Yu Haitao, Lu Ziyang, Lohse Detlef, Zhang Xuehua
Soft Matter & Interfaces Group, School of Civil, Environmental and Chemical Engineering, RMIT University , Melbourne, Victoria 3001, Australia.
Physics of Fluids group, Department of Science and Engineering, Mesa+ Institute, and J. M. Burgers Centre for Fluid Dynamics, University of Twente , P.O. Box 217, 7500 AE Enschede, The Netherlands.
Langmuir. 2015 Nov 24;31(46):12628-34. doi: 10.1021/acs.langmuir.5b03464. Epub 2015 Nov 10.
Nanoscale droplets at a solid-liquid interface are of high relevance for many fundamental phenomena and applied processes. The solvent exchange process is a simple approach to produce, e.g., oil nanodroplets over a large surface area on a substrate, by exchange oil-saturated ethanol by oil-saturated water, which has a lower oil solubility than ethanol. In this process, the size of the nanodroplets is closely related to the flow conditions. To achieve control of the droplet size, it is essential to fully understand the nucleation and growth of nanodroplets under different flow conditions. In this work, we investigate the gravitational effect on the droplet formation by the solvent exchange. We compared the droplet size as the substrate was placed on the upper or lower wall in a horizontal fluid channel or on the sides of a vertical channel with an upward or downward flow. We found significant difference in the droplet size for the three substrate positions in a wide channel with height h = 0.21 mm. The difference of droplet size was eliminated in a narrow channel with height h = 0.07 mm. The relevant dimensional control parameter for the occurrence of the gravitational effects is the Archimedes number Ar and these two heights correspond to Ar = 10 and Ar = 0.35, respectively. The gravitational effects lead to a nonsymmetric parabolic profile of the mixing front, with the velocity maximum being off-center and thus with different distances α(Ar)h and (1 - α(Ar))h to the lower and upper wall, respectively. The ratio of the total droplet volume on the lower and upper wall is theoretically found to be (α(Ar)/(1 - α(Ar)))(3). This study thus improves our understanding of the mechanism of the solvent exchange process, providing guidelines for tailoring the volume of surface nanodroplets.
固液界面处的纳米级液滴与许多基本现象和应用过程高度相关。溶剂交换过程是一种简单的方法，例如通过用比乙醇油溶解度更低的油饱和水交换油饱和乙醇，在基板的大面积上制备油纳米液滴。在这个过程中，纳米液滴的大小与流动条件密切相关。为了实现对液滴大小的控制，充分理解不同流动条件下纳米液滴的成核和生长至关重要。在这项工作中，我们研究了重力对溶剂交换形成液滴的影响。我们比较了将基板放置在水平流体通道的上壁或下壁上，或垂直通道的侧面且流体向上或向下流动时的液滴大小。我们发现在高度(h = 0.21)毫米的宽通道中，三种基板位置的液滴大小存在显著差异。在高度(h = 0.07)毫米的窄通道中，液滴大小的差异消失了。重力效应发生的相关尺寸控制参数是阿基米德数(Ar)，这两个高度分别对应(Ar = 10)和(Ar = 0.35)。重力效应导致混合前沿呈现非对称抛物线轮廓，速度最大值偏离中心，因此到下壁和上壁的距离分别为(\alpha(Ar)h)和((1 - \alpha(Ar))h)。理论上发现下壁和上壁上总液滴体积的比率为((\alpha(Ar)/(1 - \alpha(Ar)))^3)。因此，这项研究增进了我们对溶剂交换过程机制的理解，为定制表面纳米液滴的体积提供了指导。
