微流道内椭圆粒子黏弹性聚焦数值模拟研究

黏弹性聚焦技术借助微尺度黏弹性流体的惯性和弹性耦合效应，能够实现生物粒子在流道中心的单一位置聚焦排列，被认为是未来生物粒子计数以及检测的理想预处理单元，因而引起了广泛的关注．自然界中的生物粒子往往是非球形的，故而研究不同形状粒子在黏弹性流体中的迁移特性具有十分重要的价值．本文通过格子玻尔兹曼方法耦合浸入边界法，对椭球粒子在直流道内黏弹性流体中的聚焦行为进行了系统的数值模拟研究．结果表明，面积相同但长径比不同的椭圆粒子在黏弹性流体中有不同的旋转周期与迁移速度．长径比更大的粒子旋转周期更长，且长径比大于3.5 的粒子甚至不再有明显的旋转．长径比更大的粒子上下两侧的黏弹性力分布更加平缓，受到指向流道中心的弹性力更小，使得粒子横向迁移速度更慢从而导致了长径比不同的椭圆粒子聚焦至流道中心所需时间的差异．此外，Weissenberg 数Wi 的增加同样能够减弱粒子的旋转，使得长径比稍小的粒子也能和长径比为1.0 的圆形粒子产生明显的分离．上述数值模拟的结论，为不同长径比粒子在黏弹性流体中的聚焦与分选应用提供了重要的理论指导．

Numerical Simulation of Viscoelastic Focusing of Ellipsoid Particles in a Microchannel

The viscoelastic focusing technology is becoming popular as an ideal pretreatment unit for bioparticle counting and detection, which can focus the particles at the centerline of the microchannel with the help of the inertia and elastic effects. The bioparticles in nature are usually non-spherical, therefore, it is important to study the focusing mechanics of various shaped particles. This paper presents a numerical simulation study of the ellipsoid particle focusing in viscoelastic fluids through lattice Boltzmann method (LBM) coupled with immersed boundary method (IBM). In two dimensional straight microchannel, ellipsoid particles with the same area and different aspect ratios have various rotational speeds and focusing efficiencies. The particles with large aspect ratios have slower rotational speeds and migration velocities to the channel centerline. The distribution of viscoelastic force on the upper and lower sides of the particle with larger aspect ratio is more uniform, which reduces the elastic force that pushes the particles to the channel centerline and makes the lateral migration speed of the particle slower. With aspect ratio larger than 3.5, particles do not rotate any more and migrate more slower than particles with small aspect ratio. As a result, the difference of particle focusing efficiency can help us to separate the various particles with different aspect ratios. The increase of Wi can also attenuate the rotation of the particles and make the particless with lightly larger aspect ratios separate from the circular particles. The simulation results provide the theoretical basis for the ellipsoid particle separation in viscoelastic fluid.