Electroosmotic flow hysteresis and effect of nanostructure orientation in a microfluidic channel

• Main Author: Lim, An Eng
• Other Authors: Lam Yee Cheong
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Mechanical engineering
• Online Access: http://hdl.handle.net/10356/74219
• Institution: Nanyang Technological University

Electroosmotic flow (EOF) has been exploited in a wide range of microfluidic applications. Most of these applications require the driving fluid to be inhomogeneous, i.e. differing in either concentration or conductivity. There is an experimentally observed anomaly in the EOF of two fluids with different concentrations, whereby the flow behavior is found to be direction-dependent. This observation is termed as “EOF hysteresis”. The conventional theoretical models for EOF contain critical limitations for describing the hysteresis phenomenon. Thus, a generalized theoretical model was formulated in this thesis for a proper description of the two-fluid displacement flow process, with appropriate simplifications for specific experimental conditions to ease the demand of computational effort when implemented for numerical simulation. The abovementioned EOF hysteresis was demonstrated numerically to originate from the accumulation/depletion of minority pH-governing ions, such as the hydronium ion (H3O+), as a result of the imbalanced electric-field-induced flux at the two-fluid interface, which subsequently widened and spread during the displacement flow process. The resultant pH changes caused the variations of zeta potential and EOF flow rate that gave rise to the hysteretic behavior. To provide conclusive evidence on pH changes in EOF hysteresis, direct experimental observation of the pH changes and their quantifications for comparison with the numerical simulations were performed. The experimental results showed good quantitative agreements with the simulation results. Hitherto, no investigation has been performed to examine the flow direction-dependent behavior during electroosmotic displacement flow of solutions with dissimilar ionic species. Hence, EOF hysteresis involving solution pair with dissimilar cation or anion species was investigated in this thesis, both experimentally and numerically. Two different mechanisms had been identified as the causes of EOF hysteresis for dissimilar cationic solutions: (a) widening/sharpening effect of the interfacial region generated by conductivity difference between the two solutions, and (b) difference in average zeta potentials caused by the concentration adjustments in different flow directions. For the investigation of EOF hysteresis involving dissimilar anionic solutions, the displacement flow was discovered to exhibit an unusual behavior, for which the equilibrium concentration in the microchannel deviated from the initial displacing electrolyte. This was found to be caused by the ion concentration adjustment when the displacing anions migrated in opposition to EOF, and a second ion concentration adjustment due to the diffusive interface at the junction between the anode reservoir and microchannel which was convected throughout the entire channel by EOF. The resultant ion distributions (thus difference in average zeta potentials) led to the direction-dependency of the EOF flow rate. This investigation reveals that an in-depth understanding of EOF hysteresis for solutions with different concentrations and ionic species is important to the precise manipulation of fluids and analyte transports in microfluidic applications, where the fluids involved are typically inhomogeneous. Nanoscale structures are usually incorporated within a microchannel for various applications. Even though there are numerous investigations on EOF reduction due to the presence of nanostructures in microchannels, a proper study on the orientation effect of nanostructures on EOF has never been conducted. This thesis presents a novel fabrication method for microchannels with nanostructure designs that have significant orientation difference, i.e. parallel versus perpendicular indented nanolines. The fabrication process consisted of four phases: fabrication of silicon master, creation of mold insert via electroplating, injection molding with cyclic olefin copolymer (COC), and thermal bonding and integration of practical inlet/outlet ports. The effect of nanostructure orientation on EOF in a microfluidic channel was examined both experimentally and numerically. The experimental results showed that the perpendicular nanolines significantly reduced the EOF velocity (by approximately 20%). The numerical simulation revealed that the flow velocity reduction was due to the local electric field distortion at the nanostructured surface. In contrast, the parallel nanolines had no effect on the EOF, as it was deduced that the parallel nanolines did not result in distortion of the electric field. The outcomes of this investigation enhance the fundamental understanding of the effect of nanostructures on EOF behavior. These have implications on the precise EOF control in devices utilizing nanostructured surfaces for chemical and biological analyses.