Mori Masahiro, Tai Tsubasa, Nishimura Kosuke, Harada Shusaku, Yamamoto Yasufumi
Division of Sustainable Resources Engineering, Faculty of Engineering, Hokkaido University, N13-W8, Sapporo, Hokkaido 060-8628, Japan.
Department of Mechanical Engineering, Kansai University, 3-35, Yamate-cho 3-chome, Suita, Osaka 564-8680, Japan.
J Colloid Interface Sci. 2020 Jul 1;571:13-20. doi: 10.1016/j.jcis.2020.03.019. Epub 2020 Mar 7.
Relative motion of micro-sized particles suspended in liquid is governed by hydrodynamic effect, in contrast to nano-sized particle suspension in which thermal effect is significant. As a result, the mixing behavior of stratified suspensions with micro-sized particles is totally different from those obeying Fick's diffusion law for nano-sized particles. Such a "non-Fickian" mixing of micro-sized particles is determined not only by the concentration difference but also the physical properties of suspensions.
We conducted an experimental study of gravitational settling of stratified suspensions of micro-sized particles with concentration gradients opposed to gravity. We also performed point-force-type numerical simulations under the same conditions as those in the experiment. Particularly, we focused on the relative motion of particles near the concentration interface, which is an apparent interface between the upper and the lower suspensions having different concentrations.
The experimental and numerical results indicate that, if the number density of particles in suspension is sufficient, the concentration interface seemingly behaves immiscibly and the interface prevents particle mixing. However, a small number of particles cannot maintain the seal of the concentration interface then demonstrates miscible behavior. The mixing mechanism of the suspended particles at the concentration interface is strongly related to the miscible and immiscible characteristics of the interface.
与热效应显著的纳米级颗粒悬浮液不同，悬浮在液体中的微米级颗粒的相对运动受流体动力学效应支配。因此，具有微米级颗粒的分层悬浮液的混合行为与遵循纳米级颗粒菲克扩散定律的悬浮液完全不同。这种微米级颗粒的“非菲克”混合不仅取决于浓度差，还取决于悬浮液的物理性质。
我们对浓度梯度与重力方向相反的微米级颗粒分层悬浮液的重力沉降进行了实验研究。我们还在与实验相同的条件下进行了点力型数值模拟。特别地，我们关注了颗粒在浓度界面附近的相对运动，浓度界面是上下浓度不同的悬浮液之间的明显界面。
实验和数值结果表明，如果悬浮液中颗粒的数密度足够，浓度界面似乎表现出不混溶，并且该界面会阻止颗粒混合。然而，少量颗粒无法维持浓度界面的封闭，随后表现出混溶行为。浓度界面处悬浮颗粒的混合机制与界面的混溶和不混溶特性密切相关。
