Insight into the effects of smooth channels, sharp channels and channel bending angles on intra-droplet mass transfer behavior

Microdroplet technology has excellent properties and great potential for applications, especially in chemical engineering where each droplet can be used as an individually addressable microreactor, and researchers have used droplets to synthesize many types of materials, including copolymers, microbeads, molecular probes, nanostructures/particles and microgels. In this paper, we investigate the effect of different microchannel structures on the mass transfer behavior within droplets, such as the curvature angle of the channels, smooth channels and sharp channels. It was found that due to the variation of the radius of curvature of the channel, the vortex size on the side with small radius of curvature becomes smaller, while the vortex on the other side with large radius of curvature becomes larger, allowing the exchange and mixing of droplets in each half of the droplet top and bottom. In the sharp channel, the vortex size on the side with small radius of curvature becomes smaller and the vortex on the other side becomes larger, which can better promote the full mixing of the components inside the droplet. However, in the smooth channel, compared with the sharp channel, the droplets move more smoothly, and the distance traveled by the droplets in the smooth channel is larger than that in the sharp channel per unit time, so the mixing efficiency of the droplets in the smooth channel is better than that of the droplets in the sharp channel from the perspective of distance. Finally, we designed a channel structure which has a good mixing effect and a small pressure drop. In the field of droplet microreactor, this structure can be used as a unit with multiple units connected in series and then built into an efficient and integrated microfluidic chip. Graphical abstract