Experimental study on generation of non-Newtonian droplets in dripping mode in a flow focusing microchannel
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摘要: 液滴微流控是微流控领域重要分支，其所涉及的生物流体往往具有非牛顿性质。为深入理解非牛顿性质对液滴生成的影响，配置4种不同流变特性的流体，系统研究流动聚焦微通道中滴流模式下的非牛顿液滴生成。结果表明：与牛顿液滴相比，非牛顿液滴生成表现出更显著的“连珠现象”；不同非牛顿性质对液滴生成的影响截然不同，剪切稀化和弹性效应对液滴尺寸和生成频率的作用相反。液柱颈缩动力学结果显示：单一的剪切稀化效应使得非牛顿液滴液柱颈缩过程与牛顿液滴相似，均只有流动驱动阶段；单一的弹性效应则使得非牛顿液滴液柱颈缩后期出现不同于牛顿流体的毛细驱动阶段；而剪切稀化和弹性效应的共同作用会导致液柱颈缩过程中更显著的毛细驱动阶段和液柱断裂后更显著的“连珠现象”。Abstract: Droplet microfluidic is an important branch of the microfluidic field and the biological fluids involved in it often have non-Newtonian properties. In order to understand the influence of non-Newtonian properties on droplet formation, four kinds of fluids with different rheological properties were configured to systematically study the non-Newtonian droplet formation in the dripping mode in a flow focusing microchannel. The results show that compared with Newtonian droplet formation, non-Newtonian droplet formation shows a more significant “beads-on-a-string” phenomenon. Different non-Newtonian properties have different effects on droplet formation. Shear thinning effect and elastic effect have opposite effects on the droplet size and formation frequency. The results of liquid column necking dynamics show that the process of liquid column necking is similar to that of Newtonian fluid due to a single shear thinning effect. The single elastic effect makes the capillary drive stage which appears of liquid column necking different from that of Newtonian fluid. The combined effect of the elastic effect and shear thinning effect leads to more significant capillary drive stage in the process of column necking and more significant “beads-on-a-string” after column necking.
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图 4 滴流模式下4种离散相液滴生成过程实验图像
Figure 4. Experimental images of droplet formation process of four dispersed phases in dripping mode
图 5 液滴尺寸和液滴生成频率随流量比的变化
Figure 5. Droplet size and the frequency of droplet generation vary with the flow ratio
图 6 4种离散相生成液滴的相对尺寸和相对生成频率随流量比的变化
Figure 6. Relative droplet size and relative frequency vary with the flow ratio of the four dispersed phases
图 7 4种液滴生成时液柱颈缩过程图
Figure 7. Diagram of liquid column necking process during the formation of four droplets
图 8 不同流量比下扩展延伸率随时间的变化
Figure 8. Extensional strain rate
$\dot \varepsilon $ as functions of the elapsed time at different flow ratio
图 9 无量纲最小距离Wm/W随无量纲剩余时间(tp–t)/τt的变化
Figure 9. Plots of the dimensionless minimum width Wm/W of the dispersed filament to the dimentionless residual time (tp–t)/τt
图 10 无量纲最小距离Wm/W随无量纲偏移时间(tp–t)/τt的变化
Figure 10. Plot of the dimensionless minimum width Wm/W of the dispersed filament to the dimensionless shift time (tp–t)/τt
表 1 Carreau-Yasuda模型和改进Carreau模型的拟合系数
Table 1. The fitting parameters of the Carreau-Yasuda model and modified Carreau model
流体 ${\eta _0 }/({\rm{Pa} } \cdot {\rm{s} } )$ ${\eta _\infty }/({\rm{Pa} } \cdot {\rm{s} })$ ${\lambda _{\rm{C}}}/{\rm{ms}}$ a n XG 0.0187 0.00182 0.27 2 0.03 PEO 0.0106 0.00462 0.02 1.1 0.22 
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表 2 不同溶液与橄榄油之间的界面张力
Table 2. Interfacial tension between different solutions and olive oil
流体 密度ρ/（g·cm−3） 界面张力$ \sigma $/（mN·m−1） GW 1.158 20.49±0.16 PVP 1.020 18.22±0.17 XG 1.001 21.54±0.28 PEO 1.004 18.22±0.23
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表 3 不同离散相液滴的拟合系数
Table 3. Fitting coefficients of droplets of different dispersed phases
拟合系数 液滴 GW PVP XG PEO A 1.21±0.02 1.01±0.09 1.48±0.08 1.19±0.04 B −0.05±0.01 −0.04±0.01 −0.07±0.01 −0.05±0.01 C 1.10×10−3 7.00×10−4 1.37×10−3 1.20×10−3 D 0.44±0.01 0.63±0.19 0.2±0.09 0.55±0.03 E 0.11±0.01 0.14±0.01 0.08±0.01 0.06±0.01
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