Study of Heterogeneous Aggregation, Packing, and Self-Assembly of Colloids using Discrete Elements

• Main Authors: Jenn-Feng Li, 李劍鋒
• Other Authors: 陳俊杉
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/68547932612825020941

博士 === 國立臺灣大學 === 土木工程學研究所 === 94 === A colloidal dispersion is a system in which the dispersed particles through the medium are much larger than the molecules of the medium. Ceramic manufacturing has been processed by means of the colloidal system for several millennia. The colloidal system applications in this study focus on photonic bandgap (PBG) crystal related processes where the maintenance of the stability is critical. Computer simulation offers a potential means to study colloidal stability. Several simulation methods have been presented in the literature for the colloidal system simulation. However, none of them is eminent and none of the source code is available for further modification to conduct our application cases of interests. Thus, the first objective of this study is to develop a simulation program which can properly mimic the dynamic behaviors of colloids in a finite system. In this study, an Object-Oriented DEM (discrete element method) based Brownian dynamics simulation system is developed to mimic the integrated behavior of colloidal particles in a suspension. The Langevin-type equations of motion are employed in the DEM simulation program to govern the movement of colloidal particles subjected to inter-particle interactions (mainly DLVO interaction), field forces, and the Brownian motion (in diffusive scale). Several techniques are proposed to solve the relating problem during the simulation, including the overlapping of particles, formation of agglomerations, and the collision efficiency. Simulation cases of a centrifugal casting and a rapid Brownian coagulation have been conducted to verify the feasibility of the program. The simulation results agree well with the simulation results reported in the literature and the theoretical predictions of Smoluchowski’s kinetics of coagulation theory. The second objective is to seek the implications of the recipe for synthesizing the PBG crystals by monitoring transformation during simulation. To this end, three simulation cases were conducted; these include coating PBG particles by heterogeneous aggregation, packing of PBG crystals, and template-directed crystallization of PBG crystals. For the case of heterogeneous aggregation, intensive studies have been conducted to understand the importance of Brownian motion for colloidal particles in nano and sub-micron scales. The importance of Brownian motion was illustrated when the particle size is in the order of ten nm. The recipes for synthesizing core/shell structures by mean of heterogeneous aggregation were suggested. For the case of PBG crystal packing, the processes of self-assembly of the silica spheres on the solid substrate were studied. The evolution of the positional order in the process of the assembly was demonstrated by means of snapshots visualization and RDF analysis. We found that particles inside a sediment cake continue adjusting their positions by inter-particle interactions even after the height of the sediment cake has reached a stable condition. For the case of template-directed crystallization, a simple and heuristic analysis to find the most appropriate template pattern to synthesize well-ordered PBG by colloidal epitaxy was proposed. The analysis was verified by a series of simulations based on a system of 1000 SiO2 particles. The cubic array template provided only one kind of site for the upward layers. A cubic array with lattice spacing of 2.45r was illustrated to be the best designed template for synthesizing the PBG crystals. In conclusion, we have shown that computer simulation is a proper tool to study the colloidal process, for example, to monitor the transformation from the initial state to the consolidated result. The collaboration of simulations and experiments on studying the colloidal processes have been demonstrated in this study, and we believe that further novel advancing through such venue is expected in the future.