Surfactant effects on droplet formation in microfluidic systems

多相流在微尺度通道允许操作的流动模式，并提供高的传热和传质率。表面活性剂被广泛应用于从乳化到DNA提取和喷墨打印的各种应用中，它们对液滴形成的影响非常重要。在这项研究中，实验研究了在一系列表面活性剂的存在下，使用流动聚焦微通道在有机连续相中形成水滴。双色PiV系统用于识别液滴形成过程中界面的变化，并获得两相的速度场。以低粘度硅油（0.0046 Pa s）为连续相，以48% w/w的水和52% w/w的甘油为分散相。两种阳离子表面活性剂C12TAB （50 mM）和C16TAB （5 mM）在水相中加入，浓度高于CMC值。离子表面活性剂SDS （1 mM）对液滴形成的影响也进行了比较。确定了四种液滴形成方式，即挤压、滴下、喷射和穿线，当表面活性剂存在时，其边界发生了变化。对于所研究的所有溶液，确定了三个不同的液滴形成阶段：膨胀阶段、缩颈阶段和挤压阶段。正如预期的那样，表面活性剂浓度越高，液滴越小。表面活性剂的吸附动力学在液滴形成过程中起着至关重要的作用。对于CMC值较低的表面活性剂，动态界面张力对液滴形成特征时间尺度的影响更为显著，表明表面活性剂活性增加。利用这些信息，可以解释在液滴形成阶段之间的过渡期间，表面活性剂体系之间的差异。

Multiphase flows in microscale channels allow manipulation of the flow patterns and offer high heat and mass transfer rates. Surfactants are commonly used in a wide variety of applications from emulsification to DNA extraction and inkjet printing, and their effects on droplet formation is of great importance. In this study, the formation of aqueous droplets in an organic continuous phase was studied experimentally using a flow-focusing microchannel in the presence of a range of surfactants. A two-colour PiV system was used to identify changes in the interface during the drop formation and to obtain velocity fields in both phases. A low viscosity silicone oil (0.0046 Pa s) was used as the continuous phase and a mixture of 48% w/w water and 52% w/w glycerol was the dispersed phase. Two cationic surfactants, C12TAB (50 mM) and C16TAB (5 mM) were added in the aqueous phase, at concentrations above the CMC values. An ionic surfactant, SDS (1 mM) was also used to compare the effects on the drop formation. Four regimes of drop formation were identified, namely squeezing, dripping, jetting and threading, whose boundaries changed when the surfactants were present. For all solutions studied, three distinct drop formation stages were identified, expansion, necking and pinch-off. Smaller drops were observed at higher concentrations of surfactant, as expected. The dynamics of surfactant adsorption can play a vital role in the drop formation process and were studied. The effect of dynamic interfacial tension at the characteristic time scale of drop formation becomes more significant at for surfactants with low CMC values, which suggests increased surfactant activity. Using this information the differences between surfactant systems during the transitions between drop formation stages could be explained.