Theoretical and numerical studies of chaotic mixing are performed to circumvent the difficulties of efficient mixing, which come from the lack of turbulence in microfluidic devices. In order to carry out efficient and accurate parametric studies and to identify a fully chaotic state, a spectral element algorithm for solution of the incompressible Navier-Stokes and species transport equations is developed. Using Taylor series expansions in time marching, the new algorithm employs an algebraic factorization scheme on multi-dimensional staggered spectral element grids, and extends classical conforming Galerkin formulations to nonconforming spectral elements. Lagrangian particle tracking methods are utilized to study particle dispersion in the mixing device using spectral element and fourth order Runge-Kutta discretizations in space and time, respectively. Comparative studies of five different techniques commonly employed to identify the chaotic strength and mixing efficiency in microfluidic systems are presented to demonstrate the competitive advantages and shortcomings of each method. These are the stirring index based on the box counting method, Poincare sections, finite time Lyapunov exponents, the probability density function of the stretching field, and mixing index inverse, based on the standard deviation of scalar species distribution. Series of numerical simulations are performed by varying the Peclet number (Pe) at fixed kinematic conditions. The mixing length (lm) is characterized as function of the Pe number, and lm ∝ ln(Pe) scaling is demonstrated for fully chaotic cases. Employing the aforementioned techniques, optimum kinematic conditions and the actuation frequency of the stirrer that result in the highest mixing/stirring efficiency are identified in a zeta potential patterned straight micro channel, where a continuous flow is generated by superposition of a steady pressure driven flow and time periodic electroosmotic flow induced by a stream-wise AC electric field. Finally, it is shown that the invariant manifold of hyperbolic periodic point determines the geometry of fast mixing zones in oscillatory flows in two-dimensional cavity.

The goal of this research is to investigate computational and experimental techniques to effectively analyze microscale fluid dynamics, transport, and mixing of an analyte-antibody system. This work is applicable to the development of an in-plane, passive mixer component of a miniature antibody-based sensor suitable for environmental monitoring, food testing, and medical diagnostics. The computational methods allow the efficient evaluation of microchannel designs to enhance analyte-antibody binding, which may reduce the time and cost required for experimental trials. We describe a computational algorithm to solve the governing equations for microscale fluid flow and transport in complex 2-D domains created through a graphical user interface. We implement the particle strength exchange method to solve the convection-diffusion-reaction equations, coupled to the boundary element method to compute the velocity field from the steady state Stokes equations. We validate the numerical methods by comparison to analytical and finite element method solutions. Because the chosen methods require no internal mesh, our algorithm provides an efficient alternative to grid-based methods when solving transport in complex geometries with internal obstacles. We characterize two fluorescein-antibody clones through competitive ELISA experiments and demonstrate the quenching effect of the antibodies with a fluorescence spectrophotometer. We describe a microchannel flow system to image the quenching of fluorescence by the antibody when fluorescein and fluorescein-antibody solutions are injected into separate inlets of the microchannel. We correlate the fluorescence intensity of microscope images of fluorescein flowing through the microchannel to concentrations of fluorescein to establish a calibration curve. This system provides a method to visualize and quantitatively analyze the mixing and reaction in a microfluidic device. We test the numerical methods by comparing the experimentally determined fluorescein concentration to the outlet amount numerically predicted by the computational model under identical conditions and find good agreement between the two fluorescein concentration profiles. We complete the transport-reaction computation in a set of microchannels with cylindrical obstructions. We find that decreasing the channel width and increasing the fluid path length by placing the obstruction on the walls is more effective than placing free-standing obstructions within the channel to enhance the fluorescein and fluorescein-antibody reaction. / acase@tulane.edu

Abstract This thesis describes the design, fabrication and operation of a microfluidic device for the screening of biomolecule mixture surface mediated effects. The characterisation of a surface immobilisation strategy that will allow the robust attachment of candidate biomolecules on a substrate for use in cell culture applications. This is carried out in the form of a modified and optimised layer-by-layer surface immobilisation strategy and its subsequent thorough and robust characterisation. This was achieved by compiling and critically analysing large amounts of quartz crystal microbalance with dissipation (QCM-D) data and the model utilised to provide meaningful, physical data as an output. QCM-D data was combined with surface plasmon resonance (SPR) data to validate the assumptions used within the QCM-D model package. Further evidence demonstrating the presence of the multilayer, as described by QCM-D and SPR, is achieved using x-ray photoelectron spectroscopy (XPS). These results show that the multilayer surface is robustly attached to the substrate and consists of a large amount of water whilst being able to immobilise mixtures of four proteins. A custom protocol for fabricating these two layer devices was devised and is presented. Scale limitations have been overcome to provide mixing capabilities for large extracellular matrix molecules to be immobilised on the previously described, microfluidically generated surface immobilisation strategy. The optimisation and characterisation of the mixing within this microfluidic device, affected by the incorporated staggered herring bone mixer is also shown. Using dynamic force spectroscopy (DFS) along with a custom designed force curve data processing and analysis package, the spatial localisation of a mixture of four immobilised biomolecules was determined. The aim of this study was to compare the spatial localization of a mixture of four biomolecules created by; standard cell culture protocols (adsorbed from bulk onto tissue culture polystyrene) and a surface created via microfluidic deposition on top of a previously described surface immobilisation strategy. The design and robust application of this custom analysis package allows the definition of a “Barricade of Specificity” such that interactions between an antibody functionalised AFM tip and a surface composed of a mixture of proteins, to be categorised as either a “true” specific interaction, or a non-specific interaction. The application of this Barricade of Specificity thus allows the spatial localisation of four immobilized biomolecules to be determined with a large degree of accuracy as a result of the large rage of non-specific interactions surveyed and the strict definition of a valid rupture force. The final chapter details the application of the microfluidic platform to enable high throughput screening of the effects of extracellular matrix (ECM) molecules, singly and in combination, with regards to the effect on the expression of cell surface markers on umbilical cord blood (UCB) derived CD34+ cells. Careful selection of candidate ECM molecules, cytokine and oxygen concentration has resulted in little difference in the effect on UCB derived CD34+ cells differentiation state after seven days in culture. The major effect has been the maturation towards lymphocyte and leukocyte precursors. However, of the four ECM molecules tested individually, in binary and in quaternary combinations, osteopontin (Opn) and laminin (Ln) demonstrated differences compared to other surfaces tested. In order to further assess the effect of these protein surfaces on the cell surface marker expression of UCB derived CD34+ cells, further tests are warranted for increased periods of time to enable greater discrimination in marker expression and thus increase our understanding of the fundamental biology of this rare and clinically useful cell source.

Liposomes are versatile biocompatible and biodegradable carriers for a variety of medical applications. As the first nanoparticles, they have been approved for pharmaceutical use so far, and many liposome-based preparations are in clinical trials. Classical methods of liposome preparation represent potential limitations in technology transfer from laboratory to industrial scale. New, microfluidic techniques overcome these limitations and offer new possibilities for controlled, continuous preparation of liposomal particles in a laboratory and industrial scale. An important element in the development of new nanoparticle systems is their complex characterization and purification. In addition to the established chromatographic techniques, the Field flow fractionation technique, in particular the Asymmetrical flow Field-flow fractionation, is described. This relatively new technique in conjunction with the MALS/DLS/DAD-UV/dRI online detectors enables the purification and characterization of complex samples. The main advantage of this technique lies in the possibility of separation under native conditions, which plays an important role in the separation of biopolymers in particular. Separation in the “empty” channel then eliminates sample degradation due to unwanted interactions at the stationary phase-sample interface. The theoretical part of this thesis describes the possibilities of preparation, modification, and characterization of liposomal nanoparticles. For this purpose, optical methods based on dynamic light scattering, multi-angle dynamic light scattering and nanoparticle tracking analysis techniques are described, as well as a non-optical method using "particle by the particle" analysis, tunable resistive pulse sensing method. A separate chapter of the theoretical part is dedicated to the technique Asymmetrical flow Field-flow fractionation in connection with the above-mentioned detectors. Important results associated with this work are summarized in the attached scientific paper, together with the result summaries and the author's contributions.