Ge Zhouyang, Tammisola Outi, Brandt Luca
Linné FLOW Centre and SeRC (Swedish e-Science Research Centre), KTH Mechanics, SE-100 44 Stockholm, Sweden.
Soft Matter. 2019 Apr 17;15(16):3451-3460. doi: 10.1039/c8sm02479k.
Self-assembly of soft matter, such as droplets or colloids, has become a promising scheme to engineer novel materials, model living matter, and explore non-equilibrium statistical mechanics. In this article, we present detailed numerical simulations of few non-Brownian droplets in various flow conditions, specifically, focusing on their self-assembly within a short distance in a three-dimensional (3D) microfluidic channel, cf. [Shen et al., Adv. Sci., 2016, 3(6), 1600012]. Contrary to quasi two-dimensional (q2D) systems, where dipolar interaction is the key mechanism for droplet rearrangement, droplets in 3D confinement produce much less disturbance to the underlying flow, thus experiencing weaker dipolar interactions. Using confined simple shear and Poiseuille flows as reference flows, we show that the droplet dynamics is mostly affected by the shear-induced cross-stream migration, which favors chain structures if the droplets are under an attractive depletion force. For more compact clusters, such as three droplets in a triangular shape, our results suggest that an inhomogeneous cross-sectional inflow profile is further required. Overall, the accelerated self-assembly of a small-size droplet cluster results from the combined effects of strong depletion forces, confinement-mediated shear alignments, and fine-tuned inflow conditions. The deterministic nature of the flow-assisted self-assembly implies the possibility of large throughputs, though calibration of all different effects to directly produce large droplet crystals is generally difficult.
软物质（如液滴或胶体）的自组装已成为一种颇具前景的方案，可用于设计新型材料、模拟生物物质以及探索非平衡统计力学。在本文中，我们展示了在各种流动条件下对少量非布朗液滴进行的详细数值模拟，具体而言，重点关注它们在三维（3D）微流控通道中短距离内的自组装情况，参见[Shen等人，《先进科学》，2016年，3(6)，1600012]。与准二维（q2D）系统不同，在准二维系统中偶极相互作用是液滴重排的关键机制，处于3D受限环境中的液滴对下层流动产生的干扰要小得多，因此经历的偶极相互作用也较弱。以受限简单剪切流和泊肃叶流作为参考流，我们表明液滴动力学主要受剪切诱导的横向迁移影响，如果液滴受到吸引性耗尽力作用，这种迁移有利于形成链状结构。对于更紧凑的团簇，如三角形排列的三个液滴，我们的结果表明还需要不均匀的横截面流入剖面。总体而言，小尺寸液滴团簇的加速自组装是由强耗尽力、受限介导的剪切排列以及微调的流入条件共同作用的结果。流动辅助自组装的确定性意味着有可能实现高产量，尽管要校准所有不同效应以直接生成大尺寸液滴晶体通常很困难。
