圆截面直流道中微粒黏弹性聚焦机理研究∗

摘要

微粒黏弹性聚焦技术近年来受到了广泛的研究重视，但影响粒子聚焦特性的关键参数调控机理仍不清楚。基于此目的，本文量化研究了圆截面直流道中非牛顿流体诱导微粒黏弹性聚焦的行为，给出了流速和流道长度对粒子聚焦特性的调控机理。具体而言：首先，对比分析不同黏度牛顿流体(水和22 wt%甘油水溶液)和非牛顿流体(8 wt%聚乙烯吡咯烷酮水溶液)中粒子横向迁移行为，发现非牛顿流体中粒子将在弹性力主导下聚焦至流道中心区域，而牛顿流体中粒子则在惯性升力主导下迁移形成Segré–Silberberg圆环。其次，量化分析粒子尺寸和驱动流速对黏弹性聚焦效果的影响，发现随着流速的增加，粒子聚焦效果逐渐变好并最终趋于稳定，且大粒子较小粒子具有更好的聚焦效果。最后，研究粒子沿流道长度的动态聚焦过程，推导并验证了粒子聚焦所需安全流道长度的数学模型，发现大粒子聚焦所需安全流道长度显著短于小粒子。上述研究结果对于提升粒子黏弹性聚焦机理和过程的理解，实现微粒聚焦特性的灵活控制具有非常重要的意义。%Particle focusing induced by viscoelasticity of fluids has attracted increasing interest in recent years. However, the regulation mechanisms of critical parameters affecting the particle focusing behaviors are still unclear. This paper systematically characterized the dynamics of particle migration in non-Newtonian fluid flows, and analyzed the effects of flow rate and channel length on particle focusing behaviors. First, the lateral migration behaviors of particles suspended in Newtonian fluids (e.g., pure water and 22 wt%glycerol aqueous solution) are compared with those in non-Newtonian fluids (8 wt% polyvinylpyrrolidone aqueous solution). It is found that the particles suspended in non-Newtonian fluids would migrate towards the channel centerline and form a single-line particle array under the action of elastic force while the particles suspended in Newtonian fluids would migrate to form a famous Segré-Silberberg particle annular ring due to the effects of inertial lift forces. Second, the effects of particle size and driving flow rate on particle viscoelastic focusing are quantitatively analyzed. Results show that with increasing flow rate the focusing degree increases and finally stabilize at a certain value, and the large particles have better focusing quality than the small ones. Finally, the dynamic focusing process of particles along the channel length is investigated. A mathematical model of safe channel length for achieving particle focusing is derived and validated by experiments. It is found that the safe channel length for large particles is significantly shorter than that for small ones. The obtained results would improve the understanding of particle focusing processes and mechanisms, and help realize the flexible control of particle migration behaviors in non-Newtonian fluids.