Effect of local geometry on droplet formation in flow-focusing microchannel

doi:10.11949/0438-1157.20191503

Abstract

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

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

CLC Number:

O 359

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.

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.

Figures/Tables 20

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

Fig.2 Grid independence validation

Fig.3 Droplet generation cycles for different grids

Fig.4 Schematic diagram of experimental channel structure

Fig.5 Comparison of droplet morphology between simulation and experiment

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

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

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

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

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

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

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

Fig.13 Variation of vc,x for different θ1

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

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

Fig.16 Droplet formation period and diameter for different θ2

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

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

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

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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