The State Key Lab of Chemical Engineering, Department of Chemical Engineering , Tsinghua University , Beijing 100084 , China.
Langmuir. 2018 May 1;34(17):4980-4990. doi: 10.1021/acs.langmuir.8b00123. Epub 2018 Apr 18.
Microchannel emulsification requires large amounts of surfactant to prevent coalescence and improve emulsions lifetime. However, most numerical studies have considered surfactant-free mixtures as models for droplet formation in microchannels, without taking into account the distribution of surfactant on the droplet surface. In this paper, we investigate the effects of nonuniform surfactant coverage on the microfluidic flow pattern using an extended lattice-Boltzmann model. This numerical study, supported by micro-particle image velocimetry experiments, reveals the likelihood of uneven distribution of surfactant during the droplet formation and the appearance of a stagnant cap. The Marangoni effect affects the droplet breakup by increasing the shear rate. According to our results, surfactant-free and surfactant-rich droplet formation processes are qualitatively different, such that both the capillary number and the Damköhler number should be considered when modeling the droplet generation in microfluidic devices. The limitations of traditional volume and pressure estimation methods for determining the dynamic interfacial tension are also discussed on the basis of the simulation results.
微通道乳化需要大量的表面活性剂来防止聚结并提高乳液的寿命。然而，大多数数值研究都将无表面活性剂的混合物视为微通道中液滴形成的模型，而没有考虑表面活性剂在液滴表面的分布。在本文中，我们使用扩展的格子玻尔兹曼模型研究了非均匀表面活性剂覆盖对微流控流型的影响。这项数值研究得到了微粒子图像测速实验的支持，揭示了在液滴形成过程中表面活性剂分布不均匀的可能性以及停滞帽的出现。Marangoni 效应通过增加剪切率来影响液滴的破裂。根据我们的结果，无表面活性剂和富含表面活性剂的液滴形成过程在定性上是不同的，因此在对微流控装置中的液滴生成进行建模时，应同时考虑毛细管数和 Damköhler 数。还根据模拟结果讨论了传统的体积和压力估计方法在确定动态界面张力方面的局限性。
