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1.
A. M. Berezhkovskii 《Journal of statistical physics》1994,76(3-4):1089-1097
The average value and variance of the volume of the domain visited in timet byN spherical Brownian particles starting initially at the same point are presented as quadratures of the solutions of simple diffusion problems of the survival of a point Brownian particle in the presence of one and two spherical traps. As an illustration, explicit time dependences are obtained for the average volume in one and three dimensions. 相似文献
2.
The Onsager reciprocity relation and generalized efficiency of a thermal Brownian motor 总被引:1,自引:0,他引:1 下载免费PDF全文
Based on a general model of Brownian motors, the Onsager
coefficients and generalized efficiency of a thermal Brownian motor
are calculated analytically. It is found that the Onsager
reciprocity relation holds and the Onsager coefficients are not
affected by the kinetic energy change due to the particle's motion.
Only when the heat leak in the system is negligible can the
determinant of the Onsager matrix vanish. Moreover, the influence of
the main parameters characterizing the model on the generalized
efficiency of the Brownian motor is discussed in detail. The
characteristic curves of the generalized efficiency varying with
these parameters are presented, and the maximum generalized
efficiency and the corresponding optimum parameters are determined.
The results obtained here are of general significance. They are used
to analyze the performance characteristics of the Brownian motors
operating in the three interesting cases with zero heat leak,
zero average drift velocity or a linear response relation, so that
some important conclusions in current references are directly
included in some limit cases of the present paper. 相似文献
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A sphere falling in a fluid may collide with another sphere falling more slowly if, when the spheres are far apart vertically, the horizontal distance between their centers is less than or equal to a critical radius. Accurate prediction of aerosol particle coagulation requires a good understanding of this process. Previously reported optical techniques for measuring hydrodynamic effects on this phenomenon have inherent difficulties detecting grazing collisions and hence in determining the critical radius. In this work, a novel detection technique is demonstrated and it is shown that the critical radius may be determined from the sound generated by the collision of two spheres in a viscous liquid. The technique is shown to provide a more precise and decisive indication of when hard spheres collide. 相似文献
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针对气溶胶雾霾粒子在大气流中所受力的平衡体系(旋转升力平衡重力, 粒子与粒子之间依靠斥力形成稳定的网状的力平衡体系)的问题, 提出了用激光光梯度力破坏力平衡进而消除雾霾的新机理. 首先, 根据牛顿第二定理, 得到了粒子所受力的非线性方程组, 应用Runge-Kutta 法积分求解了雾霾颗粒在大气流中所受的主要力(空气曳引阻力、范德瓦耳斯斥力、旋转升力)的数值, 成功验证了西安市2013年12月17-25日、2014年2月20-26日两次雾霾检测试验结论: 在雾霾过程中, 粒径在0.5-0.835 μm径段的粒子数浓度增加最明显. 其次, 在雾霾粒子形成的均匀介质中, 计算了激光光梯度力的大小. 研究发现, 激光光梯度力的数量级恒大于雾霾颗粒所受主要力的数量级, 激光光梯度力完全可以破坏雾霾颗粒所受力的平衡体系. 因此, 用激光光梯度力消除雾霾是可行的, 这种新的解决雾霾的方法对人们的实际生活、环保及创建美丽的蓝天具有非常重要的意义. 相似文献
6.
利用T矩阵和离散坐标法研究了取向比对椭球粒子散射特性的影响, 计算了小尺度范围内椭球粒子的散射特征参量, 包括消光效率因子、不对称因子、单次散射反照率、散射相矩阵及双向反射函数(BRDF). 结果表明, 椭球粒子的散射特性与取向比密切相关, 粒子取向比会影响散射参量的振荡频率和振幅, 与球形粒子散射参量的相对差异也呈周期振荡趋势. 研究还发现, 某些特殊粒子尺寸的散射参量与粒子取向比基本无关. 在多次散射条件下, 分析不同取向比粒子群的BRDF随反射角和光学厚度的变化特性. 结果显示: 不同取向比粒子群的BRDF随反射角的变化趋势基本一致, 球形粒子群比非球形粒子群的BRDF曲线波动振幅更大; 球形-非球形粒子的BRDF相对差异随光学厚度和取向比的增大而减小, 随入射角的增大而增大. 相似文献
7.
Diffusional coagulation of superparamagnetic particles in the presence of an external magnetic field
In this paper, we investigate the diffusional coagulation of colloidal superparamagnetic (SP) latex particles that are under the influence of an external magnetic field. The cluster size distributions (CSDs) that evolve with time were determined using an optical set-up that permits the direct visualization of particle clusters. Following the dynamic scaling analysis of van Dongen and Ernst (Phys. Rev. Lett. 54 (1985) 1396), we find that the CSDs all collapse onto a master curve when properly scaled. The bell-shape of this master curve indicates that large clusters preferentially scavenge small clusters in our system. From the time evolution of the average cluster size we infer that the reactivity between large clusters diminishes with increasing cluster size. These results are consistent with a simple mathematical formulation of the coagulation rate constant, or kernel, for the Brownian coagulation of magnetic particles. Moreover, our results support a growing body of evidence that the dynamic scaling theory developed by van Dongen and Ernst is a useful framework with which to study the microscale processes governing particle coagulation. 相似文献
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This paper studies the incorporation of Casimir and van der Waals forces applied to a nanostructure with parallel configuration. The focus of this study is in a transition region in which Casimir force gradually transforms into van der Waals force. It is proposed that in the transition region, a proportion of both Casimir and van der Waals forces, as the interacting nanoscale forces, can be considered based on the separation distance between upper structure and substrate during deflection. Moreover, as the separation distance descends during deflection, the nanoscale forces could transform from Casimir to a proportion of both Casimir and van der Waals forces and so as to van der Waals. This is also extended to the entire surface of the nanostructure in such a way that any point of the structure may be subjected to Casimir, van der Waals or a proportion of both of them about its separation distance from the substrate. Therefore, a mathematical model is presented which calculate the incorporation of Casimir and van der Waals forces considering transition region and their own domination area. The mechanical behavior of a circular nano-plate has been investigated as a case study to illustrate how different approaches to nanoscale forces lead to different results. For this purpose, the pull-in phenomena and frequency response in terms of magnitude have been studied based on Eringen nonlocal elasticity theory. The results are presented using different values of the nonlocal parameter and indicated in comparison with those of the classical theory. These results also amplify the idea of studying the mechanical behavior of nanostructures using the nonlocal elasticity theory. 相似文献
10.
Optical tweezers have been successfully used in the study of colloid science. In most applications people are concerned with the behaviour of a single particle held in the optical tweezers. Recently, the ability of the optical tweezers to simultaneously hold two particles has been used to determine the stability ratio of colloidal dispersion. This new development stimulates the efforts to explore the characteristics of a two-particle system in the optical tweezers.An infinite spherical potential well has been used to estimate the collision frequency for two particles in the optical trap based on a Monte Carlo simulation. In this article, a more reasonable harmonic potential, commonly accepted for the optical tweezers, is adopted in a Monte Carlo simulation of the collision frequency. The effect of hydrodynamic interaction of particles in the trap is also considered. The simulation results based on this improved model show quantitatively that the collision frequency drops down sharply at first and then decreases slowly as the distance between the two particles increases. The simulation also shows how the collision frequency is related to the stiffness of the optical tweezers. 相似文献
11.
A. Elgarayhi 《Journal of Quantitative Spectroscopy & Radiative Transfer》2003,77(2):211-217
The Lie technique has been employed to obtain the solutions of the dynamic equation of aerosols. This has been achieved when the rate of growth due to condensation of particles, removal rates of aerosol particles and diffusion process are taken into consideration. In such a case, we have determined a family of new solutions in this paper. 相似文献
12.
The rotational Brownian motion of magnetized tri-axial ellipsoidal particles (orthotropic particles) suspended in a Newtonian fluid, in the dilute suspension limit, under applied d.c. and a.c. magnetic fields was studied using rotational Brownian dynamics simulations. The algorithm describing the change in the suspension magnetization was obtained from the stochastic angular momentum equation using the fluctuation-dissipation theorem and a quaternion formulation of orientation space. Simulation results are in agreement with the Langevin function for equilibrium magnetization and with single-exponential relaxation from equilibrium at small fields using Perrin's effective relaxation time. Dynamic susceptibilities for ellipsoidal particles of different aspect ratios were obtained from the response to oscillating magnetic fields of different frequencies and described by Debye's model for the complex susceptibility using Perrin's effective relaxation time. Simulations at high equilibrium and probe fields indicate that Perrin's effective relaxation time continues to describe relaxation from equilibrium and response to oscillating fields even beyond the small field limit. 相似文献
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聚集速率是评估胶体体系特性及稳定性的关键参数, 静态光散射和动态光散射则是测量聚集速率的两个重要方法. 然而, 用静态光散射和动态光散射测量聚集速率时, 需要知道有关单粒子和双粒子聚集体光散射特性的数据. 为此, 通常需要把动、静两种方法结合, 才能消去这个数据. 以前各种近似理论曾用来解决这个问题, 但因粒子尺寸和形状的限制, 结果并不理想. 而T矩阵方法可以不受粒子大小和形状的限制计算其光散射特性. 本工作用T矩阵方法直接计算静态光散射和动态光散射所必须的粒子散射特性, 并将该法得到的聚集速率与动静态光散射结合法得到的聚集速率进行了比较, 两者结果很接近. 本工作为简化静态光散射和动态光散射测量聚集速率, 扩展其应用范围开辟了新途径.
关键词:
T矩阵')" href="#">T矩阵
光散射法
聚集速率 相似文献
15.
利用二维粒子模拟方法研究振荡鞘层对近壁电导的影响.研究结果表明,当二次电子发射系数大于1时,鞘层处于振荡状态.在振荡鞘层状态下,电子与壁面的碰撞通量沿平行与壁面方向剧烈的周期性振荡,振荡的波长为电子静电波波长量级,电子与壁面的碰撞频率高出经典鞘层状态下电子与壁面碰撞频率1—2个数量级,此时的碰撞频率对通道中电流的贡献不可忽略.振荡鞘层相对与经典鞘层增大了电子与壁面的碰撞频率,但是振荡鞘层的存在,仍然会使一部分慢电子无法穿越鞘层的势垒而打到壁面.
关键词:
霍尔推进器
振荡鞘层
二次电子 相似文献
16.
We study the collision frequency of electron-neutral-particle in the weakly ionized plasma with the power-law velocity q-distribution and derive the formulation of the average collision frequency. We find that the average collision frequency in the q-distributed plasma also depends strongly on the q-parameter and thus is generally different from that in the Maxwell-distributed plasma, which therefore modifies the transport coefficients in the previous studies of the weakly ionized plasmas with the power-law velocity distributions. 相似文献
17.
Vahedi Tafreshi H. Piseri P. Barborini E. Benedek G. Milani P. 《Journal of nanoparticle research》2002,4(6):511-524
We describe a simulation of the nanoparticle trajectories in a pulsed cluster beam source. Clusters, formed by condensation of atomic vapor in a helium bath, and considered here as rigid spheres having a diameter of 1.5nm, were tracked during their travel inside the source cavity, an aerodynamic lens, and a cylindrical nozzle. Steady state supersonic laminar flow of helium is considered in an axi-symmetric geometry aiming to simulate, within some limitations, the conditions under which cluster formation takes place in a pulsed microplasma cluster source. In spite of the unsteady nature of the pulsed source, the time scale characterizing particle motion in the flow field is significantly smaller than the characteristic time constant for the evolution of gas pressure in the source. For this reason, a steady simulation can shed some light on the understanding of processes governing nanoparticle motion in a pulsed vaporization source. The extent to which the Brownian diffusion can affect the particle extraction from the source is investigated. Simulations have shown that the Brownian motion perturbs the clusters from the trajectories dictated by the carrier gas and increases the rate of cluster deposition on the source internal walls. However, it does not hinder the aerodynamic focalization produced by the lens even in nano-size cluster regime. This result is qualitatively confirmed by experiment. 相似文献
18.
Dynamical and structural details of traveling chemical waves in an excitable solution layer of the Belousov-Zhabotinskii reaction are sensitively detected by measurements with a two-dimensional spectrophotometer and application of perspective display techniques for data analysis. Concentration gradients of the redox couple ferroin/ferriin catalyzing the reaction are determined to a resolution of about 10m. In solutions of identical chemical composition the gradients reach peak values of 8 mM/mm in a front of a spiral and 15 mM/mm in a front of a circular wave. Variation of ferroin concentration in the center of spiral rotation is less than 6% of that outside the core region. The degree of oxidation of ferroin at which the steepest concentration gradients occur is higher inside the core than at the outward-moving fronts. The core structure and the collision area between two circular fronts are represented by three-dimensional orthographic projections. 相似文献
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