Dukhin Stanislav, Zhu Chao, Dave Rajesh N, Yu Qun
New Jersey Center of Engineered Particulate, New Jersey Institute of Technology, Newark, NJ 07102, USA.
Adv Colloid Interface Sci. 2005 Jun 30;114-115:119-31. doi: 10.1016/j.cis.2004.07.012. Epub 2005 Feb 26.
Hydrodynamic forces on a doublet of large particles or aggregates during sedimentation cause the non-inertial fragmentation of the doublet, if the doublet Reynolds number and Stokes number are small. In lio-dispersed systems, this non-inertial fragmentation is known to promote the wet classification of large particles whereas the small particles are aggregated (in the secondary minimum) and hence cannot be separated despite the electrostatic repulsion. In aero-dispersed systems with negligible electrostatic repulsion, it is possible to have a narrower separation between two interacting particles so that the attractive surface forces such as van der Waals force could increase in orders of magnitude. As a result, the doublet fragmentation by the aerodynamic detaching force becomes very difficult or even impossible in sedimentation at a small Re. However, this study shows that, when extending above analysis for the interacting fractal aggregates of nanoparticles in a suspension, it is still possible to have aggregate (doublet of two nano-aggregates) fragmentation by the aerodynamic detaching force because the surface forces for nanoparticle contact between two aggregates may be in orders of magnitude smaller than that for micron-sized particles. Even with multiple contacts between two interacting nanoparticle aggregates, this prediction of aggregate fragmentation may still be valid because the contacts may break step by step due to the aggregate rolling along each other caused by a short-range aerodynamic interaction during their differential settling. The aerodynamics of aero-dispersed nanoparticle aggregates is analogous to the hydrodynamics of lio-dispersed solid particles. Therefore the hydrodynamic fragmentation model may be used to partially interpret the stability of nanoparticle fluidization process. Our model indicates that, without continued doublet fragmentation of nano-aggregates, the fluidized nanoparticle suspension would be de-fluidized within minutes via the cascading aggregation. However, the experimental evidence of a sustained operation of fluidization of nanoparticle aggregates over a very long time period without changing the size of nanoparticle aggregates indicates that there is a dynamic balance between the aggregation and fragmentation of nano-aggregates in nanoparticle fluidization. The prediction of critical size of nanoparticle aggregates caused by fragmentation in stabilized fluidization agrees with our in situ measurements.
如果双粒子雷诺数和斯托克斯数较小，沉降过程中作用在大颗粒或聚集体双体上的流体动力会导致双体的非惯性破碎。在液固分散体系中，已知这种非惯性破碎会促进大颗粒的湿法分级，而小颗粒会聚集（在第二极小值处），因此尽管存在静电排斥力，它们仍无法分离。在静电排斥力可忽略不计的气固分散体系中，两个相互作用颗粒之间的间距可能更窄，使得诸如范德华力等吸引性表面力可能会增大几个数量级。结果，在小雷诺数下的沉降过程中，气动分离力导致双体破碎变得非常困难甚至不可能。然而，本研究表明，当将上述分析扩展到悬浮液中纳米颗粒相互作用的分形聚集体时，气动分离力仍有可能导致聚集体（两个纳米聚集体的双体）破碎，因为两个聚集体之间纳米颗粒接触的表面力可能比微米级颗粒的表面力小几个数量级。即使两个相互作用的纳米颗粒聚集体之间存在多次接触，这种聚集体破碎的预测可能仍然有效，因为在它们的差异沉降过程中，由于短程气动相互作用导致聚集体相互滚动，接触可能会逐步破裂。气固分散纳米颗粒聚集体的空气动力学类似于液固分散固体颗粒的流体动力学。因此，流体动力破碎模型可用于部分解释纳米颗粒流化过程的稳定性。我们的模型表明，如果纳米聚集体没有持续的双体破碎，流化的纳米颗粒悬浮液将在几分钟内通过级联聚集而失流化。然而，纳米颗粒聚集体在很长一段时间内持续流化且纳米颗粒聚集体尺寸不变的实验证据表明，在纳米颗粒流化过程中，纳米聚集体的聚集和破碎之间存在动态平衡。稳定流化过程中破碎导致的纳米颗粒聚集体临界尺寸预测与我们的原位测量结果一致。
