Mbanjwa Mesuli B, Harding Kevin, Gledhill Irvy M A
Technology Transfer & Innovation Support, North-West University, Potchefstroom 2520, South Africa.
School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg 2050, South Africa.
Micromachines (Basel). 2022 Apr 30;13(5):708. doi: 10.3390/mi13050708.
Droplets generated in microfluidic channels are effective self-contained micromixers and micro-reactors for use in a multiplicity of chemical synthesis and bioanalytical applications. Droplet microfluidic systems have the ability to generate multitudes of droplets with well-defined reagent volumes and narrow size distributions, providing a means for the replication of mixing within each droplet and thus the scaling of processes. Numerical modelling using computational fluid dynamics (CFD) is a useful technique for analysing and understanding the internal mixing in microfluidic droplets. We present and demonstrate a CFD method for modelling and simulating mixing between two species within a droplet travelling in straight microchannel, using a two-phase moving frame of reference approach. Finite element and level set methods were utilised to solve the equations governing the coupled physics between two-phase flow and mass transport of the chemical species. This approach had not been previously demonstrated for the problem of mixing in droplet microfluidics and requires less computational resources compared to the conventional fixed frame of reference approach. The key conclusions of this work are: (1) a limitation of this method exists for flow conditions where the droplet mobility approaches unity, due to the moving wall boundary condition, which results in an untenable solution under those conditions; (2) the efficiency of the mixing declines as the length of the droplet or plug increases; (3) the initial orientation of the droplet influences the mixing and the transverse orientation provides better mixing performance than the axial orientation and; (4) the recirculation inside the droplet depends on the superficial velocity and the viscosity ratio.
微流控通道中产生的液滴是有效的独立式微混合器和微反应器，可用于多种化学合成和生物分析应用。液滴微流控系统能够产生大量具有明确试剂体积和窄尺寸分布的液滴，为在每个液滴内复制混合以及扩大过程规模提供了一种手段。使用计算流体动力学（CFD）进行数值建模是分析和理解微流控液滴内部混合的一种有用技术。我们提出并演示了一种CFD方法，用于对在直微通道中移动的液滴内两种物质之间的混合进行建模和模拟，采用两相移动参考系方法。利用有限元和水平集方法求解控制两相流与化学物质质量传输之间耦合物理过程的方程。这种方法以前尚未用于液滴微流控中的混合问题，并且与传统的固定参考系方法相比，所需的计算资源更少。这项工作的关键结论是：（1）由于移动壁边界条件，对于液滴迁移率接近1的流动条件，该方法存在局限性，这会导致在这些条件下得到不可行的解；（2）混合效率随着液滴或柱塞长度的增加而下降；（3）液滴的初始取向影响混合，横向取向比轴向取向具有更好的混合性能；（4）液滴内部的再循环取决于表观速度和粘度比。
