  Effective mixing is essential for biochemical reactions. In droplet-based
microfluidics, immediate mixing of substances upon contact in the droplet
formation stage can greatly enhance the uniformity of chemical reactions.
Furthermore, it eliminates the need for implementing micromixers in the chip.
In our research, we conducted a comprehensive study by first employing a series
of two-dimensional (2D) numerical simulations, followed by experimental
investigations using three-dimensional-printed (3D-printed) microfluidic chips.
Our primary focus was on assessing the mixing efficiency within droplets.
Specifically, we compared the performance of three different types of droplet
generators: T-junction, cross-junction, and a novel asymmetric design with
various angles. Our evaluation criteria encompassed mixing efficiency, droplet
diameter, and droplet eccentricity. Our findings indicate that 2D numerical
simulations can serve as a valuable tool for qualitatively analyzing two-phase
flows and the droplet generation process, particularly in quasi-two-dimensional
devices. The relative simplicity of such simulations renders them readily
applicable, specifically in complex microfluidic geometries. Regarding mixing
efficiency, we observed that the asymmetric droplet generators outperformed the
cross-junction configuration but fell slightly short of the mixing efficiency
achieved by the T-junction. Additionally, while the mixing index in the
asymmetric generators closely matched that of the T-junction, these asymmetric
generators produced smaller droplets. Our study suggests that the novel
asymmetric droplet generators offer a significant advantage by simplifying the
design of microfluidic devices. This is achieved by facilitating both droplet
formation and rapid reagent mixing within the droplets while concurrently
maintaining a small droplet diameter.
