MICROFLUIDIC MIXING OF NEWTONIAN/NONNEWTONIAN FLUID THROUGH CHARGED CHANNELS

Abstract

This thesis examines the viability of adding geometric changes and surface potential heterogeneity in micro and nano uidic systems to enhance the uid ow and mixing e ciency through consideration of various micro channel designs with integrated impediments along the walls. Any mechanical system's performance may be improved by using the smartest, most e cient design, which will also reduce system energy consumption and increase output at the lowest possible cost. To conduct chemical or biological analysis, samples and reagents must be completely mixed together. Moreover, in miniature Total Analysis Systems, where mixing plays a crucial role for system analysis, this is an essential ow component. As the characteristic length scale becomes small enough, ow driving by di usion process turns into an e ective way to create homogeneous solutions while scaling down the size of micro-devices. The major portion of this thesis deals with passive mixing, which has the bene t over active mixing in terms of simplicity and ease of manufacturing. Passive mixing uses geometric modulations with surface potential non-homogeneity in micro/nano channels by using electroosmotic

ow. The mathematical model comprise of the coupled set of non-linear equations consisting the Maxwell's equation for electric potential, the Nernst-Planck equation for ion transfer, and the Navier-Stokes equation for momentum transport. The ow regulating equations are numerically solved using a control volume-based technique. The fundamental concepts and several ways to solving the electrokinetic ow regulating equations are presented in Chapter 1. The electroosmotic ow augmentation of non-Newtonian power-law uids in modulated nanochannels with hydrophilic walls is discussed in Chapter 2. The channel walls have rectangular grooves oriented vertically with the external electric eld consisting EOF of power-law uid. Grooves can substitute hydrophobic strips to compare power-law uid ow enhancement factor, which are common in nano con nements. An e ective convection di usion-based mathematical model for species movement and mixing in di erent-shaped (nozzle, di user, nozzle-di user, di user-nozzle) micro/nano channels connecting to reservoirs is presented in Chapter 3. Mixing e ciency is increased as compared to a rectangular slit channel for various geometric designs when parallel reservoirs are joined to microchannel. The physical parameters, e.g., Debye-H uckel parameter, conical angles or slope, and reservoir height/ width a ect ion transport and mixing, resulting maximum mixing e ciency in the reservoir-linked nozzle microchannel without obstacles or heterogeneous zeta potential along the channel wall. In Chapter4, an external electric eld enhances mixing e ciency and electroosmotic ow across a hydrophobic structured microchannel with nozzle-di user shaped microchannel. The interfacial surface zeta potential is modi ed to provide a substantial convection e ect between two reservoir-injected uids over a broad range of ow parameters. The mixing e ciency is scaled with average pressure drop to prevent fatigue or mechanical failures in micro/ nano domain. Hydrodynamic slip increases

ow velocity and mobility, but heterogeneous zeta potential causes back ow that inhibits driving

uids from mixing e ciently. The electroosmotic ow of non-Newtonian viscoplastic uid (Hurschel-Bulkley model) in reservoir-connected nozzle and di user microchannel with sinusoidal channel wall constriction is discussed in Chapter 5. In the fully developed situation, electroosmotic ow in slit channels has a top-hat velocity pro le, although yield stress weakens it. The proposed model addresses the behavioral change in ow velocity and the decrement in

ow rate owing to the viscoplastic model. This model can be used as a benchmark model for the fabrication of Lab-on-a-chip devices to measure blood ow and drug mixing through stenosed arteries since blood is a Hurschel-Bulkley uid with yield stress 0 < 0:3 and arteries are considered in nozzle/di user shape with sinusoidal stenosis. Chapter 6 explores the numerical micromixing quality of an electro-osmotic micro-mixer with a nozzle-di user shaped channel linked to reservoirs at both ends and a micro-chamber at the center with an inner rectangular barrier. This chapter examines the mixing chamber design (circular and triangular), inner obstacle size, and ow within an electroosmotic micromixer to analyse mixing performance. The scopes of future work for the electroosmotic ow are covered in Chapter 7, where heated blocks of di erent shapes like rectangular, sinusoidal, and triangular are put at the surface of the microchannel to induce passive mixing. Additionally, the ideal settings will be discovered to reduce Joule heating and enhance overall system e ciency.