关键构型参数对流动聚焦式微流控液滴生成的影响

INFLUENCES OF KEY CONFIGURATION PARAMETERS ON FLOW-FOCUSING MICROFLUIDIC DROPLET GENERATION

流动聚焦式液滴微流控技术借助流动聚焦效应和离散相液丝界面失稳,实现单分散微液滴的连续生成.该技术中的多相界面流动对于构型参数有较强的依赖性,表现出丰富的微尺度流动特征.本研究在前期发展的基于毛细管可变几何微流控装置的基础上,采用数值模拟方法研究关键参数对于液滴生成模态和尺寸的影响规律.经过合理简化后,研究建立实验装置的轴对称模型,并结合自适应网格加密技术,提高了数值模拟效率;通过多个实验工况的对比,验证了数值模拟的准确性.研究发现:在所选择的流体组合、几何和流量参数范围内,液滴生成过程存在滴流、串滴、射流和不稳定4个模态;在固定的离散相和连续相流量组合下,上游和下游毛细管端部间距的变化会改变滴流和串滴模态下液滴的长度,而对射流模态下液滴的大小影响很小;在固定的几何参数下流量变化时,液滴长度的变化在滴流和串滴模态转换时基本连续,而在射流模态发生时产生骤降;下游毛细管的内径对模态相图影响显著,大内径下滴流模态占主导,且射流模态下液丝的射流长度变化明显,而小内径下射流模态占主导,且在大的连续相流量下存在不稳定模态.研究结果表明关键结构参数对于流动聚焦式微流控液滴的生成有重要影响,合理改变这些参数可以控制液滴尺寸和改善液滴的单分散性,可为设计和优化流动聚焦式液滴生成装置提供依据.

The flow-focusing droplet microfluidics achieves continuous generation of monodisperse microdroplets by means of flow-focusing effects and interfacial destabilization phenomenon of discrete-phase liquid filament.The multiphase interfacial flow in this technique exhibits dependence on configuration parameters and shows rich microfluidic device developed in our previous study,numerical simulations are used to investigate the influences of key configuration parameters on droplet generation modes and droplet dimensions.After reasonable simplifications,the study establishes an axisymmetric model of the actual device and combines the adaptive mesh refinement technique to improve the efficiency of the numerical simulation.The accuracy of the numerical simulation is verified through the comparison of several experimental operating conditions.It is found that within the selected fluid combination,geometry,and flow parameters,the droplet generation process exists in four modes:dripping,streaming,jetting,and unstable.Under the fixed discrete phase and continuous phase flow rate combinations,the variation of the distance between the upstream and downstream capillary ends changes the droplet length in the dripping and streaming modes,while it has little effect on the droplet size in the jetting mode.Under the fixed geometry parameters,when the flow rates vary,the change of droplet length is nearly continuous at the transition between dripping and streaming modes,but produces a sudden drop at the onset of the jetting mode.The internal diameter of the downstream capillary has a significant effect on the phase diagram,the dripping mode dominates for the large diameter internal diameter and the jet length changes more significantly in the jetting mode,while the jetting mode dominates for the small internal diameter and unstable modes are found at large continuous phase flows.The results of this paper show that the key configuration parameters have important effects on the flow-focusing microfluidic droplet generation,and the applicable alteration of these parameters can control the droplet size and improve the droplet monodispersity,which provides a basis for the design and optimization of flowfocusing microdroplet generation devices.