局部几何构型对聚焦流微通道内液滴生成特性的影响

doi:10.11949/0438-1157.20191503

摘要/Abstract

摘要：

在微流控技术中，微通道结构的优化设计是一种被动实现液滴精确调控的有效方法。为探究分散相入口、通道下游孔口以及二者共存模式下的通道结构变化对液滴生成特性的影响，采用VOF / CSF耦合level set的界面捕捉法对聚焦流微通道内的液滴生成开展了数值模拟研究。结果表明，当孔口为单一变量时，液滴生成周期和直径随孔口宽度呈近线性增大，且颈部宽度收缩率随孔口宽度的增大而不断减小。孔口的收缩有助于强化连续相Y方向的挤压和X方向的黏性剪切作用。当孔口宽度较小，聚焦作用较强时，液滴生成周期和直径整体上对分散相入口竖直和水平边锥形角的变化并不敏感；此时，孔口对连续相的聚焦效应主要影响液滴的生成特性。当孔口和分散相入口水平边锥形角θ₂同步变化时，二者可协同影响液滴的生成。孔口宽度的增大削弱了孔口的聚焦作用，液滴挤压破裂时间在单个周期中的占比逐渐增大。此外，当孔口宽度较大时，液滴生成开始对θ₂敏感，其周期和直径随θ₂增大而增大，且液滴可从滴流向射流模式转变。

关键词: 聚焦流, 数值模拟, 锥形入口, 孔口, 协同效应

Abstract:

In microfluidic technology, the optimal design of the microchannel structure is an effective method to passively achieve precise control of droplets. To investigate the influence of the local geometries including the dispersed phase inlet, the downstream orifice and their coexistence mode on the droplet formation, this paper adopts the interface capture method coupling the VOF/CSF and level set method to numerically simulate the droplet formation in flow-focusing devices. The results show that when the orifice as a single variable, the droplet generation cycle and diameter increase linearly with the orifice width. The shrinkage rate of the neck width decreases with the increasing orifice width. The narrowing neck is helpful to strengthen the Y-direction squeezing and the X-direction shear stress. When the orifice width is relatively small, the generation cycle and diameter are not sensitive to the included angle of the vertical edge and the horizontal edge on the whole. In this case, the droplet generation is mainly affected by the orifice s focusing effect. When the orifice and the included angle θ₂vary simultaneously, they two can cooperatively affect the droplet formation. The increasing orifice width gradually weakens the orifice s focusing effect, and thus leads to an increase of the proportion of the extrusion-rupture time in a single formation cycle. In addition, when the orifice width is relatively large, the droplet generation period and diameter increase with the increase of θ₂, and the droplet flow pattern can transform from a dripping regime to a jetting regime, indicating that the included angle θ₂ at the horizontal edge begins to significantly affect the droplet generation.

Key words: flow-focusing, numerical simulation, tapered inlet, orifice, synergistic effect

中图分类号:

O 359

宋祺, 杨智, 陈颖, 罗向龙, 陈健勇, 梁颖宗. 局部几何构型对聚焦流微通道内液滴生成特性的影响[J]. 化工学报, 2020, 71(4): 1540-1553.

Qi SONG, Zhi YANG, Ying CHEN, Xianglong LUO, Jianyong CHEN, Yingzong LIANG. Effect of local geometry on droplet formation in flow-focusing microchannel[J]. CIESC Journal, 2020, 71(4): 1540-1553.

图/表 20

图1 十字聚焦微通道二维几何结构（下角标c和d分别表示连续相和分散相）

Fig.1 Schematic diagram of 2D geometric structure of cross-focusing microchannel（subscripts c and d represent continuous and dispersed phase, respectively）

图2 数值模拟过程中网格无关性验证

Fig.2 Grid independence validation

图3 不同网格所对应的液滴生成周期

Fig.3 Droplet generation cycles for different grids

图4 实验通道结构

Fig.4 Schematic diagram of experimental channel structure

图5 液滴生成的模拟与实验结果对比

Fig.5 Comparison of droplet morphology between simulation and experiment

图6 不同孔口宽度下液滴断裂时刻的两相云图

Fig.6 Two-phase cloud diagrams at droplet break-up under different orifice widths

图7 液滴生成周期tcycle和直径Ddroplet随孔口宽度的变化

Fig.7 Variation of droplet formation cycle and diameter with orifice widths

图8 各监测点速度随孔口宽度的变化

Fig.8 Velocity component of each monitoring point varies with orifice widths

图9 单个生成周期内监测点速度、液柱轮廓、速度场和压力场随时间的变化

Fig.9 Monitoring velocity, droplet profile and velocity and pressure field in a single cycle

图10 挤压阶段内颈部宽度和位置随时间的变化

Fig.10 Variations of neck width and position in squeezing stage

图11 不同竖直边锥形角下液滴断裂时刻两相云图

Fig.11 Two-phase cloud diagrams at droplet break-up under different θ1

图12 液滴生成周期tcycle和直径Ddroplet随竖直边锥形角的变化

Fig.12 Variation of droplet formation period and diameter with θ1

图13 两相交汇区内c点vc,x速度随竖直边锥形角的变化

Fig.13 Variation of vc,x for different θ1

图14 液滴断裂时刻锥形入口的局部速度场分布图

Fig.14 Local velocity field of taper inlet at droplet break-up

图15 不同水平边锥形角下液滴断裂时刻两相云图

Fig.15 Two-phase cloud diagrams at droplet break-up under different θ2

图16 液滴生成周期tcycle和直径Ddroplet随水平边锥形角的变化

Fig.16 Droplet formation period and diameter for different θ2

图17 不同水平边锥形角和孔口宽度下液滴断裂时刻两相云图

Fig.17 Two-phase cloud diagram at droplet break-up under different θ2 and wori

图18 液滴生成周期tcycle随水平边锥形角和孔口宽度的变化规律

Fig.18 Variation of droplet formation period under different θ2 and wori

图19 单个生成周期内监视点a处不同水平边锥形角下va,x速度随时间的变化

Fig.19 Change of va,x in a single generation cycle at different θ2

表1 不同通道构型下液滴生长与挤压破裂阶段所经历时间的对比

Table 1 Comparison of droplet growth and squeez fracture under different channel configurations

w_ori/μm	θ₂/ (°)	t₁/ms	t₂/ms	t₃/ms	(t₂-t₁)/ms	(t₃-t₂)/ms	(t₃-t₂)/t₃
50	0	0	1.24	1.693	1.24	0.453	26.757%
	20	0	1.195	1.621	1.195	0.426	26.280%
	40	0	1.292	1.717	1.292	0.425	24.753%
75	0	0	1.815	2.686	1.815	0.871	32.427%
	20	0	1.68	2.459	1.68	0.779	31.679%
	40	0	2.036	2.425	2.036	0.389	16.041%
100	0	0	2.12	3.865	2.12	1.745	45.148%
	20	0	1.892	3.606	1.892	1.714	47.532%
	40	—	—	—	—	—	—

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