Fluids in microchannels

In addition to the typical thermal and pressure forces exerted on fluids in microchannels, acoustic/ultrasonic and electric held forces have also been applied to manipulate huids as a body - referred to as body force driven processes. 

Capillary force valves are fluid control structures that use superficial tension at the interface between different fluids to block and/or restore the entrance of fluids in microchannels filled with a second immiscible fluid. For most of the microfluidic applications, the second fluid is air, and the liquid-air interface at a narrow hydrophobic stricture is used to prevent the liquid from entering a capillary. 

Electrokinetics is essentially the consequence of a coupling between electrostatics and hydrodynamics. Newtonian hydrodynamics is widely assumed for the classic description of electrokinetics. However, practical applications of electrokinetics frequently deal with biofluids (such as solutions of DNA, blood, and protein, polymeric solutions, and colloid suspensions) which all are complex fluids and therefore demonstrate non-Newtonian behaviors. Recently intensive efforts on electrokinetics of non-Newtonian fluids have been made after Das and Chakraborty [1] who pioneered a theoretical analysis of electroosmosis of non-Newtonian fluids. Here in this entry the example of electroosmosis of non-Newtonian fluids in microchannels is used to demonstrate the fundamental formulation of non-Newtonian electrokinetics. 

Zhao C, Yang C (2011) An exact solution for electroosmosis of non-Newtonian fluids in microchannels. J Non-Newtonian Huid Mech 166(17-18) 1076-1079... 

Berli CLA, Olivares ML (2008) Electrokinetic flow of non-Newtorrian fluids in microchannels. J Colloid Interface Sci 320(2) 582-589... 

Electrokinetics is currently the preferred method for moving and transporting fluids in microchannels due to the ease of electrode fabrication and since electrokinetic mechanisms involve no moving mechanical parts which are... 

In a more recent study. Das and Chakraborty [9] presented analytical solutions for velocity, temperature, and concentration distribution in electroosmotic flows of non-Newtonian fluids in microchannels. A brief description of their transport model is summarized here, for the sake of completeness. A schematic diagram of the parallel plate microchannel configuration, as considered by the above authors, is depicted in Fig. 2. The bottom plate is denoted as y = H and top plate as y = +H. A potential gradient is applied along the axis of the channel, which provides the necessary driving force for electroosmotic flow. The governing equations appropriate to the physical problem are the equations for conservation... 

Non-Newtonian Fluids in MicroChannel, Fig. 2 Schematic diagram depicting a parallel plate microchannel... 

Electroviscous effects on fluid flow for ionic fluids in microchannels have been evidenced over the last decade experimentally [7-9] and are still a subject of widespread theoretical research [4, 10]. Ren et al. [5] found... 

The effect of the viscous heating of fluids in microchannels is significant especially for hydraulic diameters less than 200 pm. In this case, further investigations on the combined... 

In this chapter, heat transfer from or to a fluid in microchannels with diameters of less than 1 mm was discussed. It was seen from the literature that correlations for Nusselt numbers in microcharmels show little agreement whether heat transfer is enhanced or decreased at the microscale. The findings were often restricted to the individual setup used in a particular study. This leads to the conclusion that so far, no general correlation for heat transfer coefficients in microchannels can be suggested. The comparison with heat transfer analysis in macroscopic systems... 

Elastic turbulence One final topic we will mention here before closing the chapter is the elastic turbulence. The term describes the turbulence caused by the elastic forces arising in non-Newtonian fluids. In the situations where mixing is needed, the elastic forces in a non-Newtonian fluid can be used to create turbulence. The analysis by Joo and Shaqfeh (1992) is particularly useful for the flow of non-Newtonian fluids in microchannels but is beyond the scope of the text here. 

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