluster. More specifically, we find ellipsoidal nematic clusters with an aspect ratio of about 1.7 and a homogeneous nematic director field. Our results are consistent with theoretical predictions on the shape and director field of nematic tactoids. Classical nucleation theory gives reasonable predictions for the height of the nucleation barrier and the critical nucleus size.     

Cooperative stochastic behavior in the onset of localized corrosion

Stochastic temporal and spatiotemporal models of metastable pitting on a metal surface are presented. A stochastic reaction-diffusion model accounts for the effects of local changes in concentration, potential drop, and oxide film damage on the nucleation of subsequent events. The cooperative interactions among events can lead to the formation of clusters of metastable pits and to an explosive growth in the total number of pits. Recent progress in the studies of such phenomena is reviewed. New results based on a mean-field analysis of the model and numerical simulations on critical nucleation effects are reported.     

Dynamic Scaling of Ramified Clusters Formed on Liquid Surfaces

A comprehensive simulation model -- deposition, diffusion, rotation, reaction and aggregation model is presented to simulate the formation processes of ramified clusters on liquid surfaces, where clusters can disuse and rotate easily. The mobility （including diffusion and rotation） of clusters is related to its mass, which is given by Dm = Dos^-γD and θm = θos^-γθ, respectively. The influence of the reaction probability on the kinetics and structure formation is included in the simulation model. We concentrate on revealing dynamic scaling during ramified cluster formation. For this purpose, the time evolution of the cluster density and the weight-average cluster size as well as the cluster-size distribution scaling function at different time are determined for various conditions. The dependence of the cluster density on the deposition flux and time-dependence of fractal dimension are also investigated. The obtained results are helpful in understanding the formation of clusters or thin film growth on liquid surfaces.     

Primary nucleation processes in binary oxide growth: The case of MgO

The smallest forms of stoichiometric and non-stoichiometric MgO clusters appearing on the MgO(0 0 1) surface during the growth under atomic and/or molecular deposition are investigated from first-principles and empiric potentials. The basic entities (MgO molecule and (MgO)₂ cluster) result from a very exoenergetic and spontaneous redox reaction that involves directly the deposited species (Mg and O atoms, O₂ molecule). The stoichiometric clusters, resulting from the agglomeration of MgO molecules, are very stable under non-polar forms. Their formation energy is modelized, down to very small sizes, within an independent defect model. We point out the specificity of such clusters in the framework of the classical nucleation theory. The high-energy polar isomers are associated to destabilizing macroscopic electric fields and dipoles. These forms may nevertheless be strongly stabilized by incorporating extra Mg adatoms that give part of their valence shell to the cluster and decrease the total dipole in this way, illustrating the delicate coupling between chemistry and electrostatics in growth processes of oxides. Based on these considerations, we propose a scenario describing MgO growth both in the step-flow and in the nucleation regime.     

Dynamic Effects in Nucleation of Receptor Clusters

Nucleation theory has been widely applied for the interpretation of critical phenomena in nonequilibrium systems. Ligand-induced receptor clustering is a critical step of cellular activation. Receptor clusters on the cell surface are treated from the nucleation theory point of view. The authors propose that the redistribution of energy over the degrees of freedom is crucial for forming each new bond in the growing cluster. The expression for a kinetic barrier for new bond formation in a cluster was obtained. The shape of critical receptor clusters seems to be very important for the clustering on the cell surface. The von Neumann entropy of the graph of bonds is used to determine the influence of the cluster shape on the kinetic barrier. Numerical studies were carried out to assess the dependence of the barrier on the size of the cluster. The asymptotic expression, reflecting the conditions necessary for the formation of receptor clusters, was obtained. Several dynamic effects were found. A slight increase of the ligand mass has been shown to significantly accelerate the nucleation of receptor clusters. The possible meaning of the obtained results for medical applications is discussed.     

Nucleation Based Explanation of Crystalloluminescence

In crystalloluminescence light is produced during crystallization. While there was work on crystalloluminescence during the 18th, 19th and very early 20th centuries, crystalloluminescence is a phenomena without scientific explanation. A new nucleation theory, based on a cluster population balance accounting for all possible cluster–cluster collisions, is presented that uses the energetics of clusters of various geometries determined from ab initio quantum mechanical calculations. This theory predicts cluster population dynamics during nucleation, as well as the spectrum of light emitted during nucleation due to the energy released by clusters activated by collision. Further experimental work is suggested to determine the nature of the light spectrum resulting from crystalloluminescence to validate this mechanism of light production.     

Monte-Carlo study of scaling exponents of rough surfaces and correlated percolation

In spallation neutron sources, liquid mercury, upon adsorbing the proton beam, is exhibited to large thermal and pressure shocks. These local changes in the state of mercury can cause the formation of unstable bubbles in the liquid, which can damage at their collapse the enclosing the liquid solid material. While there are methods to deal with the pressure shock, the local temperature shock cannot be avoided. In our paper we calculated the work of the critical cluster formation (for mercury micro-bubbles) together with the rate of their formation (nucleation rate). It is shown that the homogeneous nucleation rates are very low at the considered process conditions even after adsorbing several proton pulses, therefore, the probability of temperature induced homogeneous bubble nucleation is negligible.     

Thermodynamics of supersaturated vapor nucleation on molecular condensation nuclei

A key problem in the theory of a supersaturated vapor nucleation on molecular condensation nuclei (namely, the work of formation and the equilibrium concentrations of clusters) is considered. To calculate these quantities using the structural models of clusters, which are better suited for this purpose than the classical droplet model, we derive the equation connecting the work of transfer of a molecular condensation nucleus from the gas phase to a homogeneous cluster with a change in the number of contacts between molecules, occurring in the course of this transfer, and with the work of rupture of individual contacts.     

原子模拟钛中微孔洞的结构及其失效行为

六角金属由于其各向异性等特点,在塑性变形等过程中容易产生形状和构型都相对复杂的点缺陷团簇.这些团簇之间及其与运动位错等缺陷的相互作用直接影响材料的物理和力学性能.然而对相关问题的原子尺度、尤其是空位团簇的演化和微孔洞的形成乃至裂纹形核扩展等的理解还不全面.本文采用激发弛豫算法结合第一原理及原子间作用势,系统考察了钛中的空位团簇构型及不同构型间的相互转变,给出了不同尺寸空位团簇的稳定和亚稳构型、空位团簇合并分解和迁移的激发能垒等关键参数,发现较小的空位团簇形成稳定构型,较大的空位团簇呈现出空间对称分布趋势进而形成微孔洞;采用高通量分子动力学模拟系统研究了不同尺寸的空位团簇在拉应力作用下对变形过程的影响,发现这些空位团簇可以形成层错,并对微裂纹的形核产生影响.     

Stability of charged metal clusters in the vicinity of an ionic charge

Stability of highly charged metal clusters in the electric field of an external ion is investigated with the classical liquid drop model. We study the optimum shape of the cluster which has a local minimum of the total energy, taking account of the effects of the surface charge polarization on the Coulomb energy and the cluster deformation on the surface energy. We find that the cluster deformation greatly affects the total energy of the system and that a cluster with a fissility larger than some critical value 0.7-0.8 can become unstable against deformation. We investigate the local competition between the Coulomb force and the surface tension at the cluster surface and show that the surface charge polarization which is induced by the external electric field significantly affects the shape of the cluster and its stability. Received 5 November 2002 / Received in final form 27 January 2003 Published online 11 March 2003 RID="a" ID="a"e-mail: hamada@konan-u.ac.jp     

An extended master-equation approach applied to aggregation in freeway traffic

We restudy the master-equation approach applied to aggregation in a one-dimensional freeway, where the decay transition probabilities for the jump processes are reconstructed based on a car-following model. According to the reconstructed transition probabilities, the clustering behaviours and the stochastic properties of the master equation in a one-lane freeway traffic model are investigated in detail The numerical results show that the size of the clusters initially below the critical size of the unstable cluster and initially above that of the unstable cluster all enter the same stable state, which also accords with the nucleation theory and is known from the result in earlier work. Moreover, we have obtained more reasonable parameters of the master equation based on some results of cellular automata models.     

Formation of argon cluster with proton seeding

ABSTRACT

We employ force-field molecular dynamics simulations to investigate the kinetics of nucleation to new liquid or solid phases in a dense gas of particles, seeded with ions. We use precise atomic pair interactions, with physically correct long-range behaviour, between argon atoms and protons. Time dependence of molecular cluster formation is analysed at different proton concentration, temperature and argon gas density. The modified phase transitions with proton seeding of the argon gas are identified and analysed. The seeding of the gas enhances the formation of nano-size atomic clusters and their aggregation. The strong attraction between protons and bath gas atoms stabilises large nano-clusters and the critical temperature for evaporation. An analytical model is proposed to describe the stability of argon-proton droplets and is compared with the molecular dynamics simulations.     

Study of clusters and hypernuclei production with the NICA/MPD experiment

Heavy-ion collisions provide the unique possibility to create and investigate hot and dense matter in the laboratory. At the initial stage of the reaction a QGP is formed, while the final stage is driven by the hadronization process and the formation of clusters. The capture of the produced hyperons by clusters of nucleons leads to the hypernuclei formation which is a very rare process at strangeness threshold energies. We report on the first results on the dynamical modeling of cluster formation with the combined PHSD + SACA (Parton–Hadron–Strings dynamics + Simulated Annealing Clusterization Algorithm) model at Nuclotron and NICA energies. The clusters selection in SACA is realized by a simulated annealing procedure to obtain the most bound configuration of fragments and nucleons. Based on present predictions of the combined model we study the possibility to detect such clusters and hypernuclei in the NICA/MPD detector.     

Nucleation theorems applied to the Ising model

We use Monte Carlo simulations to study a single cluster of "up" spins in a sea of "down" spins in the three-dimensional Ising model. We evaluate the growth and decay rates for clusters of different sizes, identify the critical size for which these rates are equal, and obtain the internal energy of the critical size cluster. The results of the simulations at different temperatures and magnetic fields are used together with the first and second nucleation theorems to predict how the cluster nucleation rate changes when the external magnetic field and the temperature are changed. Our results are in agreement with literature values, but our method requires significantly less computational effort than the simulations reported earlier and avoids the difficult evaluation of free energies.     

Kinetic frustration of Ostwald ripening in Ge/Si(100) hut ensembles

We show that low area density Ge/Si(100) island ensembles comprised solely of hut and pyramid clusters do not undergo Ostwald ripening during days-long growth temperature anneals. In contrast, a very low density of large, low chemical potential Ge islands reduce the supersaturation causing the huts and pyramids to ripen. By assuming that huts lengthen by adding single {105} planes that grow from apex-to-base, we use a mean-field facet nucleation model to interpret these experimental observations. We find that each newly completed plane replenishes the nucleation site at the hut apex and depletes the Ge supersaturation by a fixed amount. This provides a feedback mechanism that reduces the island growth rate. As long as the supersaturation remains high enough to support nucleation of additional planes on the narrowest hut cluster, Ostwald ripening is suppressed on an experimental time scale.