重力场和电解质浓度对胶体凝聚体分形结构的影响

Effect of Gravity and Electrolyte Concentration on the Fractal Structure of Colloidal Aggregates

运用李航等提出的新方法,克服了DLVO理论中无法理论计算不同电解质浓度下颗粒的表面电位这一困难,从而可以直接计算出不同电解质浓度下胶体颗粒间的位能.同时,还运用胶体颗粒动能的玻耳兹曼分布原理和蒙特卡罗方法来模拟胶体的运动,并采用非弹性碰撞理论解决了碰撞后凝聚的有效概率问题.在改进DDA模型的基础上,成功地建立了以往的模拟中未能建立的重力场中电解质浓度与碰撞凝聚概率间的联系,结果发现,(1)重力场作用下的凝聚体分形维数随电解质浓度变化的曲线完全不同于无重力条件下的曲线.无重力作用下,凝聚结构体分形维数随电解质浓度的变化比较缓慢,曲线呈“L”形;而重力作用下的分形维数则呈明显的“S”形曲线.(2)在重力条件下,慢凝聚包括两个区域,对电解质浓度不敏感区域和敏感区域.在敏感区域存在一个电解质浓度的拐点.(3)无重力条件下,不同大小的胶体颗粒在快凝聚时的分形维数都是在1.86±0.01.当电解质浓度降低,凝聚速率变慢,分形维数增加,最大达到2.01±0.02,但不会形成重力条件下的分形维数接近3的结构体.

The new method developed by Li et al. was used to calculate the energy between two colloidal particles, which could break through the limit that the surface potential to be taken as a constant as electrolyte concentration changes in the classic DLVO theory. Also in this research, both the Boltzmann theory of kinetic energy of colloidal particles and Monte Carlo method were used to simulate the movement of colloidal particles, and the inelastic collision theory was used to solve the problem of effective collision probability. By improving the DDA model, the relationship between the cohesion efficiency and the electrolyte concentration in gravity field was established successfully. The results showed that： （1） the curves of the fractal dimension change with electrolyte concentration as gravity field presence were quite different from that as the gravity field absence. The curves were ＂L＂-shaped as the gravity field absence; however, as the gravity field presence, the curves were ＂S＂-shaped. （2） As gravity field presence, the slow aggregating process can be divided into two sections： the sensitive and non-sensitive sections to electrolyte concentration. In the sensitive section, an inflexion of electrolyte concentration was found. （3） As gravity field absence, the fractal dimension of aggregates was 1.86±0.01 for different size of colloidal particles as the aggregating process was fast under a higher electrolyte concentration condition. Comparatively, for a slow aggregating process of low electrolyte concentration, the fractal dimension increased to 2.01±0.02. However, under the same low electrolyte concentration, the fractal dimension of aggregates approached 3 as the gravity field presence.