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Schematic of microfluidic chip. (a) Schematic of the whole chip. (b) The image of flow-focusing region with orifice. (c) Schematic of orifice-void and orifice-included T-junction channels. Scale bar was 50 μm.
A single period of droplet splitting at the first T-junction channel, scale bar was 50 μm. (QW = 10 μl/h, QOil = 10 μl/h).
(a) Influence of flow rate on mother and daughter droplet volumes. The critical QOil/QW value at which mother droplets were too small to split was 4.4. At the critical QOil/QW, the volume of the mother droplet was about 71.1 pl, and the volume of the daughter droplet split in the side channel was about 6.5 fl (10−15 L). (b) Influence of flow rate on daughter droplet volume in the three side channels (the orifice-void T-junction channels). (c) The schematic of electric circuit which was an analog of the flow circuit. P0 was grounded.
Influence of geometry conformation on droplet splitting. (a) Four chips with different geometry conformations: orifice-included chip (Chip I), orifice-void chip (Chip II), side channel of 50 μm width chip (Chip III), and main channel of 100 μm width chip (Chip IV), scale bar was 50 μm. (b) Influence of geometry conformation on daughter droplet volume (the first side channel). It revealed the introduction of orifice and the decreased relative resistance of main channel to side channel facilitated the generation of femtoliter daughter droplet.
Influence of surface tension on droplet splitting (scale bars were 50 μm in insets (a)-(c), scale bar was 5 μm in inset (d), and scale bars were 2 μm in insets (e) and (f)).
(a)-(c) Influence of bead concentration and droplet volume on the distribution of multi-bead sampling and single-bead sampling in droplets. They showed the interference of multi-bead sampling to single-bead sampling was low in 100 fl droplets at the concentration of 1.00 × 109 beads/ml. And it revealed femtoliter droplets are more suitable for single bead sampling and digital analysis than picoliter droplets. (d) Fluorescent bead sampling from droplet splitting at the concentration of 2.85 × 109 beads/ml. Femtoliter droplets containing single bead (white arrow) and multiple beads (red arrow) were pointed out respectively. (e) Theoretical λ value and experimental λ value were plotted as a function of bead concentration in 100 fl droplets. It revealed there was no significant difference between experimental λ value and theoretical λ value. (f) Influence of droplet volume on the measured concentration at the concentration of 1.00 × 109 beads/ml. The experimental λ value was analyzed from about 1000 droplets at each volume.
Four different channel geometries (“+” orifice-included chip, “−” orifice-void chip).
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