Xu Sheng-Hua

Coupling effect of Brownian motion and laminar shear flow on colloid coagulation：a Brownian dynamics simulation study

Simultaneous orthokinetic and perikinetic coagulations (SOPCs) are studied for small and large Peclet numbers (P-e) using Brownian dynamics simulation. The results demonstrate that the contributions of the Brownian motion and the shear flow to the overall coagulation rate are basically not additive. At the early stages of coagulation with small Peclet numbers, the ratio of overall coagulation rate to the rate of pure perikinetic coagulation is proportional to P-e(1/2), while with high Peclet numbers, the ratio of overall coagulation rate to the rate of pure orthokinetic coagulation is proportional to P-e(-1/2). Moreover, our results show that the aggregation rate generally changes with time for the SOPC, which is different from that for pure perikinetic and pure orthokinetic coagulations. By comparing the SOPC with pure perikinetic and pure orthokinetic coagulations, we show that the redistribution of particles due to Brownian motion can play a very important role in the SOPC. In addition, the effects of redistribution in the directions perpendicular and parallel to the shear flow direction are different. This perspective explains the behavior of coagulation due to the joint effects of the Brownian motion (perikinetic) and the fluid motion (orthokinetic).

Observation of Diffusion Process of a Water Droplet Immersed in Protein Solution in Microgravity

Pure liquid - liquid diffusion driven by concentration gradients is hard to study in a normal gravity environment since convection and sedimentation also contribute to the mass transfer process. We employ a Mach - Zehnder interferometer to monitor the mass transfer process of a water droplet in EAFP protein solution under microgravity condition provided by the Satellite Shi Jian No 8. A series of the evolution charts of mass distribution during the diffusion process of the liquid droplet are presented and the relevant diffusion coefficient is determined.

Towards an Understanding of the Influence of Sedimentation on Colloidal Aggregation by Peclet Number

The Peclet number is a useful index to estimate the importance of sedimentation as compared to the Brownian motion. However, how to choose the characteristic length scale for the Peclet number evaluation is rather critical because the diffusion length increases as the square root of the time whereas the drifting length is linearly related to time. Our Brownian dynamics simulation shows that the degree of sedimentation influence on the coagulation decreases when the dispersion volume fraction increases. Therefore using a fixed length, such as the diameter of particle, as the characteristic length scale for Peclet number evaluation is not a good choice when dealing with the influence of sedimentation on coagulation. The simulations demonstrated that environmental factors in the coagulation process, such as dispersion volume fraction and size distribution, should be taken into account for more reasonable evaluation of the sedimentation influence.

Brownian Dynamics Simulation of the Influence of Hydrodynamic Interaction on Particle Coagulation

采用布朗动力学模拟方法,研究了流体动力学作用对稀溶液中悬浮粒子聚集过程的影响.模拟中忽略了一个粒子同时与多个粒子碰撞聚集的可能,引入了前人有关两粒子间流体动力学作用影响的研究成果.模拟结果证实了流体动力学的作用在比较大的幅度上减缓了粒子的聚集过程,是导致粒子聚集速率的实验值低于Smoluchowski理论值的重要原因之一.另外,在分别加入和排除重力作用,以及考虑和忽略粒子间流体动力学作用在内的各种...

A Brownian Dynamics Simulation to Study the Influence of Gravity on the Process of Particle Coagulation in Suspension

The Brownian dynamics method was used to simulate the influence of gravity on the process of particle coagulation in a dilute solution. By including or excluding the gravitational influence in a computer simulation, the relative change in the number of aggregates with time for these two gravitational conditions could be compared. Time-dependent variations of the number and size of all aggregates under the gravitational conditions were obtained. We conclude that the influence of gravity is negligible during the earlier stage of the coagulation process and that the rate of aggregation is accelerated by the gravitational force in the latter stage. The effect of gravitational force on the process of particle aggregation in suspension is discussed by using a dynamic analysis method.