An experimental study of pressure- and electroosmotically-driven flows in microchannels with surface modifications

Researchers investigating pressure-driven flows within microchannels have reported a discrepancy with classical theory. These discrepancies were attributed to various reasons; notably entrance effects, electroviscous effects, early transition to turbulence, surface roughness effects, etc. These measurements were all performed using bulk flow measurement techniques such as pressure drop and flow rate measurements. To investigate if deviations from classical theory do exist, the friction factor obtained from the current study was compared to classical theory for microchannels with a characteristic size of 300mum. The determination of the friction factor in this study differs from other researchers as it is derived from local measurements of the velocity profile using the Molecular Tagging Velocimetry (MTV) technique. This is the first time that a local measurement of the velocity profile has been used to derive the friction factor. Local measurements are advantageous as entrance effects do not affect it and it does not suffer from the high order dependence on diameter (D4) when relating pressure drop to volume flow rate. The microchannels used in this study were coated with polymer brushes (HEMA) and octadecyl-trichlorosilane (OTS) to investigate if the added hydrophobicity of these coatings would affect the pressure-driven flow.; The second part of this study relates to electroosmotically-driven flows within microchannels. Electroosmotically-driven flows are of interest as pressure-driven flows become increasingly ineffective in transporting fluid due to the increase in surface to volume ratios in smaller microchannels. The electroosmotic velocity's dependence on temperature and the applied electric potential was studied using a simultaneous velocity and temperature measurement technique (Molecular Tagging Velocimetry & Thermometry, MTV&T). The estimation of the electroosmotic velocity based only on the Helmholtz-Smoluchowski equation was found to be inadequate due to the presence of Joule heating and showcases the need for a simultaneous local measurement of both the velocity and temperature to resolve the flow physics. This local measurement technique is also used to obtain local zeta potential (zeta) measurements of the different surfaces. Zeta potential information is important in designing microfluidic devices to improve mixing characteristics within microchannels. An added capability of the measurement technique to reveal zeta potential dependence on temperature is also presented. To further our understanding of the spatio-temporal evolution of temperature within the microchannel during electroosmosis, a numerical simulation (FLUENT) was employed with boundary conditions similar to our experimental studies.