Transport Phenomena in Electrokinetically Generated Microflows

Sadeghi, Arman | 2012

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  1. Type of Document: Ph.D. Dissertation
  2. Language: English
  3. Document No: 45154 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Saeedi, Mohammad Hassan; Mozaffari, Ali Asghar; Chakraborty, Suman
  7. Abstract:
  8. This is a theoretical study on heat and mass transfer in electrokinetically driven microfluidic devices with particular consideration to the lab-on-a-chip (LOC) applications. Special attention is given to disclose the effects of the fluid rheology and the aspect ratio of the rectangular geometry on the surface reaction kinetics and the temperature rise due to the Joule heating. Along this line, two different viscoelastic constitutive equations namely PTT and FENE-P models along with the power-law viscosity model are being considered. The secondary aims also include exploring the influences of the fluid properties variations throughout the channel cross section on the hydrodynamic and thermal features as well as developing analytical solutions for the thermal features of electroosmosis under simplified circumstances. A finite difference based 3-D numerical simulation of the Joule heating effects reveals that the temperature variations throughout the channel cross section are negligible under the conditions that most LOC devices operate. This conclusion is further justified by an analysis of the temperature dependent effects revealing that the property variations throughout the channel cross section are not significant. Hence, simplified 1-D models are capable of accurately predicting the temperature distribution in LOCs. In addition, the evaluation of the entry length shows that, because of the small channel lengths encountered in LOC application, a thermally fully developed flow is hardly achieved. Accordingly, the maximum temperature rise may significantly be smaller than what is calculated based on a thermally fully developed premise. The fluid rheology effects on the temperature field are also found to be important only in the presence of a huge viscous heating parameter. In contrast to the temperature field, the mass concentration field is a strong function of the fluid type. The results reveal that the saturation time is completely affected by the fluid rheology and the relevant impact is increased significantly by increasing the Damkohler number or decreasing the dimensionless Debye-Hückel parameter. Moreover, it is observed that modeling the reactor geometry as a slit instead of a rectangular shape leads to the underestimation of the saturation time. The amount of the underestimation is dependent upon the Damkohler number. Last but not least, two analytical solutions are developed for thermal features of electroosmosis under both thermally fully developed and developing conditions. These solutions show that the temperature rise due to the Joule heating in micro heat sinks is not significant compared with that caused by external heating, thus justifying the cooling capabilities of electroosmosis.
  9. Keywords:
  10. Lab-on-a-Chip ; Microchannel ; Electroosmotic Flow ; Joul Heating ; Mass Transfer ; Electric Double Layer

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