Microchannel serpentine

A 3D serpentine microchannel was fabricated on a Si-glass chip to enhance mixing by chaotic advection (see Figure 3.44). It was found that mixing in the 3-D channel was faster and was more uniform than in either a square wave channel or straight channel [477]. 

In their pioneering work, Jensen et al. demonstrated that photochemical transformation can be carried out in a microfabricated reactor [37]. The photomicroreactor had a single serpentine-shaped microchannel (having a width of 500 pm and a depth of 250 or 500 pm, and etched on a silicon chip) covered by a transparent window (Pyrex or quartz) (Scheme 4.25). A miniature UV light source and an online UV analysis probe were integrated to the device. Jensen et al. studied the radical photopinacolization of benzophenone in isopropanol. Substantial conversion of benzophenone was observed for a 0.5 M benzophenone solution in this microflow system. Such a high concentration of benzophenone would present a challenge in macroscale reactors. This microreaction device provided an opportunity for fast process optimization by online analysis of the reaction mixture. 

A coolant channel is guided through the metal block in a serpentine fashion so that reactant and coolant flows are orthogonal [272]. A thermocouple measures the temperature at the product outlet. This microreactor was developed for the (very fast) fluorination reactions with elemental fluorine. Therefore, the surface of the microchannel was inactivated by exposure to the increasing concentration of fluorine in nitrogen. 

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) ... 

Sundarsan et al. [95, 127] reported two improved 2D serpentine micromixers, namely the planar spiral micromixer [127] and the asymmetric serpentine micromixer (ASM) [95] (Fig. 11a). Both the micromixers were able to produce effective mixing at low Re number. The mixing enhancement was due to the symergistic effect of Dean and expansion vortices, where the latter were introduced by abrupt expansions of the microchannels. 

Liu R, Stremler M, Sharp K, Olsen M, Santiago J, Adrian R, Aref H, Beebe D (2000) Passive mixing in a three-dimensional serpentine microchannel. J Microelectromech Syst 9(2) 190-197... 

Figure 3 shows the electrokinetic motion of 2.2 pm-diameter polystyrene particles in a 50 pm wide serpentine microchannel under the application of a DC-biased AC electric field [4]. When particles are suspended in 1 mM phosphate buffer, they experience negative DEP and are focused to a stream along the channel centerhne as demonstrated in Fig. 3 a. This is because particles are deflected from the inner to the outer comer when they pass the alternating turns and their equilibrium position is along the channel centerline [5, 6]. If, however, the C-iDEP is too strong, particles will bounce between the two sidewalls [7]. In contrast, when particles are suspended in pure water, they experience positive DEP and line the sidewalls of the serpentine microchannel as demonstrated in Fig. 3b. This is because particles are pulled into the inner comers of every single turn where the electric field is much greater than the outer comers. Such distinct focusing phenomena can be utilized to continuously sort particles by size in a serpentine microchannel [4].

Zhu J, Canto RC, Keten G, Vedantam P, Tzeng TJ, Xuan X (2011) Continuous flow separation of particles and cells in a serpentine microchannel via curvature-induced dielectrophoresis. Microfluid Nanofluidics 11 743-752... 

Church C, Zhu J, Nieto J, Keten G, Ibarra E, Xuan X (2010) Continuous particle separation in a serpentine microchannel via negative and positive dielectrophoretic focusing. J Micromech Microeng 20 065011... 

Zhu J, Tzeng TJ, Hu G, Xuan X (2009) Dielectrophoretic focusing of particles in a serpentine microchannel. Microfluid Nanofluidics 7 751-756... 

Church C, Zhu J, Xuan X (2011) Negative dielectrophoresis-based particle separation by size in a serpentine microchannel. Electrophoresis 32 527-531... 

Helical microchannel flow Serpentine microchannel flow Spiral mierochaimel flow... 

Integrated Microdevices for Medical Diagnostics, Fig. 2 (Left) Schematic diagram of a blood typing biochip. The device contains flow splitting microchannels, a serpentine micromixer, reaction microchambers, and detection microfilters. The reaction chamber holds 3 pi... 

An example of OCT applications in microfluidics is the investigation of laminar dispersion in a serpentine microchannel with a Y-shape inlet (Fig. 3). Transient two-fluid mixing in microfluidic devices can be clearly observed (Fig. 4) [9]. 

Apart from the microchannel cross-section, the impact of non-straight charmels is crucial, especially for mixing applications, where serpentine channels have been shown to break symmetry and enhance mixing in bubbles [106]. Note that this enhanced mixing is due to chaotic advection in Stokes flow. Dean vortices, i.e. secondary flow patterns due to centrifugal inertia, are typically not a problem in low-inertia (Re< 1) microfluidic applications. 

P.E. Geyer, N.R. Rosaguti, D.F. Fletcher, B.S. Haynes, Thermohydraulics of square-section microchannels following a serpentine path. Microfluidics Nanofluidics, 2006, 2, 195—204. 

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