Department of Mechanical Engineering , The University of Hong Kong , Pokfulam Road , Hong Kong , China.
HKU-Shenzhen Institute of Research and Innovation (HKU-SIRI) , Shenzhen , Guangdong 518000 , China.
Langmuir. 2018 Mar 6;34(9):3030-3036. doi: 10.1021/acs.langmuir.7b04168. Epub 2018 Feb 21.
The interfacial phenomena at liquid-liquid interfaces remain the subject of constant fascination in science and technology. Here, we show that fingers forming at the interface of nonequilibrium all-aqueous systems can spontaneously break into an array of droplets. The dynamic formation of droplets at the water-water (w/w) interface is observed when a less dense aqueous phase, for instance, the dextran solution, is placed on a denser aqueous phase, the polyethylene glycol solution, in a vertical Hele-Shaw cell. Because of the gradual diffusion of water from the upper phase into the lower phase, a dense layer appears at the nonequilibrium w/w interface. As a result, a periodic array of fingers emerge and sink. Remarkably, these fingers break up and an array of droplets are emitted from the interface. We characterize the wavelength of fingering by measuring the average distance between the dominant fingers. By varying the initial concentrations of the two nonequilibrium aqueous phases, we identify experimentally a phase diagram with a wide parameter space in which finger breaking occurs. Finally, plenty of droplets, spontaneously formed when one phase is continuously deposited onto another aqueous phase, further confirm the robustness of our experimental results. Our work suggests a simple yet efficient approach with a potential upscalability to generate all-aqueous droplets.
在科学和技术中，液-液界面的界面现象仍然是人们持续关注的焦点。在这里，我们表明，在非平衡全水体系的界面上形成的指状物可以自发地分裂成一系列液滴。当较稀的水相（例如葡聚糖溶液）置于密度较大的水相（聚乙二醇溶液）中时，在垂直的亥姆霍兹（Hele-Shaw）细胞中观察到水-水（w/w）界面处液滴的动态形成。由于水从上层逐渐扩散到下层，在非平衡 w/w 界面处出现致密层。结果，周期性的指状物出现并下沉。值得注意的是，这些指状物会破裂，并且一系列液滴从界面中释放出来。我们通过测量主导指状物之间的平均距离来表征指状的波长。通过改变两种非平衡水相的初始浓度，我们实验确定了一个具有广泛参数空间的相图，其中发生了指状破裂。最后，当一种相连续沉积到另一种水相上时，会自发形成大量液滴，这进一步证实了我们实验结果的稳健性。我们的工作提出了一种简单而有效的方法，具有潜在的可扩展性，可以生成全水液滴。
