Abstract Millimeter-scale W1/O/W2 compound droplets generation in a co-flowing microfluidic device with one-step structure was symmetrically observed through experiments, and the formation process is distinguished according to the variation law of thread tip velocity. Specially, under a wide range of operation conditions, W1/O/W2 compound droplets form accompanying with the generation of oil phase droplet. The formation ratio of compound droplets to oil phase droplet always depends on the outer phase Capillary number, Caw2, and the flow rate ratio, R, of middle to inner phases, and thus the flow pattern diagram is obtained. Further the influences of all phases flow rates, outer phase viscosity as well as the interfacial tension on the formation period and geometric sizes are extensively studied. Finally, a prediction correlation for dimensionless inner and outer diameters with several dimensionless numbers is also obtained. The results could provide an experimental basis for controllably preparing monodisperse compound droplets with millimeter-scale.

中文翻译：

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一步结构共流装置中毫米级W1/O/W2复合液滴生成研究

摘要 通过实验对称地观察了一级结构共流微流控装置中毫米级W1/O/W2复合液滴的生成，并根据线尖速度的变化规律区分了形成过程。特别是，在广泛的操作条件下，W1/O/W2 复合液滴的形成伴随着油相液滴的产生。复合液滴与油相液滴的形成比例始终取决于外相毛细管数 Caw2 和中相与内相的流速比 R，从而获得流动模式图。此外，还广泛研究了所有相流速、外相粘度以及界面张力对形成周期和几何尺寸的影响。最后，还获得了具有多个无量纲数的无量纲内径和外径的预测相关性。研究结果可为可控制备毫米级单分散复合液滴提供实验依据。