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

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University of Macau / Faculty of Science and Technology / Department of Computer and Information Science

The main aim and ultimate final goal of the work carried out in this thesis is a drive towards a feasible system for light harvesting, which is in short, using the Sun’s energy to create electricity or a fuel for our energy requirements here on Earth. This work will see an approach using the triple phase boundary afforded by a microdroplet array. Although light harvesting is an ambition which has seen decades of work and uncountable man-hours, approaching it from the angle of utilizing the triple phase boundary between two immiscible liquids and a solid electrode is a new, and novel concept. Before any attempts towards a light harvesting technique can be made, we will need to have characterized and fully understood the mechanisms and nuances, both for dark and light processes, that are observed at the triple phase boundary. This initial process will start by selection of a suitable redox molecule, and exploring its reactivity in microdroplets under dark conditions. Once this has been achieved, an attempt can be made to use this knowledge, and implement it towards light harvesting. This will eventually include an attempt to couple photo-excited states with other molecules, this will be an important step if energy is ever able to be stored from such a system. This early phase will also see the need to employ many other techniques other than electrochemistry in an effort to aid in the understanding and characterization of the triple phase boundary at microdroplets. This will include travelling to other laboratories in search of specialized scientific skills and apparatus, such as electron paramagnetic resonance, or photocurrent spectroscopy. It will also see the need to build new equipment needed to conduct tests such as surface tension visualization, or new electrochemical cells for photocurrent measurement. In summary, this report will see initial characterization of the processes, both light and dark, that occur within the triple phase boundary of a microdroplet for a given redox molecule dissolved within. Early attempts at coupling excited states with other molecules are also explored. Serendipity has always played a part in scientific discovery and the work outlined in this report was no different. The choice of oil used for the organic phase microdroplet deposits yielded some interesting and unexpected results, and has been implicated as one of the key aspects of the photoreactions that have been explored.

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

Abstract Application of Argon Plasma Technology to Hydrophobic and Hydrophilic Microdroplet Generation in PDMS Microfluidic Devices Brennan Graham Microfluidics has gained popularity over the last decade due to the ability to replace many large, expensive laboratory processes with small handheld chips with a higher throughput due to the small channel dimensions [1]. Droplet microfluidics is the field of fluid manipulation that takes advantage of two immiscible fluids to create droplets from the geometry of the microchannels. This project includes the design of a microfluidic device that applies the results of an argon plasma surface treatment to polydimethylsiloxane (PDMS) to successfully produce both hydrophobic and hydrophilic surfaces to create oil in water (O/W) and water in oil (W/O) microdroplets. If an argon plasma surface treatment renders the surface of PDMS hydrophilic, then O/W microdroplets can be created and integrated into a larger microdroplet emulsion device. The major aims of this project include: (1) validating previously established Cal Poly lab protocols to produce W/O droplets in hydrophobic PDMS microdroplet generators (2) creating hydrophilic PDMS microdroplet generators (3) making oil in water droplets in hydrophilic PDMS microdroplet generators (4) designing a multilayer microfluidic device to transfer W/O droplets to a second hydrophilic PDMS microdroplet generator v W/O droplets were successfully created and transferred to a second hydrophilic PDMS device. The hydrophilic PDMS device also produced O/W droplets in separate testing from the multilayered microfluidic PDMS device. The ultimate purpose of this project is to create a multilayer microdroplet generator that produces water in oil in water (W/O/W) microdroplet emulsions through a stacked device design that can be used in diagnostic microdroplet applications. Thesis Supervisor: Dave Clague Title: Professor of Biomedical Engineering

Recently, considerable attention has been focused on the generation of nano- and micrometer scale multicomponent polymer particles with specifically tailored mechanical, electrical and optical properties. As only a few polymer-polymer pairs are miscible, the set of multicomponent polymer systems achievable by conventional methods, such as melt blending, is severely limited in property ranges. Therefore, researchers have been evaluating synthesis methods that can arbitrarily blend immiscible solvent pairs, thus expanding the range of properties that are practical. The generation of blended microparticles by evaporating a co-solvent from aerosol droplets containing two dissolved immiscible polymers in solution seems likely to exhibit a high degree of phase uniformity. A second important advantage of this technique is the formation of nano- and microscale particulates with very low impurities, which are not attainable through conventional solution techniques. When the timescale of solvent evaporation is lower than that of polymer diffusion and self-organization, phase separation is inhibited within the atto- to femto-liter volume of the droplet, and homogeneous blends of immiscible polymers can be produced. We have studied multicomponent polymer particles generated from highly monodisperse micrordroplets that were produced using a Vibrating Orifice Aerosol Generator (VOAG). The particles are characterized for both external and internal morphology along with homogeneity of the blends. Ultra-thin slices of polymer particles were characterized by a Scanning Electron Microscope (SEM), and the degree of uniformity was examined using an Electron Dispersive X-ray Analysis (EDAX). To further establish the homogeneity of the polymer blend microparticles, differential scanning calorimeter was used to measure the glass transition temperature of the microparticles obtained. A single glass transition temperature was obtained for these microparticles and hence the homogeneity of the blend was concluded. These results have its significance in the field of particulate encapsulation. Also, better control of the phase morphologies can be obtained by simply changing the solvent/solvents in the dilute solutions. Evaporation and drying of a binary droplet containing a solute and a solvent is a complicated phenomenon. Most of the present models do not consider convection in the droplet phase as solvent is usually water which is not very volatile. In considering highly volatile solvents the evaporation is very rapid. The surface of the droplet recedes inwards very fast and there is an inherent convective flow that is established inside the solution droplet. In this dissertation work, a model is developed that incorporates convection inside the droplet. The results obtained are compared to the size obtained from experimental results. The same model when used with an aqueous solution droplet predicted concentration profiles that are comparable to results obtained when convection was not taken into account. These results have significance for more rigorous modeling of binary and multicomponent droplet drying.

Dans cette thèse, nous avons développé et caractérisé un nouveau type de source de mise en phase gazeuse de biomolécules qui repose sur une désorption laser non résonnante sur des microgouttelettes directement sous vide. Ce dispositif nous permettra à terme d’ouvrir une voie d’étude originale pour appréhender les effets des rayonnements ionisants sur des molécules organiques d'un point de vue physique.Nous présenterons en détail ce dispositif avec lequel nous avons réussi à transférer sous vide, de façon non destructive, des biomolécules et des complexes non-covalents dans une gamme de masse de l’ordre du kDa et à les identifier par spectrométrie de masse par temps de vol. Nous montrerons notamment les défis techniques qu’il a fallu relever pour permettre le transfert des microgouttelettes sous vide et comment par simulation du spectromètre, nous sommes parvenus à optimiser fortement les paramètres de collection des espèces moléculaires désorbées et la résolution en masse de notre système, en remplaçant, dans la zone de désorption, l’extraction électrostatique à potentiels retardés par un piège électrostatique quadripolaire. Nous exposerons enfin la façon dont ce dispositif, couplé à une plateforme d'irradiation d'ions simplement chargés possédant une énergie de l'ordre du keV, nous permettra de caractériser à une échelle moléculaire l’endommagement lié à des mécanismes de chimie radicalaire radio-induits. / In this thesis, we have developed and characterized a new type of gas phase source of biomolecules which based on non-resonant laser desorption on microdroplets directly under vacuum. This device will eventually allow us to open an original way to study the effects of ionizing radiation on organic molecules from a physical point of view. We will present in detail this device with which we transfered with sucess under vacuum,in a non-destructively way, biomolecules and non-covalent complexes in a mass range of the order of kDa and we assigned them with time-of-ight mass spectrometry. We will show in particular the technical challenges that we had to overcome in order to allow the transfer of microdroplets under vacuum and how by simulation of the spectrometer, we have been able to highly optimize the collection parameters of the desorbed molecular species and the mass resolution of our system, by replacing, in the desorption zone,delayed extraction by a quadrupole electrostatic trap. Finally, we will describe the way in which this device, coupled to a simply charged ion irradiation platform with an energy of the order of the keV, will enable us to characterizeon a molecular scale the damage due to radio-induced radical chemistry mechanisms.

There is widespread use of ionization sources (ambient and non-ambient) for a variety of applications. More recently, charged microdroplets generated by electrospray ionization and paper spray have been used to conduct chemistry at faster rates compared to bulk volumes. Uncharged droplets such as those generated by the Leidenfrost technique have also been used to explore chemistry and study the degradation of drugs in an accelerated manner. These microdroplets serve as reaction vessels in which in which some reactions are known to occur at accelerated rates. Such chemistry can be particularly useful in pharmaceutical settings to rapidly synthesize small amounts of materials in relatively short amount of time. Additionally, microdroplets may also be used to perform high throughput screening analysis. While several parameters influencing the rate of reaction in microdroplets have been explored (such as spray distance and reagent concentration), the mechanism of reaction acceleration has not been probed to a significant extent. A major portion of my dissertation describes the use of charged and uncharged microdroplets to perform quick chemistry, guide microfluidic synthesis of drugs such as diazepam, perform scale up of copper catalyzed C-O and C-N coupling reactio<a></a>ns and screen reaction conditions for pharmaceutically relevant reactions such as the Suzuki cross-coupling reaction. Additionally, work discussed here also describes development and use of existing techniques such as structured illumination microscopy to measure droplet sizes, explore the role of distances on droplet size, and study the effect of surfactants on the rate of reactions in microdroplets generated by nano-electrospray ionization. A mathematical model to understand the mechanism of increased reaction rates in microdroplets has also been presented. Additionally, this dissertation also describes ways to manipulate ions in air using various designs of 3D-printed electrodes that operate with DC potentials only and which can be easily coupled with nano-electrospray ionization sources to transmit ions over long distances

Recent reports on the formation of hydrogen peroxide (H$_2$O$_2$) in water microdroplets produced via capillary condensation or pneumatic spraying have garnered significant attention. How covalent bonds in water could break under such conditions challenges our textbook understanding of physical chemistry and the water substance. While there is no definitive answer, it has been speculated that ultrahigh electric fields at the air-water interface are responsible for this chemical transformation. This thesis documents the findings of our exploration of this mystery via a comprehensive experimental investigation of H$_2$O$_2$ formation in (i) water microdroplets condensed on hydrophobic and hydrophilic substrates formed via hot water in the 50–70 ℃ range or ultrasonic humidifier under controlled air composition, and (ii) water microdroplets sprayed over a range of liquid flow-rates, the (shearing) air flow rates, and the air composition. Our glovebox experiments, with controlled gas composition, revealed that no H$_2$O$_2$(aq) was produced in water microdroplets condensed via heating water (detection limit ≥ 0.25 μM), regardless of the droplet size or the substrate wettability. In contrast, water droplets condensed via ultrasonic humidification contained significantly higher (~1 μM) H$_2$O$_2$ concentration. We pinpointed that ultrasonic humidifiers induced cavitation of tiny bubbles in water, which is known to form H$_2$O$_2$(aq) and other reactive species. Next, in the case of sprayed water microdroplets, also, we did not detect H$_2$O$_2$(aq) unless O$_3$(g) was present in the ambient atmosphere. In contrast, water microdroplets (sprayed or condensed) exposed to O$_3$(g) concentration in the range 2–5000 ppb formed 2–100 μM H$_2$O$_2$(aq); increasing the gas–liquid surface area, mixing, and contact duration enhanced the H$_2$O$_2$(aq) concentration. Therefore, we submit that the original reports suffered from experimental artifacts due to the high regional O$_3$(g) giving rise to H$_2$O$_2$(aq), and the air–water interface does not spontaneously produce H$_2$O$_2$(aq). The water surface merely facilitates the O$_3$(g) mass transfer, which then undergoes chemical transformations in the water to form H$_2$O$_2$(aq). Taken together, these findings offer an alternative explanation to the mysterious production of H$_2$O$_2$ in water microdroplets; These findings should also advance our understanding of the implications of this chemistry in natural and applied contexts.