Department of Chemistry and Applied Biosciences, Institute for Chemical and Bioengineering, ETH Zurich, Zurich 8093, Switzerland.
Langmuir. 2011 May 17;27(10):5739-52. doi: 10.1021/la1046589. Epub 2011 Apr 20.
In this work we present experimental and simulation analysis of the breakage and restructuring of colloidal aggregates in dilute conditions under shear. In order to cover a broad range of hydrodynamic and interparticle forces, aggregates composed of primary particles with two sizes, d(p) = 90 and 810 nm, were generated. Moreover, to understand the dependence of breakage and restructuring on the cluster structure, aggregates grown under stagnant and turbulent conditions, having substantially different initial internal structures with fractal dimension d(f) equal to 1.7 and 2.7, respectively, were used. The aggregates were broken by exposing them to a well-defined elongational flow produced in a nozzle positioned between two syringes. To investigate the evolution of aggregate size and morphology, respectively, the mean radius of gyration, <R(g)>, and d(f) were monitored during the breakup process using light scattering and confocal laser scanning microscopy. It was found that the evolution of aggregates' fractal dimension during breakage is solely controlled by their initial structure and is independent of the primary particles size. Similarly, the scaling of the steady-state <R(g)> vs the applied hydrodynamic stress is independent of primary particle size, however, depends on the history of aggregate structure. To quantitatively explain these observations, the breakage process was modeled using stokesian dynamics simulations incorporating DLVO and contact interactions among particles. The required flow-field for these simulations was obtained from computational fluid dynamics. The complex flow pattern was simplified by considering a characteristic stream line passing through the zone with the highest hydrodynamic stress inside the nozzle, this being the most critical flow condition experienced by the clusters. As the flow-field along this streamline was found to be neither pure simple shear nor pure extensional flow, the real flow was approximated as an elongational flow followed by a simple shear flow, with a stepwise transition between them. Using this approach, very good agreement between the measured and simulated aggregate size values and structure evolution was obtained. The results of this study show that the process of cluster breakup is very complex and strongly depends on the initial aggregate structure and flow-field conditions.
在这项工作中，我们展示了在剪切作用下稀相条件下胶体聚集体破裂和重构的实验和模拟分析。为了涵盖广泛的流体动力和粒子间力，我们生成了由两种尺寸的初级粒子组成的聚集体，d(p)=90nm 和 810nm。此外，为了了解聚集体结构对破裂和重构的依赖性，使用了在静止和湍流条件下生长的聚集体，它们具有显著不同的初始内部结构，分形维数 d(f)分别为 1.7 和 2.7。聚集体通过暴露于位于两个注射器之间的喷嘴中产生的明确定义的拉伸流来破裂。为了分别研究聚集体尺寸和形态的演化，在破裂过程中使用光散射和共焦激光扫描显微镜监测了旋转半径的平均值<R(g)>和 d(f)。发现聚集体破裂过程中分形维数的演化仅由其初始结构控制，而与初级粒子尺寸无关。同样，稳态<R(g)>与施加的流体动力应力的标度与初级粒子尺寸无关，但取决于聚集体结构的历史。为了定量解释这些观察结果，使用包含 DLVO 和颗粒间接触相互作用的 Stokesian 动力学模拟来模拟破裂过程。这些模拟所需的流场是通过计算流体动力学获得的。通过考虑穿过喷嘴内具有最高流体动力应力的区域的特征流线，简化了复杂的流场，这是聚集体经历的最关键的流场条件。由于沿该流线的流场既不是纯简单剪切也不是纯拉伸流，因此真实流被近似为拉伸流随后是简单剪切流，它们之间有一个逐步过渡。使用这种方法，测量值和模拟值之间的聚集体尺寸值和结构演化非常吻合。这项研究的结果表明，聚集体的破裂过程非常复杂，强烈依赖于初始聚集体结构和流场条件。
