Chemical reaction and mixing inside a coalesced droplet after a head-on collision

• Published in: Microfluidics and nanofluidics Vol. 18; no. 5-6; pp. 1355 - 1363
• Main Authors: Yeh, Szu-I, Sheen, Horn-Jiunn, Yang, Jing-Tang
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.05.2015; Springer Nature B.V
• Subjects: Analytical Chemistry; Biomedical Engineering and Bioengineering; Chemical reactions; Coalescence; Engineering; Engineering Fluid Dynamics; Nanotechnology and Microengineering; Research Paper; Microfluidic mixing; Neck curvature; Droplets coalescence; Chemical reaction front
• ISSN: 1613-4982; 1613-4990
• DOI: 10.1007/s10404-014-1534-4

We investigated the phenomena of a chemical reaction inside a coalesced droplet after a direct (head-on) collision. A droplet containing an alkaline solution collided with a droplet containing a pH indicator on a surface with a wettability gradient. We used a high-speed camera to observe the color-changing reaction inside the coalesced droplet. Compared with a traditional dye-mixing test, the chemical reaction inside the coalesced droplet facilitated the mixing of two counter-reactive fluids and was more than 100 times as efficient as for unreactive fluids mixing inside the coalesced droplet. Instead of mere mixing, a chemical reaction inside a coalesced droplet is valuable for applications in a digital microfluidic open system. In droplet coalescence, the characteristics of the fluids and the ratio of volumes of two droplets caused a varied profile of the droplet coalescence, especially the neck curvature that affects the shape of the material interface between the two droplets at an initial phase. We observed the evolution of the chemical reaction with a varying radius of neck curvature inside the coalesced droplet. For the case of a small radius of neck curvature, the small interfacial area between two reactive fluids accumulated an intense heat of reaction and induced a rapid growth of the fingers. For the case of a large radius of neck curvature, the growth of fingers was slight and the interface was uniform across the large interfacial area. Our work illustrates a correlation between the rate of chemical reaction and the profile of a coalesced droplet, which is a significant reference in droplet-based microfluidic systems for biochemical applications.