Micromixers based on chaotic advection - Micromixers Fundamentals Design and Fabrication - page 234

If the application requires a single phase of the mixed fluid, the two immiscible phases need to be
separated. One of the drawbacks of droplet-based micromixers is the difficult separation of the carrier
fluid and the mixed fluid due to the lower interfacial tension. The higher surface tension of a liquid/gas
system would allow easy separation by capillary effect. G ¨ nther et al. replaced the immiscible oil by
a gas phase [46] . The liquid phase is separated by gas bubbles. Mixing in a liquid plug occurs in the
same manner as discussed above. Curved channels induce chaotic advection and improve mixing
significantly. The large surface tension of the gas/liquid system allows the design of a capillary
separator at the end of the micromixer. A single mixed liquid can be collected ( Fig. 6.40 (a)).
Garstecki et al. [47] introduced the loop design shown in Fig. 6.40 (b). The curved sections of the
loops also cause chaotic advection similar to the design shown in Fig. 6.40 (a). However, due to
increased fluidic resistance in the branch with the air bubble, the pattern of the bubbles becomes
chaotic, leading to further improvement of mixing inside the liquid plug.
Besides the passive formation process described above, droplets can be generated and transported
actively by hydrodynamic force [48,49] or surface effects, such as thermocapillary [50] and electro-
wetting [51] . Hosokawa et al. [48] reported the earliest droplet-based micromixer, which was fabri-
cated in PDMS. The concept utilized a hydrophobic microcapillary vent, which joins the solute and the
solvent droplet.
Paik et al. reported different droplet-based mixing schemes with the electrowetting concept [51] .
The active actuation allows droplets to be merged and split repeatedly. Subsequently, the merged
droplet can be transported with different motion patterns to induce chaotic mixing. Fig. 6.41 (a) depicts
the device concept. The aqueous droplet is surrounded by immiscible oil. The droplet is aligned with
FIGURE 6.41
Droplet-based micromixer based on electrowetting on dielectric: (a) device concept; (b) droplet dispensing;
(c-e) droplet merging.