共查询到20条相似文献,搜索用时 15 毫秒
1.
H. Brune H. Röder C. Romainczyk C. Boragno K. Kern 《Applied Physics A: Materials Science & Processing》1995,60(2):167-171
We have used variable-temperature scanning tunneling microscopy to study the aggregation of two-dimensional Ag clusters on Pt(111). A transition from randomly ramified to dendritic fractal growth is observed in the diffusion-limited regime. Atomic-scale observations have identified the anisotropy of edge diffusion as microscopic origin of this crossover. Dependent on the deposition flux, this anisotropy is either amplified to the macroscopic-cluster shape and trigonal dendrites result, or it is degenerated and randomly ramified fractals occur. Our study elucidates the close relation between fractal and dendritic pattern formation in diffusion-limited aggregation on a two-dimensional lattice. 相似文献
2.
Colloidal aggregation with sedimentation: concentration effects 总被引:1,自引:0,他引:1
The results of computer models for colloidal aggregation, that consider both Brownian motion and gravitational drift experienced by the colloidal particles and clusters, are extended to include concentrations spanning three orders of magnitude. In previous publications and for a high colloidal concentration, it was obtained that the aggregation crosses over from diffusion-limited colloidal aggregation (DLCA) to another regime with a higher cluster fractal dimension and a speeding up followed by a slowing down of the aggregation rate. In the present work we show, as the concentration is decreased, that we can still cross over to a similar regime during the course of the aggregation, as long as the height of the sample is increased accordingly. Among the differences between the mentioned new regimes for a high and a low colloidal concentration, the cluster fractal dimension is higher for the high concentration case and lowers its value as the concentration is decreased, presumably reaching for low enough concentrations a fixed value above the DLCA value. It is also obtained the fractal dimension of the sediments, arising from the settling clusters that reach the bottom and continue a 2D-like diffusive motion and aggregation, on the floor of the container. For these clusters we now see two and sometimes three regimes, depending on concentration and sedimentation strength, with their corresponding fractal dimensions. The first two coming from the crossover already mentioned, that took place in the bulk of the sample before the cluster deposition, while the third arises from the two-dimensional aggregation on the floor of the container. For these bottom clusters we also obtain their dynamical behavior and aggregation rate.Received: 7 January 2004, Published online: 25 March 2004PACS:
61.43.Hv Fractals; macroscopic aggregates (including diffusion-limited aggregates) - 82.70.Dd Colloids - 05.10.Ln Monte Carlo methods 相似文献
3.
González AE 《Physical review letters》2001,86(7):1243-1246
A computer model for colloidal aggregation is presented that considers both the Brownian motion and the gravitational drift experienced by the colloidal particles and clusters. It is shown that the aggregation crosses over from diffusion-limited aggregation to another type with a higher cluster fractal dimension, a speeding up followed by a slowing down of the aggregation rate, an algebraically decaying cluster size distribution, and a higher concentration required for gelation. Although these findings are in accordance with the experimental results, some interpretations are different. 相似文献
4.
5.
We report a perplexing behavior of fractal shape transition that results from a change in the annealing temperature and time or the film thickness ratio. We find that a compact-to-open fractal shape transition can be induced by increasing the annealing temperature and time or decreasing the thickness ratio of the Au and Ge films. This behavior is not completely consistent with what is predicted by theories based on diffusion-limited aggregation and previous experimental observations. In this new system, we find that fractal shape transitions are truly dominated by the random-successive nucleation and growth mechanism. PACS 61.43.Hv; 68.55.-a; 81.05.Gc 相似文献
6.
Fractal Aggregation Under Rotation 总被引:1,自引:0,他引:1
WUFeng-Min WULi-Li LUHang-Jun LIQiao-Wen YEGao-Xiang 《理论物理通讯》2004,42(6):909-912
By means of the Monte Carlo simulation, a fractal growth model is introduced to describe diffusion-limited aggregation (DLA) under rotation. Patterns which are different from the classical DLA model are observed and the fractal dimension of such clusters is calculated. It is found that the pattern of the clusters and their fractal dimension depend strongly on the rotation velocity of the diffusing particle. Our results indicate the transition from fractal to non-fractal behavior of growing cluster with increasing rotation velocity, i.e. for small enough angular velocity ω; thefractal dimension decreases with increasing ω;, but then, with increasing rotation velocity, the fractal dimension increases and the cluster becomes compact and tends to non-fractal. 相似文献
7.
Chessboard-like substrates are introduced in this paper, in order to study the diffusion-limited aggregation (DLA)and the motion of poly-atoms on heterogeneous surfaces. The effect of morphology of such substrates upon the cluster aggregation is investigated using the Monte Carlo simulation. It is found that the growth process and the cluster morphology are governed by the energetic topography of the substrates. Our simulation also indicate that the island density and the fractal dimension of the clusters depend strongly on the substrate topography and the activation energy. 相似文献
8.
Macroscopic fractal aggregates of cobalt are obtained by thermal evaporation of cobalt metal in an argon atmosphere and subsequent
deposition on a silicon substrate heated to 1000 K. It is established that the fractal structure is formed by diffusion-limited
aggregation of cobalt particles. The macroscopic fractal cobalt aggregates are ferromagnetic.
Pis’ma Zh. éksp. Teor. Fiz. 66, No. 8, 556–558 (25 October 1997) 相似文献
9.
AlSunaidi A Lach-Hab M Gonzalez AE Blaisten-Barojas E 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》2000,61(1):550-556
The structure and aggregation kinetics of three-dimensional clusters composed of two different monomeric species at three concentrations are thoroughly investigated by means of extensive, large-scale computer simulations. The aggregating monomers have all the same size and occupy the cells of a cubic lattice. Two bonding schemes are considered: (a) the binary diffusion-limited cluster-cluster aggregation (BDLCA) in which only the monomers of different species stick together, and (b) the invading binary diffusion-limited cluster-cluster aggregation (IBDLCA) in which additionally monomers of one of the two species are allowed to bond. In the two schemes, the mixed aggregates display self-similarity with a fractal dimension d(f) that depends on the relative molar fraction of the two species and on concentration. At a given concentration, when this molar fraction is small, d(f) approaches a value close to the reaction-limited cluster-cluster aggregation of one-component systems, and when the molar fraction is 0.5, d(f) becomes close to the value of the diffusion-limited cluster-cluster aggregation model. The crossover between these two regimes is due to a time-decreasing reaction probability between colliding particles, particularly at small molar fractions. Several dynamical quantities are studied as a function of time. The number of clusters and the weight-average cluster size display a power-law behavior only at small concentrations. The dynamical exponents are obtained for molar fractions above 0.3 but not at or below 0.2, indicating the presence of a critical transition between a gelling to a nongelling system. The cluster-size distribution function presents scaling for molar fractions larger than 0.2. 相似文献
10.
I. E. Grachova A. I. Maksimov V. A. Moshnikov 《Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques》2009,3(5):761-768
Tin dioxide-based gas-sensitive layers with a fractal structure were prepared by the sol-gel method. The properties of nanocomposite
layers were studied by atomic-force microscopy and x-ray diffraction. The main stages of the fractal system’s evolution, i.e.,
diffusion-limited aggregation, cluster-cluster aggregation, percolation transition, and network nanostructure formation were
studied. 相似文献
11.
We propose a model for growing fractal networks based on the mechanisms learned from the diffusion-limited aggregation (DLA) model in fractal geometries in the viewpoint of network. By studying the DLA network, our model introduces multiplicative growth, aging and geographical preferential attachment mechanisms, whereby featuring topological self-similar property and hierarchical modularity. According to the results of theoretical analysis and simulation, the degree distribution of the proposed model shows a mixed degree distribution (i.e., exponential and algebraic degree distribution) and the fractal dimension and clustering coefficient can be tuned by changing the values of parameters. 相似文献
12.
Diffusion in disordered systems does not follow the classical laws which describe transport in ordered crystalline media, and this leads to many anomalous physical properties. Since the application of percolation theory, the main advances in the understanding of these processes have come from fractal theory. Scaling theories and numerical simulations are important tools to describe diffusion processes (random walks: the 'ant in the labyrinth') on percolation systems and fractals. Different types of disordered systems exhibiting anomalous diffusion are presented (the incipient infinite percolation cluster, diffusion-limited aggregation clusters, lattice animals, and random combs), and scaling theories as well as numerical simulations of greater sophistication are described. Also, diffusion in the presence of singular distributions of transition rates is discussed and related to anomalous diffusion on disordered structures. 相似文献
13.
Diffusion in disordered systems does not follow the classical laws which describe transport in ordered crystalline media, and this leads to many anomalous physical properties. Since the application of percolation theory, the main advances in the understanding of these processes have come from fractal theory. Scaling theories and numerical simulations are important tools to describe diffusion processes (random walks: the ‘ant in the labyrinth’) on percolation systems and fractals. Different types of disordered systems exhibiting anomalous diffusion are presented (the incipient infinite percolation cluster, diffusion-limited aggregation clusters, lattice animals, and random combs), and scaling theories as well as numerical simulations of greater sophistication are described. Also, diffusion in the presence of singular distributions of transition rates is discussed and related to anomalous diffusion on disordered structures. 相似文献
14.
A. B. Ryabov E. B. Postnikov A. Yu. Loskutov 《Journal of Experimental and Theoretical Physics》2005,101(2):253-258
Mean field theory is used as a basis for a new approach to analyzing fractal pattern formation by diffusion-limited aggregation. A coarse time scale is introduced to take into account the discrete nature of DLA clusters. A system of equations is derived and solved numerically to determine the fractal dimension and density of a cluster as a function of distance from its center. The results obtained are in good agreement with direct numerical simulations. 相似文献
15.
Based on diffusion-limited aggregation process, a convenient nanotechnique is demonstrated to obtain large silver fractal networks for a surface-enhanced Raman scattering (SERS)-active substrate. The silver fractal networks are of high SERS enhancement factor, large dynamical range. The observed SERS efficiency can be explained in terms of strongly localized plasmon modes relative to the single particle plasmon resonance. 相似文献
16.
在扩散限制凝聚模型的基础上引入粒子的自旋自由度(包括自旋向上和向下),并假设粒子间存在幂次Ising磁相互作用,采用Monte Carlo方法研究了在不同相互作用力程情况下磁性粒子的分形生长规律.模拟结果表明,当粒子间以反铁磁方式耦合时,凝聚体中的粒子自旋交替凝聚.当粒子间以铁磁方式耦合时,凝聚体中粒子的自旋分布与相互作用力程有关:对于短程作用系统,凝聚体中存在大小不同的自旋畴块,即为铁磁生长;而对于长程相互作用系统,凝聚体中的自旋出现反常分布,即中心区域是近似反铁磁生长的结构,其外围后续生长的粒子却保持
关键词:
幂次相互作用
扩散限制凝聚模型
自旋 相似文献
17.
The fractal structure of clusters formed by diffusion-limited aggregation of rodlike particles is characterized over three decades of the scattering vector q, and displays an unexpected dependence on the aspect ratio of the constituent monomers. Monte Carlo simulations of aggregating Brownian rods corroborate the experimental finding that the measured fractal dimension is an increasing function of the monomer aspect ratio. Moreover, increasing the rod aspect ratio eliminates the structural distinction between diffusion- and reaction-limited cluster aggregation that is observed for spheres. 相似文献
18.
《中国物理 B》2015,(5)
We present a directional region control(DRC) model of thermal diffusion fractal growth with active heat diffusion in three-dimensional space. This model can be applied to predict the space body heat fractal growth and study its directional region control. When the nonlinear interference term and the inner heat source term are generalized functions, the relationship between the particle aggregation probability and the interference terms can be obtained using the norm theory. We can then predict the aggregation form of particles in different regions. When the nonlinear interference terms in the model are expressed as a trigonometric function and its composite function, our simulations show that the DRC method of thermal fractal diffusion is effective and has reference value for the directional control of actual fractal growth systems. 相似文献
19.
Finite size effect of harmonic measure estimation in a DLA model: Variable size of probe particles 总被引:1,自引:0,他引:1
A finite size effect in the probing of the harmonic measure in simulation of diffusion-limited aggregation (DLA) growth is investigated. We introduce a variable size of probe particles, to estimate harmonic measure and extract the fractal dimension of DLA clusters taking two limits, of vanishingly small probe particle size and of infinitely large size of a DLA cluster. We generate 1000 DLA clusters consisting of 50 million particles each, using an off-lattice killing-free algorithm developed in the early work. The introduced method leads to unprecedented accuracy in the estimation of the fractal dimension. We discuss the variation of the probability distribution function with the size of probing particles. 相似文献
20.
Summary Diffusion of a fractal cluster of dimensiond
f
in a three-dimensional space is investigated. The diffusion process is assumed to be modelled by a standard parabolic diffusion
equation, although a more general case represented by the Fokker-Planck-Kolmogoroff equation is also introduced. The mean-square
displacement of the cluster mass centre is analysed and its anomalous behaviour is presented and critically discussed. The
results obtained can be applied to describe some effects which may occur during the diffusion-limited cluster-cluster aggregation
process, especially when the viscosity of the solvent is changed in time and/or a directed transport of clusters is present
in the system.
Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994. 相似文献