6
1 Mixing at Microscale
electrical energy, pressure disturbance, and ultrasonic mixing can be used to stir
the fluids. Therefore, active micromixers provide higher mixing performance, while
passive micromixers offer the advantage of simple design and fabrication, and can
be easily integrated to a complex system.
Hardt et al. [12] categorized passive micromixers based on the hydrodynamic
principle employed, viz. chaotic advection, flow separation, hydrodynamic focusing,
and split-and-recombination of flows. The review aimed to provide the micromixer
designers to select the most favorable concept for a particular application. The
review by Nguyen and Wu [19] reported the development of micromixers based
on different working principles. Both active and passive types were discussed, but
the major emphasis was laid on the design and mechanism of passive micromixers.
A brief discussion on operating conditions, fabrication techniques and mixing
characterization was also included in the review.
Hessel et al. [20] conducted a detailed review on passive and active mixing principles and described the typical mixing element designs, methods for mixing characterization, and application fields. The review also discussed the mixing of gases
in microchannels. Kumar et al. [21] reported the operating ranges of passive and
active micromixers in terms of Reynolds and Peclet numbers using an extensive
literature survey, as shown in Fig. 1.5. Compared to the active counterpart, passive
micromixers were reported for wider ranges of Reynolds (0.001 ≤ Re ≤ 1000) and
Peclet (0.01 ≤ Pe ≤ 1000, 000) numbers. Micromixers designed for diffusive mixing
work well only at low Reynolds and Peclet numbers, but micromixers designed for
chaotic advection can be used for a wide range of Reynolds numbers.
1.3 Mechanism of Mixing
Micromixers can operate over a wide Reynolds number range [20, 21]. The
mixing mechanism and performance strongly depend on Reynolds number regime.
Figure 1.6 shows the variations of mixing index with Reynolds number for three
micromixers with different planar serpentine channels: square-wave, zig-zag and
curved channels. For low Reynolds numbers (Re < 1), mixing occurs primarily by
diffusion, which is dominated by the residence time of the co-flowing fluids in the
micromixer. As Reynolds number increases, the residence time of the fluids in the
mixer decreases, thereby reducing mixing index. After the mixing index reaches a
minimum around Re = 10, transverse flow starts to occur in the channel and the
mixing index increases rapidly with further increase in Reynolds number. This rapid
increase in mixing index is attributed to enlargement of fluid interface caused by
stretching and folding of fluid layers due to chaotic advection. The concentration
contours at Re = 10 show that the concentration layers are sharp and well aligned
in the microchannel, and the mass transfer takes place by diffusion at the interface
between the fluid layers of different concentrations. However, at Re = 50, the layers
in the channel no longer remain aligned due to stretching and folding of the fluid
interface.
1 Mixing at Microscale
electrical energy, pressure disturbance, and ultrasonic mixing can be used to stir
the fluids. Therefore, active micromixers provide higher mixing performance, while
passive micromixers offer the advantage of simple design and fabrication, and can
be easily integrated to a complex system.
Hardt et al. [12] categorized passive micromixers based on the hydrodynamic
principle employed, viz. chaotic advection, flow separation, hydrodynamic focusing,
and split-and-recombination of flows. The review aimed to provide the micromixer
designers to select the most favorable concept for a particular application. The
review by Nguyen and Wu [19] reported the development of micromixers based
on different working principles. Both active and passive types were discussed, but
the major emphasis was laid on the design and mechanism of passive micromixers.
A brief discussion on operating conditions, fabrication techniques and mixing
characterization was also included in the review.
Hessel et al. [20] conducted a detailed review on passive and active mixing principles and described the typical mixing element designs, methods for mixing characterization, and application fields. The review also discussed the mixing of gases
in microchannels. Kumar et al. [21] reported the operating ranges of passive and
active micromixers in terms of Reynolds and Peclet numbers using an extensive
literature survey, as shown in Fig. 1.5. Compared to the active counterpart, passive
micromixers were reported for wider ranges of Reynolds (0.001 ≤ Re ≤ 1000) and
Peclet (0.01 ≤ Pe ≤ 1000, 000) numbers. Micromixers designed for diffusive mixing
work well only at low Reynolds and Peclet numbers, but micromixers designed for
chaotic advection can be used for a wide range of Reynolds numbers.
1.3 Mechanism of Mixing
Micromixers can operate over a wide Reynolds number range [20, 21]. The
mixing mechanism and performance strongly depend on Reynolds number regime.
Figure 1.6 shows the variations of mixing index with Reynolds number for three
micromixers with different planar serpentine channels: square-wave, zig-zag and
curved channels. For low Reynolds numbers (Re < 1), mixing occurs primarily by
diffusion, which is dominated by the residence time of the co-flowing fluids in the
micromixer. As Reynolds number increases, the residence time of the fluids in the
mixer decreases, thereby reducing mixing index. After the mixing index reaches a
minimum around Re = 10, transverse flow starts to occur in the channel and the
mixing index increases rapidly with further increase in Reynolds number. This rapid
increase in mixing index is attributed to enlargement of fluid interface caused by
stretching and folding of fluid layers due to chaotic advection. The concentration
contours at Re = 10 show that the concentration layers are sharp and well aligned
in the microchannel, and the mass transfer takes place by diffusion at the interface
between the fluid layers of different concentrations. However, at Re = 50, the layers
in the channel no longer remain aligned due to stretching and folding of the fluid
interface.