|Alternative Title||Crystallization of Charged-stablized Colloids That Passes through Metastable Phase: a Molecular Dynamics Study|
|Place of Conferral||北京|
|Keyword||结晶 带电胶体 分子动力学 硬核-yukawa作用势 亚稳态bcc|
深入理解物质结晶过程及相关机制，具有重要的科学意义和应用价值。近年来，将胶体悬浮液作为模型体系来研究物质的结晶过程已经成为一个研究热点。因为胶体悬浮液和原子、分子组成的物质在物理上有许多类似的行为表现，胶体粒子可以看成在空间和时间上都高度放大了的原子或分子。比起原子或分子，胶体粒子在形状、大小以及相互作用等方面都更容易精确控制而且可以采用的测量手段也丰富得多。带电胶体，作为一种典型的胶体模型体系，其粒子的作用参数灵活可调，来对应一些别的物质或体系，使研究结果具有一定的普适性。本课题组前期的实验发现，带电胶体结晶不是由一步完成，而是按照Ostwald分步结晶规律和Alexander-McTague理论所预测的那样，先经过亚稳态BCC，然后才结晶形成最终的热力学稳定态FCC。而且，实验还发现，这个亚稳态BCC结构只有在特定体积分数条件下才能观察到。进一步结合理论模型与实验数据，推理认为，实验观察不到亚稳态并不是它不存在，当亚稳态向稳态转换率远大于由液态向亚稳态转换率时，亚稳态BCC一出现，就立即转换为稳态FCC，因而会使亚稳态BCC变得难以检测得到。所以，Ostwald分步结晶规律在带电胶体结晶过程中具有普适性。为了验证这一推理，同时更深入地了解带电胶体结晶经由亚稳态的相变机理和动力学过程，本论文采用分子动力学方法，带电胶体相互作用为硬核-Yukawa势函数，从微观角度研究了带电胶体的结晶过程。主要研究内容如下： 1. 模拟了不同状态点条件下的带电胶体结晶过程。通过键序参数方法判断前驱体结构和晶体结构，并得到两者随时间的演变过程。结果表明，不同状态点条件下所形成的前驱体总是倾向于BCC对称性。 2. 观察了晶体结构的演变过程，发现带电胶体结晶过程中晶体的主要结构与状态点有关。当状态点满足压强P（或者过饱和度）较低、Debye常数κ较小和ε较大的条件时，更容易出现大量的BCC亚稳态结构，这为实验观测亚稳态结构提供了明确的方向指引。 3. 计算了不同状态点条件下晶体结构的自由能。当系统的过饱和度较小时，结晶的自由能势垒较大，更容易观察到BCC亚稳态。当系统的作用强度较小时，BCC亚稳态自由能较大，在溶液中难以稳定，因此只有ε较大时，溶液中才能观察到亚稳态的存在。这为之前得出的宏观观察亚稳态的条件提供了热力学证据。 4. 分析了不同状态点条件下的结晶动力学过程，发现结晶的主要转变途径很好地符合Ostwald分步结晶规律的预测，为液体BCC FCC-like前驱体FCC，即液体倾向于先形成亚稳态BCC晶体，然后BCC晶体在FCC-like前驱体的帮助下转化为FCC晶体。而且，这种转变路径与系统的状态点无关，说明Ostwald分步律在带电胶体结晶过程中确实具有普适性。
It is of great scientific significance and valuable application to have a deep understanding of the crystallization process and related mechanism. In recent years, it has become popular to study the crystallization process using colloidal suspensions as a good model system. As the physical behaviors of colloidal suspensions are similar to those of systems with atoms or molecules, colloidal particles can be considered as highly enlarged atoms or molecules in both space and time scale. Compared with atoms or molecules, the shape, size and interaction of colloidal particles can be controlled precisely as well as there are much more detection methods. The charged colloids, as a typical model system, can correspond well to a lot of other systems for the colloidal particles’ interaction parameters are able to be tuned freely. Thus the research results of charged colloids are somewhat universal. In the previous experiment of our group, the crystallization of charged colloids is not one-step but in accord with the Ostwald's step rule and Alexander-McTague theory, i.e., the system first passes through a metastable BCC and then transforms into the thermodynamically stable FCC. Moreover, such a metastable BCC can only observed under some particular volume fractions. For a further inference combining theoretic model and experimental data, that the metastable BCC phase is not detected does not mean its nonexistence. When the transformation rate of metastable-stable transition is much larger than that of liquid-metastable transition, the metastable BCC immediately transforms into stable FCC as soon as it emerges, making it hardly to be detected. Therefore, it can be concluded that the Ostwald's step rule has a universality in the crystallization of charged colloids. In order to verify this inference, as well as to profoundly understand the mechanism and dynamic process of liquid-solid phase transition in charged colloids where a metastable phase is passed through, we use molecular dynamics simulation and hard-core Yukawa potential in this thesis to microscopically study the crystallization of charged colloids. The main research contents are as follows: 1. Simulations at different state points have been performed to investigate the crystallization process of charged colloids. The precursor and crystal structures are determined by the analysis of bond-orientational order parameters and their temporal evolution is obtained. The results of simulation show that the precursors always have a preference of BCC symmetry despite different state points. 2. The evolution of different crystal structures shows that the dominant crystal structures during crystallization process are dependent on the state point. When the state point meets the conditions of low enough pressure P (or supersaturation), small enough Debye constant κ and large enough interaction intensity ε, the formation of large amount of metastable BCC crystals is more likely to occur during the crystallization process, which provides an explicit guidance for the detection of metastable structures in experiments. 3. The free energies of different crystal structures at different state points have been calculated. When the supersaturation of system is small, the free-energy barrier of crystallization becomes large so that the metastable BCC is easier to be observed. When the interaction intensity ε is small, the metastable BCC has a large free energy so as to hardly maintain stability in solution. Therefore, only when ε is large, can the metastable BCC be observed, which provides a thermodynamic evidence for the previous conditions of macroscopically detecting metastable BCC. 4. The dynamic process of crystallization at different state points has been analyzed. The main pathway of crystal nucleation, in agreement with the prediction of Ostwald's step rule, is liquidBCC FCC-like precursorFCC, i.e., the liquids tend to first form the BCC crystals, then the BCC crystals transform FCC crystals with the aid of FCC-like precursors. Such a pathway is independent on the state point of system, indicating that the Ostwald's step rule is indeed universal in the crystallization of charged colloids.
|纪鑫强. 带电粒子胶体体系经由亚稳态结晶的分子动力学研究[D]. 北京. 中国科学院大学,2018.|
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