Date of Award

August 2021

Document Type


Degree Name

Master of Science (MS)


Mechanical Engineering

Committee Member

Xiangchun Xuan

Committee Member

Rodrigo Martinez-Duarte

Committee Member

Phanindra Tallapragada


Insulator-based dielectrophoresis (iDEP) has been increasingly used to focus, trap and separate particles and cells for various microfluidic applications in the fields of analytical chemistry, bioanalysis, cell biology and clinics etc. It exploits insulating structure(s) to create electric field gradients for dielectrophoretic manipulation of particles and cells in microchannels. However, iDEP has thus far been demonstrated to work with Newtonian fluids only in the majority of the reported applications. As many of the biological (e.g., blood, saliva, DNA solutions, etc.) and chemical (e.g., polymer and colloidal solutions) fluids exhibit non-Newtonian behaviors, it is important and necessary to understand how the fluid rheological properties may affect iDEP. This thesis is aimed to investigate experimentally the effects of fluid elasticity and shear thinning on the iDEP focusing and trapping of particles as well as the electroosmotic flow pattern in two types of microchannels.

In the first experiment we study the fluid rheological effects on iDEP focusing and trapping of polystyrene particles in polyethylene oxide (PEO), xanthan gum (XG) and polyacrylamide (PAA) solutions through a constricted microchannel. Such a geometry is the simplest while the most often used structure in iDEP microdevices. Particle focusing and trapping in the mildly viscoelastic PEO solution are found to be slightly weaker than in the Newtonian buffer. They are, however, significantly improved in the strongly viscoelastic and shear thinning PAA solution. These observed particle focusing behaviors exhibit a similar trend with respect to electric field, which is consistent with a revised theoretical analysis for iDEP focusing in non-Newtonian fluids. No apparent focusing of particles is achieved in the XG solution though the iDEP trapping can take place under a much larger electric field than the other fluids. This is attributed to the strong shear thinning-induced influences on both the electroosmotic flow and electrokinetic/dielectrophoretic motions.

In the second experiment we investigate the iDEP focusing and trapping of polystyrene particles in the same three types of polymer solutions in a post-array microchannel. Such a geometry has been frequently used to manipulate small particles with submicron and even nanometer sizes. The array of posts causes continuous changes in the non-Newtonian fluid properties. Similar electroosmotic flow pattern and slightly reduced particle focusing are observed in the PEO solution as compared to the Newtonian buffer, which is consistent with the observations in the constricted microchannel. The iDEP focusing and trapping of particles in the PAA solution are only available when the applied DC field is smaller than a certain threshold value. Both effects are, however, weaker than in the buffer solution, opposite to the observations in the constricted microchannel. This phenomenon may be associated with the elongation and relaxation of long PAA molecules as they are advected through the array of posts. Similar to that in the constricted microchannel, the XG solution does not exhibit an apparent iDEP effect on particles in the post-array channel. Interestingly, electroosmotic flow instability occurs under high DC electric fields in the post-array channel only.



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