Chaotic advection micromixers

Other micromixers based on various principles have also been constructed. These principles include vortex [492], eddy diffusion [493-501,654,955], rotary stirring [502], turbulence [495,503], EK instability [504—506], chaotic advection [248,507-513], magnetic stirring [514], bubble-induced acoustic mixing [515], and piezoelectric actuation [516,517]. 

On the other hand, passive chaotic micromixers typically use complex three-dimensional twisted conduits fabricated in various substrates such as silicon [13], polydimethylsiloxane (PDMS) [14], ceramic tape [15], or glass [13] to create 3-D steady flow velocity with a certain complexity to achieve chaotic advection. T q)ical examples of the aforementioned two routes to achieve chaotic advection and mixing in LOC devices are presented in the following. 

Various active micromixers using 2-D time-dependent flow to achieve chaotic advection have been developed [16-19]. Since electroosmosis is very attractive for manipulating fluids in LOC devices, a chaotic electroosmotic stirrer developed by Qian and Bau [20] is described as an example to achieve chaotic advection and mixing by 2-D time-dependent electroosmotic flow. 

Passive micromixers rely on the mass transport phenomena provided by molecular diffusion and chaotic advection. These devices are designed with a channel geometry that increases the surface area between the different fluids and decreases the diffusion path. By contrast, the enhancement of chaotic advection can be realized by modifying the design to allow the manipulation of the laminar flow inside the channels. The modified flow pattern is characterized by a shorter diffusion path that improves the mixing velocity. In this section, an overview of the different types of passive micromixers is provided. Mixed phase passive micromixers can be categorized as ... 

Advection is the transport of a substance within a moving fluid. In the micromixers discussed above, advection generally occurs in the direction of the flow, hence it has no effect on the transversal transport of the substance. However, advection in other directions, so-called chaotic advection [110], can generate a transverse... 

Chaotic advection can be induced with a 2D alternatively curved microchannel (2D serpentine) [112, 113] or zigzag channel shape [111]. In the first case, the chaotic advection is induced in the curved microchannel by consecutive generation of Dean vortices (Fig. 11a). Typically such type of micromixer can provide an effective mixing only for high Re in the range of few hundreds. These micromixers are generally described using another dimensionless number, the Dean number De) ... 

Another interesting planar structure able to induce chaotic advection has been reported by Hong et al. [126] (Fig. lid). This micromixer comprised a modified tesla structure that redirected the streams, by exploiting the Coanda effect. The authors demonstrated an efficient mixing at relative low Re number Re < 10). 

Fig. 13 Micromixer combining SAR and chaotic advection approaches (a) Serpentine laminating micromixer (SLM) and (b) concentration contours along the mixers channels, (Reproduced from [131] by permission of The Royal Society of Chemistry), (c) Staggered overlapping crisscross micromixer (SOC p-mixer) and (d) corresponding cross-section view showing concentration profiles after flowing through two junctions (Adapted from [132] with permission. Copyright lOP Publishing)...

Mixing in micromixers relies primarily on molecular diffusion or chaotic advection (laminar chaos) mechanisms. As discussed above, the diffusive mixing effect can be improved by increasing the interfacial contact area between the different fluids and reducing the diffusion length between them. The use of unstable electrokinetic flow fields to achieve chaotic mixing effect can also be adopted. 

A passive micromixer is one of the microfluidic devices. It utilizes no energy input except the mechanism (pressure head) used to drive the fluid flow at a constant rate. Due to the dominating laminar flow on the microscale, mixing in passive micromixers relies mainly on chaotic advection realized by manipulating the laminar flow in microchannels or molecular diffusion with increasing the contact surface and time between the different fluid flows. 

Figure 7.8 Modification of mixing channel for chaotic advection at low Pe (a) slanted ribs, (b) slanted grooves [74, 75], (c) staggered-herringbone grooves [74, 75], (d) patterns on both top and bottom of the mixing channel, (e) groove pattern vertical to mail flow, and (f) one of patterns for surface modification in a micromixer with electrokinetic flows [80].

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