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1.
Polymeric particles with complex shapes are required for biomedical therapies, colloidal self-assembly, and micro-robotics. It has been challenging to synthesize particles beyond simple shapes (e.g., spheres, cubes) with high structural accuracy using existing methods. Here, a method for fabricating polymeric microparticles of complex 3D shapes is reported using two-photon lithography, and dispersing the particles in an aqueous solution on a glass substrate. The fabrication of polyhedrons (e.g., tetrahedron, pyramid), polypods (e.g., tetrapod, hexapod), and other shapes of 5–10 µm in size is demonstrated. Confocal microscopy is used to track the motion of the sphere, tetrahedron, tetrapod, and screw-shaped particles near the substrate, and determine their translational diffusion coefficients. HYDRO++ is used to simulate the motion of the particles far from the substrate. The influence of particle size and substrate effects on diffusion in the spherical particles is determined and finds that the non-spherical particles have increased hindrance at the substrate compared to the spherical particles.  相似文献   

2.
We examine the behaviour of the concentration profiles of particles with repulsive interactions diffusing on a host lattice. At low temperature, the diffusion process is strongly influenced by the presence of ordered domains. We use mean field equations and Monte-Carlo simulations to describe the various effects which influence the kinetic behaviour. An effective diffusion coefficient is determined analytically and is compared with the simulations. Finite gradient effects on the ordered domains and on the diffusion are discussed. The kinetics studied is relevant for superionic conductors, for intercalation and also for the diffusion of particles adsorbed on a substrate. Received: 26 June 1997 / Revised: 18 September 1997 / Accepted: 10 November 1997  相似文献   

3.
In this paper, we investigate the diffusion process of interacting Brownian particles on stepped surfaces through a Langevin dynamic simulation method. Our primary interest is the investigation of the dynamics properties by calculating the collective diffusion coefficient for non-regularly spaced stepped structures using the Frenkel–Kontorova repulsive interactions. In particular, we have studied the effects of the Ehrlich–Schwoebel barrier $E_{S}$ and the additional binding energy $E_{B}$ on the diffusion process. Overall, our simulation results show that the value of the diffusion coefficient $D$ is reduced with increasing $E_{S}$ and $E_{B}$ . This reduction is also observed when decreasing the size of terraces. This diminution is well interpreted by calculating the effective potential which includes the effect of both potentials of Frenkel–Kontorova and the substrate.  相似文献   

4.
It is shown that, when a drop of a biological fluid dries out on a substrate, diffusion processes to a great extent suppress the transfer of the salt toward the periphery of the drop by capillary flows and still have a minor effect on the motion of colloid particles.  相似文献   

5.
Recently, a new mechanism was proposed by Fisch and Rax which should lead to amplification of lower hybrid (LH) waves at the expense of alpha particles energy, improving thus the LH current drive efficiency. The present contribution investigates the possibility of achieving the conditions on which this mechanism — based on the spatial diffusion of fusion alpha particles — could become operative. The analysis performed shows that in reactor grade tokamaks, the penetration of LH waves into the plasma column is very poor due to the intense electron Landau damping and the complicated geometry of the equilibrium magnetic fields. Consequently, at acceptable LHW input power levels, the waves do not reach the regions where fusion alpha particle power could reasonably compensate the losses of LHW energy due to the damping by electrons. The model of Fisch and Rax is based on special features of the induced diffusion of alpha particles in the energy-configuration space. Using suitable Hamiltonian formalism, the LHW induced radial diffusion of alpha particles and the energy transport between LHW and alpha particles in the frame of generalized quasilinear lines of diffusion constraint is also investigated. A rather strong change ofk of LHW along the ray trace can contribute to the change of energy transport between LHW and alpha particles as well.The authors are indebted to Dr. R. Klíma for valuable discussions.  相似文献   

6.
薄膜外延生长的计算机模拟   总被引:8,自引:0,他引:8       下载免费PDF全文
以Cu膜为例,用Monte-Carlo算法模拟了薄膜生长的随机过程,并提出了更加完善的模型.在合理选择原子间相互作用计算方法的基础上,考虑了原子的吸附、在生长表面的迁移及迁移所引起的近邻原子连带效应、从生长表面的脱附等过程.模拟计算了薄膜的早期成核情况以及表面粗糙度和相对密度.结果表明,随着衬底温度的升高或入射率的降低,沉积在衬底上的原子逐步由离散型分布向聚集状态过渡形成一些岛核,并且逐步由二维岛核向三维岛核过渡.在一定的原子入射率下,存在三个优化温度,成核率最高时的最大成核温度Tn、薄膜的表面粗糙度最低 关键词: Monte-Carlo算法 计算机模拟 薄膜生长  相似文献   

7.
A model for self-propulsion of a colloidal particle--the osmotic motor--immersed in a dispersion of "bath" particles is presented. The nonequilibrium concentration of bath particles induced by a surface chemical reaction creates an osmotic pressure imbalance on the motor causing it to move. The ratio of the speed of reaction to that of diffusion governs the bath particle distribution which is employed to calculate the driving force on the motor, and from which the self-induced osmotic velocity is determined. For slow reactions, the self-propulsion is proportional to the reaction velocity. When surface reaction dominates over diffusion the osmotic velocity cannot exceed the diffusive speed of the bath particles. Implications of these features for different bath particle volume fractions and motor sizes are discussed. Theoretical predictions are compared with Brownian dynamics simulations.  相似文献   

8.
The nucleation and growth of particles on the 1–100 nm length scale is a phenomenon of increasing interest in the areas of electronics and optoelectronics. Dealing with the preparation of nanoparticle distribution on a substrate by self-organization, we wish to determine whether conditions exist where the distributions approach an array structure which can be compared to so-called quantum dot organized structures. This means that it is fundamental to obtain particles of the same shape with a low dispersion in size and an isotropic distribution on the substrate. This work is devoted to the numerical investigation of growth phenomena on a plane substrate in the case where diffusion and coalescence are mainly taken into account. Experiments on a model system, i.e. Sn/SiOx, are compared to simulations.  相似文献   

9.
Subdiffusion-limited A+A reactions   总被引:1,自引:0,他引:1  
We consider the coagulation dynamics A+A-->A and A+A <==> A and the annihilation dynamics A+A-->0 for particles moving subdiffusively in one dimension. This scenario combines the "anomalous kinetics" and "anomalous diffusion" problems, each of which leads to interesting dynamics separately and to even more interesting dynamics in combination. Our analysis is based on the fractional diffusion equation.  相似文献   

10.
邱孝明 《物理学报》1983,32(8):1027-1034
本文基于Misguich和Balescu提出的轨道的随机处理发展了漂移波湍流中“clumps”的一种理论。指出clump效应不仅存在于非磁化湍性等离子体中也存在于磁化湍性等离子体中。它的存在使得实际横越磁场的扩散比Dupree理论预言的要小得多。导出了clumps寿命的一个明晰的表达式,它正比于横越磁场扩散的特征时间并对数地依赖于粒子的初始相对位置。 关键词:  相似文献   

11.
许晓军  王凤飞 《计算物理》2009,26(5):758-762
引入基底表面束缚能、最近邻粒子间的耦合能以及应力场,对粒子扩散势垒进行修正,采用Monte Carlo方法对不同温度下薄膜生长过程进行模拟研究.结果表明,当400 K≤T ≤ 480 K,所得团簇的平均分支宽度基本保持不变,其值近似为单粒子直径.当500 K≤T ≤ 680 K,团簇分支宽度随着温度的升高而逐渐增大至约4个粒子.随着温度的继续升高,由于粒子较高的活跃性而无法凝聚形成大团簇,团簇包含粒子数的平均值小于2.还研究了不同温度下团簇在生长过程中的形貌演化过程以及团簇数变化规律.  相似文献   

12.
G. Prévot  B. Croset 《Surface science》2007,601(9):2017-2025
We have studied by Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) the growth of gold particles on the N/Cu(0 0 1) self-organized surface. This template consists of nitrogen islands separated by bare Cu lines and forming a regular 2D array of period 5 nm. When Au is evaporated onto this surface, it mainly grows at the intersections between the Cu lines. The islands organization reproduces then the substrate 2D ordering.However, if the substrate temperature is too low, islands form everywhere. On the contrary, if the substrate temperature is too high, some nucleation sites are empty. By following the intensity of the diffraction satellites during the growth, we have observed that the ordering of the Au particles is optimum when the substrate temperature is between 210 and 290 K. Using both an analytical treatment based on the rate equations and kinetic Monte-Carlo simulations, we have determined the activation energy for the diffusion process and the energy of the traps.  相似文献   

13.
Synthesis of nanophase silica (SiO2) from hexamethyldisiloxane (HMDS) oxidation in a co-flow diffusion flame reactor at atmospheric pressure is investigated focusing on high production rates of powder. A new experimental set-up is introduced, including a diffusion burner which is operated with a ring-shape double diffusion flame. Significantly high HMDS concentrations are used resulting in SiO2 production rates of up to 130g/h. Deposition of silica powder on the burner face is eliminated by the design of a special diffusion burner and higher collection rates are achieved using a baghouse filter. The specific surface area and the product powder composition are analyzed. Carbon black coated silica particles were produced at high production rates (130g/h) at low oxygen flow rates or using a mixture of nitrogen and oxygen as oxidant. The size of the product particles was controlled in the range of 15–170nm.  相似文献   

14.
F.D.A. Aar  o Reis 《Physica A》2005,350(2-4):407-417
We simulated models of diffusion and aggregation in long pores of small widths in order to represent the basic mechanisms of coke deposition in catalysts’ pores. Coke precursors are represented by particles injected at the pore entrance. Knudsen diffusion, which is usually expected inside the pores, is modeled by ballistic motion of those particles. The regime of molecular diffusion is also analyzed via models of lattice random walks biased along the pores. The aggregation at the surface or near previously aggregated particles was modeled by different probabilistic rules, accounting for the possibilities of more compact or more ramified deposits. In the model of Knudsen diffusion and in some cases of molecular diffusion, there is an initial regime of uniform deposition along the pore, after which the deposits acquire an approximately wedge shape, with the pore plugging near its entrance. After the regime of uniform deposition and before that of critical pore plugging, the average aggregation position slowly decreases with the number N of deposited particles approximately as N-0.25. The apparently universal features of deposits generated by microscopic models are compared with those currently adopted in continuum models.  相似文献   

15.
薄膜生长的随机模型   总被引:16,自引:0,他引:16       下载免费PDF全文
利用Monte Carlo模型研究了薄膜生长初始阶段岛的形貌与基底温度之间的关系,同时还研究了它们与汽相粒子入射剩余能量之间的关系.模型中考虑了三种动力学过程:粒子入射、吸附粒子扩散和粒子脱附,与以前薄膜生长模型的不同之一在于把入射过程看作独立于其他过程,而扩散和脱附过程是相互关联的.结果表明随基底温度的升高,岛的形貌经历了一个从分散生长、分形生长到凝聚生长的变化过程.低温下随汽相粒子入射和剩余能量增加,岛的形貌也经历了同样的变化过程. 关键词:  相似文献   

16.
The aim of this paper is to study the laser-induced backside wet cleaning techniques for glass substrates. Two kinds of laser cleaning techniques are proposed in this study. The first involves applying an Nd:YAG laser to the backside of the substrate which is submerged in water. A metal plate is placed below the glass substrate. Most of the laser energy will be absorbed by the metal plate. The metal then vaporizes the water and generates a turbulent bubble flow. The bubble flow removes the alumina particles from the surface of the glass substrate. The second involves using a CO2 laser to generate turbulent bubble flow to remove the particles. Both methods were successfully demonstrated for the removal of submicron particles of 0.5 μm in size. The phenomena of bubble generation and diffusion are presented in the paper. Because the laser is applied to the backside of the substrate, the damage due to the laser heat can be significantly reduced. The quality and efficient of the backside processing is better than those of the front side processing. The proposed techniques have great potential to provide an improved solution for glass cleaning.  相似文献   

17.
Long-duration experiments with clouds of microparticles are planned for the ICAPS facility on board the International Space Station ISS. The scientific objectives of such experiments are widespread and are ranging from the simulation of aerosol behaviour in Earths atmosphere to the formation of planets in the early solar system. It is, however, even under microgravity conditions, impossible to sustain a cloud of free-floating, microscopic particles for an extended period of time, due to thermal diffusion and due to unavoidable external accelerations. Therefore, a trap for dust clouds is required which prevents the diffusion of the particles, which provides a source of relative velocities between the dust grains and which can also concentrate the dust to higher number densities that are otherwise not achievable. We are planning to use the photophoretic effect for such a particle trap. First short-duration microgravity experiments on the photophoretic motion of microscopic particles show that such an optical particle-cloud trap is feasible. First tests of a two-dimensional trap were performed in the Bremen drop tower.  相似文献   

18.
The diffusion of Ge dimers on the Ge(001) surface has been studied with scanning tunneling microscopy. We have identified three different diffusion pathways for the dimers: diffusion of on-top dimers over the substrate rows, diffusion across the substrate rows, and diffusion of dimers in the trough. We report on a heretofore unknown phenomenon, namely, diffusion driven concerted motion of substrate atoms. This concerted motion is a direct consequence of the rearrangement of substrate atoms in the proximity of the trough dimer adsorption site.  相似文献   

19.
Brownian motion on a symmetric vibrated periodic substrate is shown to be extremely sensitive to the particle mass even in the regime of large damping. This phenomenon is the most apparent for high vibration frequencies, a condition of technological interest, which is investigated here both analytically and numerically. When plotted versus the damping constant, both the particle mobility and the diffusion coefficient develop sharp (correlated) peaks, thus suggesting efficient schemes for separating submicron particles according to their mass or geometry.  相似文献   

20.
Carbon nanotubes (CNTs) have been grown directly on a Si substrate without a diffusion barrier in ethanol diffusion flame using Ni as the catalyst after a photoresist-assisted catalyst annealing process. The growth mechanism of as-synthesized CNTs is confirmed by scanning electron microscopy, high resolution transmission-electron microscopy and energy-dispersive spectroscopy. The photoresist is the key for the formation of active catalyst particles during annealing process, which then result in the growth of CNTs. The catalyst annealing temperature has been found to affect the morphologies and field electron emission properties of CNTs significantly. The field emission properties of as-grown CNTs are investigated with a diode structure and the obtained CNTs exhibit enhanced characteristics. This technique will be applicable to a low-cost fabrication process of electron-emitter arrays.  相似文献   

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