Monte Carlo simulation of the enantioseparation process

By means of Monte Carlo simulation, a study of enantioseparation by capillary electrophoresis has been carried out. A simplified system consisting of two enantiomers S (R) and a selector chiral C, which reacts with the enantiomers to form complexes RC (SC), has been considered. The dependence of Δμ$\Delta \mu$ (enantioseparation) with the concentration of chiral selector and with temperature have been analyzed by simulation. The effect of the binding constant and the charge of the complexes are also analyzed. The results are qualitatively satisfactory, despite the simplicity of the model.     

Monte Carlo simulation of Hamaker nanospheres coated with dipolar particles

Parallel tempering Monte Carlo simulation is carried out in systems of N attractive Hamaker spheres dressed with n dipolar particles, able to move on the surface of the spheres. Different cluster configurations emerge for given values of the control parameters. Energy per sphere, pair distribution functions of spheres and dipoles as function of temperature, density, external electric field, and/or the angular orientation of dipoles are used to analyse the state of aggregation of the system. As a consequence of the non-central interaction, the model predicts complex structures like self-assembly of spheres by a double crown of dipoles. This interesting result could be of help in understanding some recent experiments in colloidal science and biology.     

Aggregation modeling of calcium carbonate particles by Monte Carlo simulation

The mechanism on aggregation of spindle granular particles of calcite was investigated for the carbonation of calcium hydroxide in aqueous suspension for the purpose of controlling morphology of CaCO₃. The experimental carbonation process was carried out in a semi-batch bubble column reactor under different conditions. Although, fine rhombic nano-particles diameter ranged from 100 to 200 nm were obtained at 291 K, a higher temperature of 300 K provided spindle granular particles with a length of 1.0–1.5 μm and a width of 0.3–0.5 μm. The average crystallite size was 28 nm for the fine rhombic nano-particles and 43 nm for the spindle granules. Zeta potential measurement for the spindle granules indicated that the suspension tended to be aggregated during the carbonation process. The effect of the degree of particle aggregation on the shape of the obtained calcite particles was studied by Monte Carlo simulations. Our simulation results elucidated the dependence of aggregation on unit particles, i.e., primary particles, on the experiment carbonation condition where the spindle granules were formed out of the unit particles under the same condition as the experiments. In addition, the formation mechanism of the granules was investigated by applying classical nucleation theory to the present simulations.     

超薄膜生长的Monte Carlo模拟研究

利用Monte Carlo(MC)方法模拟研究了薄膜生长的初始阶段岛的形貌和岛的尺寸与基底温度和入射粒子剩余能量之间的关系.模型中考虑了粒子的沉积、吸附粒子的扩散和蒸发等过程.结果表明当基底温度从200K变化到260K时,岛的形貌经历了一个从分散生长逐渐过渡到分形生长的过程,并且在较低温度(200K)下,随入射粒子剩余能量的增加,岛的形貌也经历了同样的变化过程.进一步研究证明,随着基底温度的升高或入射粒子剩余能量的增加,沉积粒子的扩散能力显著增强,从而使岛的形貌发生了改变.     

Monte Carlo simulation of mictomagnets

Monte Carlo simulations of a binary alloy with impurity concentrations between 20 and 45 at.% have been carried out. The proportion of large clusters relative to that of small clusters increases with the number of MC diffusion steps as well as impurity concentration. Magnetic susceptibility peaks become more prominent and occur at higher temperatures with increasing impurity concentration. The different peaks in the susceptibility and specific heat curves seem to correspond to different sized clusters. A freezing model would explain the observed behaviour with the large clusters freezing first and the small clusters contributing to susceptibility (specific heat) peaks at lower temperatures.Contribution No. 153 from the Solid State and Structural Chemistry Unit     

A Monte Carlo model of sputtered vitreous selenium thin films

The molecular arrangements in sputtered amorphous selenium thin films have been determined by transmission electron diffraction. The radial distribution function shows a strong resemblance to that of liquid selenium at 350° C. A model of the atomic arrangement has been obtained by a Monte Carlo perturbation of an array of randomly positioned atoms within a 20 Å sphere. The final configuration may be described as a mixture of approximately 40% distorted chains and 60% distorted rings.     

Efficient kinetic Monte Carlo simulation

This paper concerns kinetic Monte Carlo (KMC) algorithms that have a single-event execution time independent of the system size. Two methods are presented—one that combines the use of inverted-list data structures with rejection Monte Carlo and a second that combines inverted lists with the Marsaglia–Norman–Cannon algorithm. The resulting algorithms apply to models with rates that are determined by the local environment but are otherwise arbitrary, time-dependent and spatially heterogeneous. While especially useful for crystal growth simulation, the algorithms are presented from the point of view that KMC is the numerical task of simulating a single realization of a Markov process, allowing application to a broad range of areas where heterogeneous random walks are the dominate simulation cost.     

Monte Carlo study of small magnetic particles

In earlier computer simulation small magnetic particles with nearest neighbor Heisenberg interactions in zero magnetic field have been studied. We now continue these investigations including next nearest neighbor exchange and non zero magnetic fieldsH. The particles treated have spherical shape with a number of spinsN in the range from 33 to 3071. It is shown that the spontaneous magnetization of the particles is rather different from the bulk magnetization. The magnetization process can be accounted for by the Néel theory, if the correct spontaneous magnetization of the particle is used. The distribution of local magnetizations (the magnetic “profile”) was also obtained in various cases. It is shown that the magnetization of very small particles is much more depressed than predicted by the mean field approximation. We introduce an “effective magnetic radius” \(\hat R\) accounting for the reduction of the local magnetization. This magnetic radius is important for the interpretation of experimental results. A distinct dependence of \(\hat R\) on the magnetic field, temperature and the fraction of next nearest neighbor exchange is found. Finally a brief comparison is made with the recent study of magnetic surface properties by Binder and Hohenberg.     

Monte Carlo simulation of nanometric cutting

Nanometric cutting of single-crystal materials at conventional cutting speeds (5?m?s^?1) is simulated for the first time using a new Monte Carlo method that is applicable to systems that are neither canonical nor microcanonical. This is accomplished by defining a local temperature in the cutting zone using the thermal analysis developed by Komanduri and Hou for conventional machining. Extension of this method to the nanometric regime permits an accurate estimate of the local temperature in cutting. This temperature is then employed in the Boltzmann probability distribution function that is used to determine the acceptance–rejection of Monte Carlo moves in the simulation. Since cutting speed is closely related to cutting temperature, the cutting speed enters the calculation via the thermal analysis equations. The method is applied to nanometric cutting of single-crystal aluminium with the crystal oriented in the (001) plane and cut in the [100] direction. Three positive rake cutting tools, namely 10°, 30° and 45°, are employed to investigate the effect of the rake angle on the forces, the specific energy and the nature of the chip formation. The method is evaluated by direct comparison with corresponding molecular dynamics simulations conducted under the same conditions.     

Monte Carlo simulation of exposure factor

In this paper, we simulate the exposure factor by a simple model of a free-air ionization chamber with the Monte Carlo programme Geant4. Special emphasis is placed on the discussion of the exposure factor related to parameters of the chamber model. The reason for the variation in exposure factor with incident ray energy is also analysed in terms of reaction cross section for different types of reactions. The obtained results indicate that our simulation is accurate in the calculation of the exposure factor and can serve as a reference in designing air ionization chambers.     

Monte Carlo simulation of liquid neopentane

Monte Carlo simulations are reported for liquid neopentane at 25°C and 0·592 g cm^-3 using three different potential models. These results are compared with X-ray diffraction data and simulations for other tetrahedral molecules.     

A Monte Carlo simulation of coagulation

A Monte Carlo simulation technique is described for the study of the coagulation of suspended particles. The method is computationally efficient since the particle trajectories are not used to determine coagulations. Instead, pairs of particles are assigned probabilities to coagulate and the evolution is computed as a stochastic Markov game. We also describe a simple analytic method to obtain the stationary distribution of sizes for the various mechanisms of relative particle motion. It is demonstrated that the simulation yields the correct stationary size distribution independent of initial condition.     

Monte Carlo simulation of aorta autofluorescence

Results of numerical simulation of autofluorescence of the aorta by the method of Monte Carlo are reported. Two states of the aorta, normal and with atherosclerotic lesions, are studied. A model of the studied tissue is developed on the basis of information about optical, morphological, and physico-chemical properties. It is shown that the data obtained by numerical Monte Carlo simulation are in good agreement with experimental results indicating adequacy of the developed model of the aorta autofluorescence.     

Multilayer adsorption by Monte Carlo simulation

Adsorption phenomena are characterized by models that include free parameters trying to reproduce experimental results. In order to understand the relationship between the model parameters and the material properties, the adsorption of small molecules on a crystalline plane surface has been simulated using the bond fluctuation model. A direct comparison between the Guggenheim–Anderson–de Boer (GAB) model for multilayer adsorption and computer simulations allowed us to establish correlations between the adsorption model parameters and the simulated interaction potentials.     

Modeling and kinetic Monte Carlo simulations of the metallographic etching process of second-phase particles

Kinetic Monte Carlo simulations of the chemical and electrolytic etching processes of nano-scale particles in two-phase materials were performed. Etching produces a surface relief, which can subsequently be studied by optical, scanning electron and atomic force microscopy to obtain quantitative information on the size, shape and spatial arrangement of the particles. The present simulations yield insight into the dependence of the etched relief on the strengths of the atomic bonds in the two phases and on the shape of the particles. Lower limits for the difference in bond energies necessary (i) to reveal the particles and (ii) to avoid over-etching are established. The results of the simulations are discussed with reference to own actual etching experiments performed for nano-scale precipitates.     

Special features of the crystallization process in Monte Carlo simulation of nanowhisker growth

The Monte Carlo method is used to simulate the kinetics of growth and the morphology of one-component nanowhiskers. It is demonstrated that for a proper choice of catalyst and substrate materials, hollow nanowhiskers can grow. The nanotube growth conditions are established. In the process of vapor-liquid-crystal crystallization, the dependence of the activation energy of chemical reactions on the neighboring species and their number in the first coordination sphere of reagent atoms is considered.     

Monte Carlo simulation of magnetic multi-core nanoparticles

In this paper, a Monte Carlo simulation is carried out to evaluate the equilibrium magnetization of magnetic multi-core nanoparticles in a liquid and subjected to a static magnetic field. The particles contain a magnetic multi-core consisting of a cluster of magnetic single-domains of magnetite. We show that the magnetization of multi-core nanoparticles cannot be fully described by a Langevin model. Inter-domain dipolar interactions and domain magnetic anisotropy contribute to decrease the magnetization of the particles, whereas the single-domain size distribution yields an increase in magnetization. Also, we show that the interactions affect the effective magnetic moment of the multi-core nanoparticles.