The multilevel microfluidic module for the generation of uniform fine droplets. (a) A schematic view of the two levels of the module. Geometric channels in each level bifurcate and equally distribute lipid solution and liquid perfluoropentane to the eight hydrodynamic flow-focusing units of the radial array. (b) Overhead view of the assembled PDMS module relative to a quarter. (c) Side view of the module relative to the quarter.
Geometry of the multi-array module. Bottom and top levels are aligned using transitional spacers and bonded via air plasma treatment. Overall footprint of the module measures 20.9 × 29.9 mm. Channels are rectangular with a height of 25 μm. Droplets are generated into a central collection reservoir, denoted here by the star (*). (inset) Image of a flow-focusing region. Lipid solution and liquid perfluoropentane distribution channels measure 38 μm in width and direct flows to 8.1 ± 1.4 μm orifices, succeeded by a post-orifice channel 30 μm in width.
Fabrication process of the parallel module. Coating the module with a 5:1 ratio of PDMS prepolymer to curing agent, and baking overnight, acts as a sealant to accommodate the elevated flows required to access the dripping regime.
Geometry-controlled vs. dripping droplet formation in the module. An elevated capillary number is necessary to transition to the dripping regime, modulated by the superficial velocity (e.g., flow rate) of the continuous lipid phase. A protrude-and-retract mechanism of the dispersed phase finger characterizes the geometry-controlled mode, whereas in dripping droplets break off the end of the dispersed phase tip, which remains at a fixed location in the orifice, due to Rayleigh capillary instability. Images are successive in time; image height is 156.25 μm.
Generation images for droplets of liquid perfluoropentane formed in the geometry-controlled and dripping regimes. In the dripping regime, per-channel generation rate dramatically quickens (to 4.61 × 104 s−1 per orifice, or 3.69 × 105 s−1 overall) and droplet diameter decreases (to 9.8 ± 1.0 μm). Scale bar is 50 μm.
Size distributions of droplets of liquid perfluoropentane in geometry-controlled mode and dripping, over the eight flow-focusing units of the multi-array module. Each unit generates quite uniform droplets. In geometry-controlled mode, QL = 100 μl min−1, PP = 22.5 PSI, Dp = 19.6 ± 1.1 μm, σp = 5.6%, σmean = 2.3%, fp = 9.50 × 104 s − 1. In dripping, QL = 180 μl min−1, PP = 33 PSI, Dp = 9.8 ± 1.0 μm, σp = 10.0%, σmean = 5.6%, fp = 3.69 × 105 s−1. Unit 7, the outlier, contributed the peak at 12.2 μm in dripping; ignoring the data from this unit, Dp = 9.6 ± 0.6 μm, σp = 6.1%, σmean = 5.7%, fp = 3.35 × 105 s−1.
Comparison of microfluidic scale-up devices using droplet volume and frequency of generation as metrics of merit. Data points 1–4 represent past shear-based systems; these systems generate droplets in the nanoliter range for applications in the food industry, high-throughput screening, and bioassays. This work forms a cluster with data points 5–7 (current MCE chips) in the sub-picoliter region for such applications as in vivo therapeutics and next generation sequencing. Our parallel module operated in the dripping regime generates droplets at a higher rate than past shear-based systems and reduces droplet volume by 3-log. 1—Mulligan and Rothstein (2012). 14 2—Nisisako and Torii (2008). 15 3—Zeng et al. (2010). 16 4—Romanowsky et al. (2012). 17 5—Kobayashi et al. (2008). 19 6—Kobayashi et al. (2010). 20 7—van Dijke et al. (2009). 21
Generation data for droplets of liquid perfluoropentane formed in the geometry-controlled and dripping regimes. The transition to the dripping regime occurred at a lipid phase flow QL ∼ 160 μl min−1.
Detailed generation data for droplets of liquid perfluoropentane formed in the dripping regime (QL = 180 μl min−1, PP = 33 PSI). Unit 7 emerges as the outlier in the data.
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