12
2 Active and Passive Micromixers
under pulsed flow conditions. The results indicated that mixing was significantly
improved using time pulsing compared to the steady flow in all tested micromixers.
In a recent study, Afzal and Kim [3] showed that a convergent-divergent channel
with sinusoidal walls (space-periodic) represented the most effective coupling with
pulsatile (time-periodic) flow. The mass fraction distributions show that the interfacial
area increases rapidly, and separates to produce discrete puffs of fluids for enhanced
mixing performance as shown in Fig. 2.1. Within two periods of the sinusoidal walls,
the micromixer with pulsing inputs showed a mixing efficiency of 92%.
The micromixers based on electrokinetic flow utilize fluctuating electric field for
effective mixing [4–6] as an alternative to a pressure-driven flow. The coupling of
electric field and ionic conductivity results in an electric body force which generates
flow instabilities, and enhances mixing. Oddy et al. [4] developed an electrokinetic
process to mix solutions for bioanalytical applications. The instability caused by
oscillating electroosmotic flow was used to stir fluid streams at Reynolds numbers
of order unity, which was defined using the channel depth and rms electroosmotic
velocity. They designed and fabricated two micromixing devices using electrokinetic
instability for rapid mixing of fluid streams. Figure 2.2 shows the development of
mixing with time after onset of the instability. After sufficient time, approximately
homogeneous fluorescence intensity indicating good mixing is seen in the mixing
chamber.
Magneto-hydrodynamics micromixers utilize a magnetic field and magnetic particles suspended in a fluid to enhance mass transport [8–11]. To improve the performance of a Y-shaped micromixer, Tsai et al. [9] and Fu et al. [10] used ferrofluid
(a liquid with a suspension of magnetic nanoparticles) and permanent magnets to
induce magnetoconvective flow in the channel. Hejazian et al. [11] used non-uniform
magnetic field, diluted ferrofluid, and hydrodynamic flow-focusing configuration
to improve mass transport in a microfluidic device shown in Fig. 2.3. The system
consists of a core stream and two sheath streams. A magnetoconvective secondary
flow occurred in the channel promoted the mass transport of the non-magnetic
fluorescent dye.
A mixing technique based on bubble-induced acoustic microstreaming principle
was developed by Liu et al. [12]. The proposed mixer included a piezoelectric disk
attached to a reaction chamber, which was designed in such a way that a desired
number of air bubbles with desirable size are trapped in the solution. Experiments
showed that steady circulatory flows were generated through vibrations by the sound
field on air bubbles resting on a solid surface, and rapid mixing was achieved
(Fig. 2.4). The time for complete mixing in a 100 μL chamber was significantly
reduced from hours (a pure diffusion-based mixing) to tens of seconds. Haeberle
et al. [13] introduced a novel active mixing concept using centrifugal force generated by a fixed rotating drive (also known as ‘player’) to pump and mix the liquid
educts as shown in Fig. 2.5. Fast mixing with high volume throughput was obtained
as a result of induced Coriolis force.
Brief descriptions of typical designs of active micromixers and their applications
can be found in reviews of Nguyen and Wu [14], Hessel et al. [15, 16] and Kumar
et al. [17].
2 Active and Passive Micromixers
under pulsed flow conditions. The results indicated that mixing was significantly
improved using time pulsing compared to the steady flow in all tested micromixers.
In a recent study, Afzal and Kim [3] showed that a convergent-divergent channel
with sinusoidal walls (space-periodic) represented the most effective coupling with
pulsatile (time-periodic) flow. The mass fraction distributions show that the interfacial
area increases rapidly, and separates to produce discrete puffs of fluids for enhanced
mixing performance as shown in Fig. 2.1. Within two periods of the sinusoidal walls,
the micromixer with pulsing inputs showed a mixing efficiency of 92%.
The micromixers based on electrokinetic flow utilize fluctuating electric field for
effective mixing [4–6] as an alternative to a pressure-driven flow. The coupling of
electric field and ionic conductivity results in an electric body force which generates
flow instabilities, and enhances mixing. Oddy et al. [4] developed an electrokinetic
process to mix solutions for bioanalytical applications. The instability caused by
oscillating electroosmotic flow was used to stir fluid streams at Reynolds numbers
of order unity, which was defined using the channel depth and rms electroosmotic
velocity. They designed and fabricated two micromixing devices using electrokinetic
instability for rapid mixing of fluid streams. Figure 2.2 shows the development of
mixing with time after onset of the instability. After sufficient time, approximately
homogeneous fluorescence intensity indicating good mixing is seen in the mixing
chamber.
Magneto-hydrodynamics micromixers utilize a magnetic field and magnetic particles suspended in a fluid to enhance mass transport [8–11]. To improve the performance of a Y-shaped micromixer, Tsai et al. [9] and Fu et al. [10] used ferrofluid
(a liquid with a suspension of magnetic nanoparticles) and permanent magnets to
induce magnetoconvective flow in the channel. Hejazian et al. [11] used non-uniform
magnetic field, diluted ferrofluid, and hydrodynamic flow-focusing configuration
to improve mass transport in a microfluidic device shown in Fig. 2.3. The system
consists of a core stream and two sheath streams. A magnetoconvective secondary
flow occurred in the channel promoted the mass transport of the non-magnetic
fluorescent dye.
A mixing technique based on bubble-induced acoustic microstreaming principle
was developed by Liu et al. [12]. The proposed mixer included a piezoelectric disk
attached to a reaction chamber, which was designed in such a way that a desired
number of air bubbles with desirable size are trapped in the solution. Experiments
showed that steady circulatory flows were generated through vibrations by the sound
field on air bubbles resting on a solid surface, and rapid mixing was achieved
(Fig. 2.4). The time for complete mixing in a 100 μL chamber was significantly
reduced from hours (a pure diffusion-based mixing) to tens of seconds. Haeberle
et al. [13] introduced a novel active mixing concept using centrifugal force generated by a fixed rotating drive (also known as ‘player’) to pump and mix the liquid
educts as shown in Fig. 2.5. Fast mixing with high volume throughput was obtained
as a result of induced Coriolis force.
Brief descriptions of typical designs of active micromixers and their applications
can be found in reviews of Nguyen and Wu [14], Hessel et al. [15, 16] and Kumar
et al. [17].