Enhanced mixing efficiency and reduced droplet size with novel droplet generators

Abstract

Nowadays, droplet microfluidics has become widely utilized for high-throughput assays. Efficient mixing is crucial for initiating biochemical reactions in many applications. Rapid mixing during droplet formation eliminates the need for incorporating micromixers, which can complicate the chip design. Furthermore, immediate mixing of substances upon contact can significantly improve the consistency of chemical reactions and resulting products. This study introduces three innovative designs for droplet generators that achieve efficient mixing and produce small droplets. The T-cross and cross-T geometries combine cross and T junction mixing mechanisms, resulting in improved mixing efficiency. Numerical simulations were conducted to compare these novel geometries with traditional T and cross junctions in terms of mixing index, droplet diameter, and eccentricity. The cross-T geometry exhibited the highest mixing index and produced the smallest droplets. For the flow rate ratio of 0.5, this geometry offered a 10% increase in the mixing index and a decrease in the droplet diameter by 10% compared to the T junction. While the T junction has the best mixing efficiency among traditional droplet generators, it produces larger droplets, which can increase the risk of contamination due to contact with the microchannel walls. Therefore, the cross-T geometry is highly desirable in most applications due to its production of considerably smaller droplets. The asymmetric cross junction offered a 8% increase in mixing index and around 2% decrease in droplet diameter compared to the conventional cross junction in flow rate ratio of 0.5. All novel geometries demonstrated comparable mixing efficiency to the T junction. The cross junction exhibited the lowest mixing efficiency and produced larger droplets compared to the cross-T geometry (around 1%). Thus, the novel geometries, particularly the cross-T geometry, are a favorable choice for applications where both high mixing efficiency and small droplet sizes are important.

Keywords: Droplet; Droplet-generator; Finite element method (FEM); Lab-on-a-chip; Microchannel; Microfluidics; Mixing efficiency; Mixing index; Two-phase.

Figure 1

The schematic illustration of the study. There are five different geometries investigated: (A) cross junction, (B) T junction, (C) asymmetric cross junction, (D) cross-T junction, and (E) T-cross junction.

Figure 2

Droplet generator geometries. The dimensions of the (A) cross junction, (B) T junction, (C) cross-T, (D) T-cross, and (E) asymmetric cross junction. The dimensions are in $\mu m$.

Figure 3

Validation of the simulations in different flow rate ratios. (A) Comparative analysis of the mixing index and (B) Simulation of the droplet formation and mixing for the flow rate ratio of 0.2. The continuous phase flow rate (Q_c) is $1\mu l/min$.

Figure 4

2D simulation of the droplet formation. Velocity distribution during the filling stage in (A) cross junction, (B) T junction, (C1) T junction of the T-cross, (C2) cross junction of the T-cross, (D1) cross junction of the cross-T, (D2) T junction of the cross-T, (E1) first junction of the asymmetric cross junction, and (E2) second junction of the asymmetric cross junction. The velocity ratio is 0.5, and color bars show the velocity magnitude in m/s.

Figure 5

Diameter and eccentricity variations of droplets with flow rate ratio. Comparative analysis of (A) droplet diameter and (B) eccentricity in various geometries. The total flow rate of the continuous phase is $10\mu l/min$.

Figure 6

Mixing efficiency within droplets. Mixing efficiency in the filling stage in different junctions: (A) cross junction, (B) T junction, (C1) cross junction of the cross-T, (C2) T junction of the cross-T, (D1) T junction of the T-cross, (D2) cross junction of the T-cross, (E1) first junction of the asymmetric cross junction, and (E2) second junction of the asymmetric cross junction. Continuous phase flow rate is $10\mu l/\text{min}$ and the flow rate ratio is 0.2 for all cases. The color bars demonstrate the relative dye concentration.

Figure 7

Mixing efficiency variations within droplets with flow rate ratio in different geometries. Comparative analysis of mixing in various Geometries. The overall flow rate of the continuous phase is $10\mu l/min$.