Date of Award

7-2008

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Mechanical Engineering

Advisor

Xuan, Xiangchun

Committee Member

Beasley , Donald

Committee Member

Figliola , Richard

Abstract

New micro-fabrication techniques have allowed for the recent development of micro-scale lab on a chip devices which can be utilized for on chip biological analysis as well as electrokinetic energy conversion. This thesis aims to theoretically examine the current thermodynamic efficiency discrepancies in electrokinetic energy conversion devices as well as explore the potential for efficiency increase in a device which supports fluid slip. Also, a micro-fluidic lab on a chip device is fabricated and demonstrated to be capable of continuously separating multiple particles of 1μm, 5μm and 10μm out of a bulk solution. Our device employs a DC dielectrophoretic technique, which previously has only been shown to be capable of separating binary mixtures. A thermoelectrohydrodynamic model is developed to analytically account for the effects of Stern layer conductance and fluid slip on electrokinetic energy conversion in nanofluidic channels. For both cases the optimum electrokinetic devices performance is dependent on a non-dimensional figure of merit. The figure of merit is defined in terms of three phenomenological coefficients which characterize the hydrodynamic conductance, streaming effects and electrical conductance. Stern layer conductance is found to significantly reduce the figure of merit and thus the efficiency and power output. This finding may explain why the recently measured electrokinetic device performances are far below the theoretical predictions. Our results also show that nanochannels with higher zeta potentials are more favorable to electrokinetic energy conversion due to the effects of Stern layer conductance, which is contrary to the previous understanding.
To consider the effects of employing a slip nanochannel for energy conversion, we apply Naviers's slip boundary condition to our model and re-examine the phenomenological co-efficients. The phenomenological coefficients are all enhanced by the fluid slip as compared to those without slip. The net result is an increased figure of merit and thus an enhanced electrokinetic devices performance, particularly in nanochannels with a high ratio of slip length to channel height.
We also successfully demonstrate that DC dielectrophoresis (DEP) can be utilized to separate particles of diameters 1μm, 5μm and 10 μm in a micro-fluidic chip with an applied electric field. The electric field is created within the chip by applying a specific combination of voltages at 5 separate locations. Furthermore, this field is manipulated by the presence of an insulating hurdle within the channel. As the particles pass through the hurdle area and thus the distorted region of the electric field, they are separated into distinct streams by a negative DEP force, which scales with particle size. Once the particles are separated, they flow into respective branches.

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