Microfluidic heat/mass transfer

T. Bayraktar, S.B. Pidugu, Characterization of liquid flow in microfluidic systems, Int. J. Heat Mass Transfer, 2006, 49, 815-824. 

R. -D. Chien, Hot embossing of microfluidic platform, Int Commun Heat Mass Transfer, 33 (5), 645-653, 2006. 

As Table 5 illustrates, compared to batchwise reactions the increased heat and mass transfer obtained within the microfluidic reactor afforded enhanced cis/trans selectivity for reactions employing alkynes and more generically a dramatic reduction in reaction time (cf. conventional batch reactions). 

In microfluidic-based systems, material is transported within microstructures (of typical dimensions of 10-500 pm) where separations, reactions, and other processes occur. Focus has been on the realization of the traditional separation techniques (electrophoresis, chromatography, isoelectric focusing, etc.) and reactions in the microchip format. The principles of separation, as in the conventional formats, are based on differences in mass and charge (thus mobility) and partitioning between phases. However, advantages associated with the small dimensions provide superior performance. For example, the higher surface to volume ratio arising from the smaller dimensions results in lower heat and mass transfer resistances and thus an improved performance. 

Fluid dynamics, heat transfer, and mass transfer in developing moderate Reynolds number flows (1 < Re < 1,000) in complex microfluidic geometries... 

Greater control over reactions. The diffusion paths for heat and mass transfer in microfluidic systems are very short, making such systems ideal candidates for heat- or mass-transfer-limited reactions. The surface-to-volume ratio of microscopic structures is very high. Thus, surface effects are likely to dominate over volumetric effects, increasing selectivity and yield. 

The Reynolds number in microreaction systems usually ranges from 0.2 to 10. In contrast to the turbulent flow patterns that occur on the macroscale, viscous effects govern the behavior of fluids on the microscale and the flow is always laminar, resulting in a parabolic flow profile. In microfluidic reaction systems, where the characteristic length is usually greater than 10 pm, a continuum description can be used to predict the flow characteristics. This allows commercially written Navier-Stokes solvers such as FEMLAB and FLUENT to model liquid flows in microreaction channels. However, modeling gas flows may require one to take account of boundary sUp conditions (if 10 < Kn < 10 , where Kn is the Knudsen number) and compressibility (if the Mach number Ma is greater than 0.3). Microfluidic reaction systems can be modeled on the basis of the Navier-Stokes equation, in conjunction with convection-diffusion equations for heat and mass transfer, and reaction-kinetic equations. 

These reactors are literally the new kids in town. The heat transfer problem that is so inhibitory in packed bed systems becomes a nuisance in the small sizes of the microfluidic reactors. The mass transfer problem can also be relinquished in the microdomains. The microdomains provide excellent flow control, as well as bring in the surface tension as an additional force field to inertial and viscous forces that we are used to dealing with so far. The surface and interface forces at this level of miniaturization become driving forces for better mixing domains. 

No smart solution is available to store the gaseous hydrogen used in miniature fuel cell (PEMFC) and hence DMFC is receiving enormous interest due to system simplicity. However, specialised air-breathing DMFC components have to be developed. New materials have to be developed in addition to optimisation of structure and operating conditions to take care of performance decay modes. New membrane/electrode assemblies appropriate for the microscale to be developed exploiting the enhanced heat and mass transfer on the microscale for improved performance, and developing microfluidic components for micro fuel cells. 

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