Soft Matter & Interfaces Group, School of Engineering, RMIT University, Melbourne, VIC 3001, Australia.
Lab Chip. 2017 Apr 11;17(8):1496-1504. doi: 10.1039/c6lc01555g.
Surface nanodroplets are important units for lab-on-a-chip devices, compartmentalised catalytic reactions, high-resolution near-field imaging, and many others. Solvent exchange is a simple solution-based bottom-up approach for producing surface nanodroplets by displacing a good solvent of the droplet liquid by a poor one in a narrow channel in the laminar regime. The droplet size is controlled by the solution composition and the flow conditions during the solvent exchange. In this paper, we investigated the effects of local microfluidic structures on the formation of surface nanodroplets. The microstructures consist of a microgap with a well-defined geometry, embedded on the opposite microchannel wall, facing the substrate where nucleation takes place. For a given channel height, the dimensionless control parameters were the Peclet number of the flow, the ratio between the gap height and the channel height, and the aspect ratio between the gap length and the channel height. We found and explained three prominent features in the surface nanodroplet distribution at the surface opposite to the microgap: (i) enhanced volume of the droplets; (ii) asymmetry as compared to the location of the gap in the spatial droplet distribution with increasing Pe; (iii) reduced exponent of the effective scaling law of the droplet size with Pe. The droplet size also varied with the aspect and height ratios of the microgap at a given Pe value. Our simulations of the profile of oversaturation in the channel reveal that the droplet size distribution may be attributed to the local flow patterns induced by the gap. Finally, in a tapered microchannel, a gradient of surface nanodroplet size was obtained. Our work shows the potential for controlling nanodroplet size and spatial organization on a homogeneous surface in a bottom-up approach by simple microfluidic structures.
表面纳米液滴是芯片实验室设备、分隔催化反应、高分辨率近场成像等的重要单元。溶剂交换是一种简单的基于溶液的自下而上的方法，通过在层流状态下在狭窄通道中用不良溶剂置换液滴液体的良溶剂来产生表面纳米液滴。液滴尺寸由溶液组成和溶剂交换过程中的流动条件控制。在本文中，我们研究了局部微流控结构对表面纳米液滴形成的影响。这些微结构由嵌入在相反微通道壁上的具有明确定义几何形状的微间隙组成，面向发生成核的基底。对于给定的通道高度，无量纲控制参数是流动的佩克莱数、间隙高度与通道高度之比以及间隙长度与通道高度之比。我们在与微间隙相对的表面上发现并解释了表面纳米液滴分布中的三个突出特征：（i）液滴体积增加；（ii）随着 Pe 的增加，相对于空间液滴分布中间隙位置的不对称性；（iii）有效液滴尺寸与 Pe 的标度律的指数减小。在给定的 Pe 值下，微间隙的纵横比和高度比也会影响液滴尺寸。我们对通道中过饱和度分布的模拟表明，液滴尺寸分布可能归因于间隙引起的局部流动模式。最后，在锥形微通道中，获得了表面纳米液滴尺寸的梯度。我们的工作表明，通过简单的微流控结构，有可能在自下而上的方法中控制均质表面上的纳米液滴尺寸和空间组织。
