Cain Department of Chemical Engineering, Louisiana State University , Baton Rouge, Louisiana 70802, USA.
Center for Advanced Microstructures and Devices (CAMD), Louisiana State University , 6980 Jefferson Highway, Baton Rouge, Louisiana 70806, USA.
Biomicrofluidics. 2014 Oct 30;8(5):054125. doi: 10.1063/1.4900939. eCollection 2014 Sep.
Droplet-based microfluidics has gained extensive research interest as it overcomes several challenges confronted by conventional single-phase microfluidics. The mixing performance inside droplets/slugs is critical in many applications such as advanced material syntheses and in situ kinetic measurements. In order to understand the effects of operating conditions on the mixing performance inside liquid slugs generated by a microfluidic T-junction, we have adopted the volume of fluid method coupled with the species transport model to study and quantify the mixing efficiencies inside slugs. Our simulation results demonstrate that an efficient mixing process is achieved by the intimate collaboration of the twirling effect and the recirculating flow. Only if the reagents are distributed transversely by the twirling effect, the recirculating flow can bring in convection mechanism thus facilitating mixing. By comparing the mixing performance inside slugs at various operating conditions, we find that slug size plays the key role in influencing the mixing performance as it determines the amount of fluid to be distributed by the twirling effect. For the cases where short slugs are generated, the mixing process is governed by the fast convection mechanism because the twirling effect can distribute the fluid to the flow path of the recirculating flow effectively. For cases with long slugs, the mixing process is dominated by the slow diffusion mechanism since the twirling effect is insufficient to distribute the large amount of fluid. In addition, our results show that increasing the operating velocity has limited effects on improving the mixing performance. This study provides the insight of the mixing process and may benefit the design and operations of droplet-based microfluidics.
液滴微流控技术因其克服了传统单相微流控所面临的许多挑战而引起了广泛的研究兴趣。液滴/液段内部的混合性能在许多应用中至关重要，例如先进材料合成和原位动力学测量。为了了解操作条件对微流控 T 形交叉口产生的液体段内部混合性能的影响，我们采用了体积流法并结合物种传输模型来研究和量化段内的混合效率。我们的模拟结果表明，通过旋流效应和回流的紧密协作，可以实现有效的混合过程。只有当试剂通过旋流效应横向分布时，回流才能引入对流机制，从而促进混合。通过比较不同操作条件下段内的混合性能，我们发现段尺寸是影响混合性能的关键因素，因为它决定了通过旋流效应分配的流体量。对于生成短段的情况，由于旋流效应可以有效地将流体分配到回流的流动路径中，因此混合过程受快速对流机制的控制。对于长段的情况，由于旋流效应不足以分配大量的流体，因此混合过程受缓慢扩散机制的控制。此外，我们的结果表明，增加操作速度对改善混合性能的效果有限。本研究提供了对混合过程的深入了解，可能有助于基于液滴的微流控的设计和操作。
