Microfluidics is the science of processing microliters or less of fluid at a time in a channel with dimensions on the order of microns. The small size of the channels allows fluid properties to be studied in a world dominated by viscosity, surface tension, and diffusion rather than gravity and inertia. Microfluidic droplet generation is a well studied and understood phenomena, which has attracted attention due to its potential applications in biology, medicine, chemistry and a wide range of industries. This dissertation adds to the field of microfluidic droplet studies by studying individual droplet deformation and the process of scaling-up microfluidic devices for industrial use. The study of droplet deformation under extensional and mixed shear and extensional flows was performed within a microfluidic device. Droplets were generated using a flow-focusing device and then sent through a hyperbolic contraction downstream of the droplet generator. The hyperbolic contraction allowed the smallest droplets to be deformed by purely extensional flows and for the larger droplets to experience mixed extensional and shear flows. The shear resulted from the proximity of the droplet to the walls of the microfluidic channel. The continuous phase in all of these devices was oil and the dispersed phase was water, an aqueous surfactant solution, or an aqueous suspension of colloidal particles. Droplet deformation dynamics are affected by the use of surfactants and colloidal particles, which are commonly used to stabilize emulsion droplets again coalescence. Microfluidic droplet generating devices have many potential industrial applications, however, due to the low output of product from a single droplet generating device, their potential has not been realized. Using six parallel flow-focusing droplet generators on a single chip, the process of microfluidic droplet formation can be scaled up, thus resulting in a higher output of droplets. The tuning of droplet size and production frequency can be achieved on chip by varying the outlet tubing lengths, thus allowing for a single device to be used to generate a range of droplet sizes.

The presence of bubbles within liquid pools is ubiquitous in many natural and industrial settings. Plants and other living systems can release gas bubbles which detach and rise up through lakes and the ocean. Degassing also forms gas bubbles on solid surfaces inside the liquids, like that from champagne or poured soda drinks. The bubbles eventually rise to the pool surface, where they can bounce or pop into the air. The detailed dynamical interaction of the bubble and the free surface can be greatly affected by any impurities on their surface, which can affect the mobility of the free surface. In this dissertation, we use both experiments and numerical simulations to study these hydrodynamics. First, we study the rise and bouncing of bubbles or water droplets from the free surface inside a perfluorocarbon liquid. From all four different configurations of mobile/immobile interface pairs we show that the mobile interface always induces stronger bouncing but faster coalescence. The bouncing enhancement ratio between mobile and immobile interface is $1.8\pm0.1$ for bubbles and $1.5\pm0.1$ for water droplets, with the size range from $250 \, \mu m - 550 \, \mu m$ for bubble and $600 \, \mu m - 1200 \, \mu m$ for droplet. Then the top phase is replaced with a glass plate to eliminate the influence from other internal properties besides surface mobility. Since our numerical simulations perfectly reproduce the experiments, we extend our simulations to the free frontal collision of two equivalent droplets. The results not only support our previous conclusions but also predict another peculiar second-collision phenomenon under certain conditions. Then we replace the surrounding liquid with more practical ones of water and ethanol. In extra-pure water, we find that a millimeter-sized bubble has a mobile interface. We add arachidic acid on the top surface to further investigate bouncing from an immobile interface without changing the interfacial tension. The bouncing enhancement by mobile vs immobile interfaces is once again verified for the water-air interface. For millimeter-sized bubbles, as we increase the bubble size from $780 \, \mu m - 1550 \, \mu m$ the bouncing enhancement ratio decreases from 1.8 to 1.2. Finally, we look into the bubble shape and evolution of the liquid film profile during the bouncing from a top glass substrate, using interferometry and numerical simulations. We use 640 nm laser interferometry with a maximum thickness resolution of 120 nm. The center-of-mass trajectory and film profiles are measured for the first bounce of bubbles between 0.8 mm to 1.2 mm. Then we compare the 1.48 mm bubble impact on a no-slip top wall with the SRYL model prediction, where they shared the same dimple diameter but have a non-trivial deviation in dimple depth. Lastly, we simulate the frontal collision between two identical 1.45 mm bubbles, which have complex multi-dimple formations during the bouncing process.

Optical microlasers have been used in different engineering fields and for sensing various applications. They have been used in biomedical fields in applications such as for detecting protein biomarkers for cancer and for measuring biomechanical properties. The goal of this work is to propose a microfluidic-based fabrication method for fabricating optical polymer based microlasers, which has advantages, over current methods, such us the fabrication time, the contained cost, and the reproducibility of the microlaser's size. The microfluidic setup consisted of microfluidic pumps and a flow focusing droplet generator chip made of polydimethylsiloxane (PDMS). Parameters such as the flow rate (Q) and the pressure (P) of both continuous and dispersed phases are taken into account for determining the microlaser's size and a MATLAB imaging tool is used to reduce the microlaser's diameter estimation. In addition, two applications are discussed: i) electric field measurements via resonator doped with Di-Anepps-4 voltage sensitive dye, and ii) strain measurements in a 3D printed bone-like structure to mimic biomedical implantable sensors.

The application of pesticides in agriculture production systems is a complex process and involves a series of factors that dynamically interact to impact overall pesticide application efficacy. Spray droplet formation, target impaction and deposition, plant uptake, and subsequent biological response are all functions of pesticide active ingredient, nozzle selection, application pressure, and carrier volume. Smaller spray droplets with a lower kinetic energy result in greater spray droplet retention on the leaf surface relative to larger droplets. Consequently, larger spray droplets with higher kinetic energy are poorly retained on the leaf surface and yield minimal coverage of leaf surface tissue. While smaller droplets maximize target coverage and spray droplet retention, larger spray droplets minimize off-target movement and agrichemical transport. Consequently, application factors that maximize pesticide efficacy and minimize off-target movement are often incongruous elements. Therefore, the objective of this research was to evaluate the impact of carrier volume and spray droplet size on the efficacy of various pesticide applications in Mississippi. These data indicate that 1) for thrips and tarnished plant bug control, acephate and sulfoxaflor are superior dicamba application partners relative to dimethoate and thiamethoxam, respectively, insecticide efficacy did not vary due to spray droplet size when applied without dicamba, and maximum efficacy was achieved with a carrier volume of 187 L ha-1; 2) dicamba + acephate and dicamba + thiamethoxam or sulfoxaflor are efficacious options to control Palmer amaranth relative to dicamba applied alone, and maximum Palmer amaranth control was achieved with a carrier volume of 187 L ha-1; 3) cotton defoliation efficacy is positively and negatively correlated with carrier volume and spray droplet size, respectively, and maximum efficacy was achieved with cotton defoliation programs consisting of two-applications, each with a carrier volume of 187 L ha-1 and 200 µm droplets; 4) soybean harvest aid efficacy is primarily a function of the harvest aid applied and that the impact of carrier volume and spray droplet size varies across harvest aids. However, when paraquat is applied, a carrier volume between 47 and 187 L ha-1 should be utilized with droplets of 200-500 µm to maximize harvest aid efficacy.

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Abstract Multiphase Droplet Interactions with a Single Fiber By: Noor M. Farhan A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Virginia Commonwealth University. Virginia Commonwealth University, 2019 Director: Hooman V. Tafreshi, Professor, Department of Mechanical and Nuclear Engineering Formulating the physics of droplet adhesion to a fiber is interesting intellectually and important industrially. A typical example of a droplet–fiber system in nature is the dew droplets on spider webs, where droplets first precipitate and grow on the fibers, but they eventually fall when they become too heavy. Obviously, quantifying the force of adhesion between a droplet and a fiber is crucial in designing fog harvesting devices or manufacturing filtration media for liquid–gas or liquid–liquid separation, among many other industrial applications. This study is aimed at developing a mathematical framework for the mechanical forces between a droplet and a fiber in terms of their physical and wetting properties. To this end, a series of experiments were conducted to detach ferrofluid droplets of varying volumes from fibers with different diameters and Young–Laplace contact angles (YLCAs) in a controlled magnetic field. The force of detachment was measured using a sensitive scale and used along with the results of numerical simulations to develop a semi-analytical expression for the force required to detach a droplet from a fiber. This universally-applicable expression allows one to predict the force detachment without the need to run an experiment or a computer simulation. This work also reports on the use of magnetic force to measure the force of detachment for nonmagnetic droplets for the first time. This is accomplished by adding a small amount of a ferrofluid to the original nonmagnetic droplet to create a compound droplet with the ferrofluid nesting inside or cloaking the nonmagnetic droplet. The ferrofluid is then used to induce a body force to the resulting compound droplet and thereby detach it from the fiber. The recorded detachment force is used directly (the case of nesting ferrofluid) or after scaling (the case of cloaking ferrofluid) to obtain the force of detachment for the original nonmagnetic droplet. The accuracy of these measurements was examined through comparison with numerical simulations as well as available experimental data in the literature. In addition, a simple method is developed to directly measure the intrinsic contact angle of a fiber (i.e., Young–Laplace Contact angle of the fiber material) with any arbitrary liquid. It is shown that the intrinsic contact angle of a fiber can be obtained by simply measuring the angle between the tangent to the fiber surface and the tangent to the droplet at the contact line, if the droplet possesses a clamshell conformation and is viewed from the longitudinal direction. The novelty of the proposed method is that its predictions are not affected by the volume of the droplet used for the experiment, the wettability of the fiber, the surface tension of the liquid, or the magnitude of the body force acting on the droplet during the experiment. Also, a liquid droplet interaction with granular coatings is simulated and the droplet apparent contact angle (ACA) and the transition from Cassie (fully dry) to Wenzel (fully wet) state as a function to the roughness wavelength have been studied. For a fixed droplet volume, two different granular coatings have been used, spherical and hemispherical bumps. It is demonstrated that the chemistry (YLCA) and geometrical parameters for the granular microtexture play an important effect on the droplet ACA and its transition from Cassie to Wenzel state.

Introduced in 1991, solid ink color printing technology is widely used in the office printing, prepress proofing, and wide format color printing markets. Ink droplets are first deposited on a rotating drum and then transferred to paper to reproduce images with high print quality. The objective of this thesis is to develop scientific knowledge of ink droplet deposition, which is needed for precise image buildup on the drum surface. The first problem studied in the thesis is droplet formation from the printhead with varied working voltages and jetting frequencies. Attention is paid to the formation of satellite droplets, the contraction of ligaments and the startup of high frequency jetting. The jetting conditions for obtaining consistent droplet generation with satellites are determined. A theoretical model is developed to predict the lifetime of ligaments. The second problem we studied is the deposition of single droplets on solid surfaces. The surface texture and final shape of deposited droplets are correlated with impact conditions, which include printhead temperature, substrate temperature, distance from printhead to substrate, and the type of substrate surface. An analytical model is developed to evaluate the interaction of oscillation and viscous damping in the droplet during impact. The third problem covered in the thesis is the deposition of multiple ink droplets on the drum surface. Interaction between droplets causes drawback effect, which degrades print quality. We define a parameter to quantify the drawback effect with varied deposition conditions. A simple model is provided to predict conditions for making continuous lines based on the results of two ink droplets deposition. To understand the hydrodynamics in causing drawback effect, a series of experiments using large liquid droplets are carried out. Focus is put on the evolution of spread length and dynamics of contact line. Correlations for maximum and minimum spread lengths are developed, which are used to reveal the cause of drawback effect in the deposition of ink droplets.

Introduced in 1991, solid ink color printing technology is widely used in the office printing, prepress proofing, and wide format color printing markets. Ink droplets are first deposited on a rotating drum and then transferred to paper to reproduce images with high print quality. The objective of this thesis is to develop scientific knowledge of ink droplet deposition, which is needed for precise image buildup on the drum surface. The first problem studied in the thesis is droplet formation from the printhead with varied working voltages and jetting frequencies. Attention is paid to the formation of satellite droplets, the contraction of ligaments and the startup of high frequency jetting. The jetting conditions for obtaining consistent droplet generation with satellites are determined. A theoretical model is developed to predict the lifetime of ligaments. The second problem we studied is the deposition of single droplets on solid surfaces. The surface texture and final shape of deposited droplets are correlated with impact conditions, which include printhead temperature, substrate temperature, distance from printhead to substrate, and the type of substrate surface. An analytical model is developed to evaluate the interaction of oscillation and viscous damping in the droplet during impact. The third problem covered in the thesis is the deposition of multiple ink droplets on the drum surface. Interaction between droplets causes drawback effect, which degrades print quality. We define a parameter to quantify the drawback effect with varied deposition conditions. A simple model is provided to predict conditions for making continuous lines based on the results of two ink droplets deposition. To understand the hydrodynamics in causing drawback effect, a series of experiments using large liquid droplets are carried out. Focus is put on the evolution of spread length and dynamics of contact line. Correlations for maximum and minimum spread lengths are developed, which are used to reveal the cause of drawback effect in the deposition of ink droplets.

Droplet microfluidics is a promising field since it overcomes many of the limitations of single phase microfluidic systems. The improved mixing time scale, the increase of number of samples and the isolation of droplets are some of its virtues. The core of droplet microfluidics is a two-phase flow condition that is subjected to scaling of the confining geometry. With the scaling the complexities of the flow phenomena arise. For that reason both the processes of droplet generation and transport are not fully understood for various flow and fluid conditions. The work in this thesis aims to experimentally examine droplet generation and transport in microchannels for flow and fluid conditions that are experimentally challenging to perform. Examination of droplet generation in a T-junction microchannel design was performed with a quantitative velocity field approach known as micro particle image velocimetry (μPIV). The studies on droplet generation focused on very fast generation regimes, namely transition and dripping that have not been studied for a T-junction design. This achievement was accomplished because of the development of a fast optical detection and triggering system that allowed for acquiring images of different identical droplets at the same position. μPIV results indicate that the quantitative velocity field patterns of different regimes share some similarities. The filling stage in the transition and dripping regimes had some resemblance in their velocity patterns. The velocity patterns for the start of droplet pinch-off were alike for the squeezing and transition regimes. Furthermore, the presence of a surfactant in the droplet phase above the critical micelle concentration (CMC) did not have an effect on the general velocity patterns as long as the capillary number Ca was matched with the no-surfactant condition. The studies of hydrodynamic properties of droplet transport were performed in hard materials to avoid cumulative error sources, such as material pressure compliance and swelling effects. The project had several parts: designing a microchannel network that allowed studying the hydrodynamic properties of small droplets, surface treatments of the channel material for stable droplet generation and examining the hydrodynamics of small liquid droplets with sizes that have not been reported in the literature. The studies examined effects of changing the interfacial tension, viscosity, and flow conditions on the transport of droplets. The experimental results from the hydrodynamic transport studies indicated that for the droplet sizes that were examined the pressure drop of droplets was affected by the capillary number Ca and length of the droplet Ld. Also, the presence of surfactants altered the hydrodynamic properties of droplets. At a high concentration of surfactants the droplets pressure drop was reduced significantly. Moreover, the type of surfactant affected the magnitude of the pressure drop. Experimental results indicate that if the concentration of surfactants was very low (below CMC) it did not have an effect on the droplet excess pressure. These findings are important to consider in designing droplet microfluidic systems with complex channel networks that involve droplet sorting, splitting, and merging for droplets that contain surfactants.

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