Chaotic flow micromixers

In the preceding two sections, it was shown how to measure local micromixing intensity using intermaterial area distribution [H(log p)] and length scale distributions [H(logs)]. These tools are not directly applicable to 3D flows, because the resolution of the smallest length scales in 3D mixture structures is, to date, computationally prohibitive. In this section we present a predictive method for striation thickness distributions applicable to either 2D or 3D chaotic flows. 

The real power of using stretching computations to characterize chaotic flows lies in the fact that stretching is the link between the macro- and micromixing intensities in laminar mixing flows. In this section we describe the method for computing striation thickness distribution in our 3D example, the Kenics mixer. 

A micromixer suggested by Stroock et al., consisting of pressure flow in straight rectangular channels with ridges on one of the walls [Reprinted by permission from A. D. Stroock, S. K. W. Dertinger, A. Ajdari, I. Mezic, H. A. Stone, and G. M. Whitesides, Chaotic Mixer for Microchannels, Science, 295, 364-651 (2002).]... 

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... 

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)...

Chaotic mixing based on viscoelastic flow instability in rather simple microfluidic channel geometries was successfully demonstrated. These viscoelastic micromixers bypass the limitation of low Reynolds number in microfluidic flows and could potentially be implemented in a Lab-on-a-Chip platform with minimum requirements for design and fabrication. However, this type of micromixer is yet to be optimized. 

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. 

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