Thesis (M.S.)--Oregon State University, 2011. / Printout. Includes bibliographical references (leaves 106-107). Also available on the World Wide Web.

A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy Johannesburg, 2019 / This work aimed to investigate dynamics involving mass transport in droplet flowing in microchannels system using numerical modelling and simulation. Droplet-based microfluidics or droplet microfluidics is a branch of microfluidics that deals with generation, manipulation and control of droplets in microchannels. Droplets flowing in microfluidic channels are effective self-contained micro-reactors for use in biological and chemical applications. The ability to generate multitudes of droplets with narrow size distribution and to control reagent volumes within individual droplets, allows for parallelisation of chemical processes in microfluidic channels. Droplet microfluidics is, thus, an exceptional tool for manufacturing and analysis in biological, chemical and nanotechnology applications. Abstract Mixing processes in droplet microfluidics often involve three-way coupled physics of two-phase flow and mass transport convection and diffusion of chemical species, governed by a set of partial differential equations which require simultaneous solution. For two-phase flow, the equations for the movement and evolution of the interface are coupled to the Navier-Stokes of flow. In the case of transport of chemical species, the scalar mass transport equation is coupled to two-phase flow. The effects experienced in these systems are both multiscale and multiphase. Finite Element and Level Set simulations have been investigated and validated for modelling mass transport in droplet microfluidics systems. A set of benchmark cases has been developed for the purpose. Using Finite Element and Level Set simulations, a 2D two-phase moving-frame-of-reference modelling approach has been introduced and has been demonstrated to be an appropriate technique for investigation of mixing within droplets travelling in straight microchannels. This approach had not been previously demonstrated for the problem of mixing in droplet microfluidics, and requires less computational resources compared to the fixed frame-of-reference approach. Key conclusions of this work are A limitation of the method exists for flow conditions where the droplet mobility approaches unity due to the moving wall boundary condition which results in an untenable solution under those conditions. Abstract. As the size of the plug increases , the efficiency of the mixing is reduced.The initial orientation of the droplet influences the mixing and the transverse orientation provides better mixing performance than the axial orientation. The recirculation inside the droplet depends on the superficial velocity and the viscosity ratio. / E.K. 2020

Paper-based microfluidic devices have been a promising device platform for point-of-care diagnostic applications. It is known for its cost-effective and simple nature. Various analytes, such as metabolites, electrolytes, and pathogens, have been quantified in various applications. One of the key advantages of paper-based devices is the inherent property of capillary flow, which allows for the transport of fluids without the need for external pumping. However, since their discovery, applications of paper-based microfluidics have become increasingly complicated. Using expensive materials or complex surface modification to push the device's performance to its limits. Many have lost sight of what made paper-based microfluidic devices attractive in the first place. Despite these advances and recently reported devices having extremely low sensitivity, none have been commercialized, like the first testing strips. This thesis presents a novel material that can mimic the properties of paper. The proposed material can be easily printed onto carbon electrodes to form a printed microfluidic device. The fabrication process is cheap and scalable for mass production. Unlike existing devices, the proposed material allows for a completely new area of design and optimization. This technology successfully demonstrated that it could similarly transport fluids to paper and the electrochemical detection of glucose. The results show that the printed microfluidic devices could detect glucose with high sensitivity and low detection limits, making them potentially useful for medical diagnostics. The material was also integrated onto commercially available screen-printed electrodes and shown to improve performance. The composition of the material is flexible and capable of being tuned for specific needs. For example, semi-conductive microparticles can be integrated for an improvement in electrochemical performance. The results show that the printed microfluidic devices offer a cost-effective, easy-to-produce and reliable alternative to current methods, with potential application in point-of-care testing. / Thesis / Master of Applied Science (MASc) / This thesis presents a novel material that can mimic the properties of paper. The proposed material can be easily printed onto carbon electrodes to form a printed microfluidic device. The fabrication process is cheap and scalable for mass production. The material can transport fluids via capillary action much like paper can, and can also handle glucose detection using electrochemical methods. The results show that the printed microfluidic devices could detect glucose with high sensitivity and low detection limits, making them potentially useful for medical diagnostics. This technology, also successfully integrated other microparticles into its composition to boost its electrochemical performance and improve its sensitivity. This novel material offers a cost-effective, easy-to-produce, and reliable alternative to current methods, with potential application in point-of-care testing.

Droplet microfluidics generates and manipulates microdroplets in microfluidic devices at high manufacturing efficiency and controllability. Microdroplets have proven effective in biomedical applications such as single-cell analysis, DNA sequencing, protein partitioning and drug delivery. Conventionally, a series of aqueous microdroplets containing biosamples is generated and controlled in an oil environment. One of the critical challenges in this system is that recovery of the aqueous samples from the oil phase is very difficult and often requires expensive and cumbersome post-processing. Also, the low Reynolds (Re) number characteristic of this system results in low throughput of droplet generation. To circumvent challenges and fully utilize microdroplets for practical clinical applications, this research aims to unpack the fundamental physics that governs droplet generation in oil-free systems including an aqueous two-phase system (ATPS) and a high inertial liquid-gas system. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2019. / FAU Electronic Theses and Dissertations Collection

Includes bibliographical references (p. 94-108).

This thesis presents the development of a digital microfluidic system to achieve automated sample preparation for the vitrification of mammalian embryos for clinical in vitro fertilization (IVF) applications. This platform included micro devices fabrication, an imaging system, a high voltage control system, and a LabVIEW interface. Individual micro droplets manipulated on the digital microfluidic device were used as micro-vessels to transport a single embryo through a complete vitrification procedure. The device showed cell survival and development rates of 77% and 90%, respectively, which are comparable to the control groups that were manually processed. Technical advantages of this approach, compared to manual operation and channel-based microfluidic vitrification, include automated operation, cryoprotectant concentration gradient generation, and feasibility of loading and retrieval of embryos.

This paper will discuss the design, fabrication, and testing of a Poly(dimethylsiloxane) (PDMS) microfluidic pump. PDMS is commonly described as a soft polymer with very appealing chemical and physical properties such as optical transparency, low permeability to water, elasticity, low electrical conductivity, and flexible surface chemistry. PDMS microfluidic device fabrication is done easily with the use of soft lithography and rapid prototyping. PDMS microfluidic devices make it easier to integrate components and interface devices with particular users, than using typically harder materials such as glass and silicon. Fabrication and design of single and multilayer PDMS microfluidic devices is much easier and straightforward than traditional methods. A novel design of a PDMS micropump with multiple vibrating membranes has been developed for application in drug delivery and molecule sorting. The PDMS micropump consists of three nozzle/diffuser elements with vibrating membranes, which are used to create pressure difference in the pump chamber. Preliminary analysis of the fluidic characteristics of the micropump was analyzed with ANSYS to investigate the transient responses of fluid velocity, pressure distributions, and flow rate during the operating cycle of the micropump. The design simulation results showed that the movement of the wall membranes combined with rectification behavior of three nozzle/diffuser elements can minimize back flow and improve net flow in one direction. To prove that the theoretical design is valid, the fabrication and testing process of the micropump has been carried out and completed. This paper will discuss in depth the design, fabrication, and testing of the PDMS micropump.

"Microfluidic technologies are popular for biological research, enabling precise physical and chemical control of the microenvironment surrounding living cells and small organisms. Caenorhabditis elegans, a 1 mm long nematode, is capable of olfactory associative learning using the classical conditioning paradigm of pairing an unconditioned stimulus that elicits an innate response, such as food, with a second stimulus, such as an odor, which then elicits a learned behavioral response to this conditioned stimulus alone. Conventional chemotaxis assays on agar petri-plates have been widely used to observe behavioral changes indicative of associative learning; however, reproducibility of these behavioral assays is a major challenge. Here, we describe a microfluidic system that improves the reproducibility of chemotactic behavioral assays by providing better spatiotemporal control of stimuli, gentler worm handling, and more detailed behavioral quantification. Specifically, the microfluidic designs in this study present a uniform conditioning environment followed by a temporally stable linear odor gradient to assess changes to olfactory preference. Stimuli are presented in an enclosed environment to multiple worm populations whose locomotory patterns are analyzed using machine vision. Furthermore, we established an optimized protocol for a positive associative learning paradigm in which animals increase their preference for an odorant, butanone, when previously paired with bacterial food. We reproduced plate-based learning results in wild-type and learning-deficient genetic mutant animals, and demonstrated how developmental stages and chemicals alter the plasticity of olfactory preference. "

Microfluidics is the science and technology of manipulating fluids at small scales ranging from microlitres (10$^{-6}$) to picolitres (10$^{-12}$). The fundamental physics is distinct from fluid behaviour on bulk scales and laminar flow is the key characteristic on this scale. Microfluidic systems have a wide range of applications in many disciplines from engineering to physics, chemistry and biotechnology. In this thesis, I explore different strategies exploiting the capabilities of microfluidic devices for manipulating and analysing biomolecules. A particular focus of the work is on the study of amyloid fibrils. These species are protein aggregates related to a wide range of human diseases and functional materials. In chapter 3 I demonstrate an efficient way to separate particles in different sizes based on a microfluidic diffusion method. This method enables us to explore the properties of amyloid fibrils, such as their growth kinetics and interaction with small molecules. Rapid binding information could be obtained with only microlitres of sample in tens of seconds time scale. A further manipulation method for charged particles is introduced in chapter 4, based on the integration of microfluidics and free flow electrophoresis. I present a very effective and simple way to overcome one of the most critical problem in this situation. High electric filed can be applied through two streams of conductive solutions, with all the electrolysis by products, e.g., gas bubbles and other deposits, removed simultaneously without interfering with the system. In addition to microfluidic devices made by soft lithography in PDMS, I also set up a hot embossing fabrication process with the Teflon material (chapter 5). Teflon has many advantages compared with PDMS, such as lower protein adsorption, higher mechanical strength and better chemical compatibility. With different materials and structures, microfluidic devices can be expanded to more applications.

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