Droplet evaporation and condensation are two important topics of interest, since these two phase-change phenomena not only occur in the cycle of global water, e.g., the formation of rain, fog, dew, and snow in nature, but also play a critical role in a variety of applications including phase-change heat transfer enhancement, surface chemistry and energy system optimization. Especially, in the past two decades, the rapid development of the nature-inspired non-wetting surfaces has promoted the applications of droplet-based phase change phenomena in various scenarios. However, most previous studies focused on the sessile droplets on one flat surface in the open space, and the effects of surface topography, i.e., surface curvature or configurations, on droplet evaporation and dropwise condensation are still elusive. This dissertation aims to explore droplet-based evaporation and condensation in more complex spaces and to elucidate how the surface topography affects the evaporating or coalescing droplet dynamics during these phase-change processes. The coalescence-induced jumping of nanodroplet on curved superhydrophobic surface is modeled via molecular dynamic simulations. As the surface curvature increases from 0 to 2, the corresponding energy conversion efficiency of jumping droplet during the coalescence process could be significantly improved about 20 times. To explain this curvature-enhanced jumping effect, the contact line dissipation, i.e., an important source of energy dissipation in nanoscale, is considered in our scaling energy analysis. And this energy-effective jumping of coalesced droplet could be mainly attributed to the reduction of contact line dissipation due to the decrease of contact line length and contact time on curved surface. As the droplets are confined between two parallel or non-parallel low-energy surfaces, i.e., hydrophobic or superhydrophobic surfaces, with a narrow gap, the total evaporation time of the squeezed droplets would be dramatically prolonged about two times. An ellipsoidal segment diffusion-driven model is established to successfully predict the evolution of contact radius and volume of the squeezed droplets during the evaporation process and to clarify it is the vapor enrichment inside the confined space giving rise to the mitigated evaporation. If two hydrophobic surfaces are configured as non-parallel, the confined droplet inside the V-shaped grooves would be self-transported towards the cusp/corner during the evaporation. Based on our energy and force analyses, the asymmetrically confined droplet would move towards an equilibrium location le, where the Laplace pressure induced force is balanced with normal adhesion force, to minimize its Gibbs surface energy. As le decreases during the evaporation, this equilibrium location would directionally shift towards the cusp, which could be regarded as the origin of this evaporation-triggered unidirectional motion. For the first time, the solvent transport and colloidal extraction could be accurately controlled in a combined manner. / Doctor of Philosophy / Droplet evaporation and condensation are two important topics of interest, since these two phase-change phenomena not only occur in the global cycle of water including the formation of rain, fog, dew, and snow in nature, but also play a critical role in a variety of applications including heat transfer enhancement, surface chemistry, and the energy system optimization. Generally, the droplets in these scenarios are deposited on one flat surface opened to the atmosphere. and the effects of surface topography on droplet evaporation and dropwise condensation are still elusive. This dissertation aims to explore droplet-based evaporation and condensation in more complex spaces and to clarify how the surface curvature or configurations affects evaporating or condensing droplet dynamics accompanying these phase change processes. As the coalesced droplet jumps off the curved superhydrophobic surfaces during dropwise condensation, the corresponding energy conversion efficiency would be significantly improved about 20 times due to the increases of curvature. It is demonstrated that the decrease of contact line length and contact time would give rise to the reduction of contact line dissipation, which should be the main factor driving this energy-effective jumping of the coalesced droplets. As the droplets are confined between two parallel or non-parallel low-energy surfaces, i.e., hydrophobic or superhydrophobic surfaces, with a narrow gap, the total evaporation time of the squeezed droplets would be dramatically prolonged about two times in the small space. An ellipsoidal segment diffusion-driven model is established to successfully predict the evolution of contact radius and volume of the squeezed droplets during the evaporation and to clarify it is the vapor enrichment in the confined space giving rise to the mitigated evaporation. If two hydrophobic surfaces are configured as non-parallel, the confined droplet inside the V-shaped grooves would be self-transported towards the cusp/corner of the structure during evaporation. Based on our energy and force analyses, the asymmetrically confined droplet would move towards an equilibrium location le, where the Laplace pressure induced force is balanced with normal adhesion force, to minimize its Gibbs surface energy. As le decreases in the scale of during the evaporation, this equilibrium location would directionally shift towards the cusp, which could be regarded as the origin of this evaporation-triggered unidirectional motion.

Electric charges are often found in naturally or artificially formed droplets, such as raindrops and those generated by Kelvin's water dropper. In contrast to the impact of neutral droplets on a flat solid surface upon which a thin convex lens shape layer of the gas film is typically formed, I show that the delicate gas thin film can be fundamentally altered for even weakly charged droplets both experimentally and numerically. As the charge level is raised above a critical level of about 1% of the Rayleigh limit for representative impact conditions, the Maxwell stress overcomes the gas pressure buildup to deform the droplet bottom surface. A conical liquid tip forms and pierces Through the gas film, leading to a circular contact line moving outwards that does not trap any gas. The critical charge level only depends on the capillary number based on the gas viscosity. The deformation applies to common liquids and molten alloy droplets. Even dielectric surfaces can also induce conical deformation. The charged droplets can also deform upon hydrophobic surfaces, and increase the contact time on hydrophobic surfaces or even avoid bouncing. / Doctor of Philosophy / Electric charges are often found in naturally or artificially formed droplets, such as raindrops, waterfall, and inkjet printer. Neutral droplets impact on flat surfaces will usually trap a bubble inside because of the viscosity of air. The air bubble entrapped can be ignored if the droplet is water because the air bubble will eventually pinch-off. However, if the droplet is metal or some other viscous liquid, the air bubble will stay inside the liquid. This entrapped air bubble is undesired under some circumstances. For example, the existence of air bubble during metal 3D printing can influence the physical property. I show that the delicate gas thin film can be fundamentally altered for even weakly charged droplets both experimentally and numerically. As the charge level is raised above a critical level of about 1% of the maximum charges a droplet can carry for representative impact conditions, the electric stress will dominate the deformation of droplet. A conical liquid tip forms at the droplet bottom, avoiding the entrapment of air bubble. The critical charge level is experimentally proved to be only dependent on the gas viscosity and impact velocity. The deformation applies to common liquids and molten alloy droplets. Even dielectric surfaces can also induce conical deformation. The charged droplets can also deform upon hydrophobic surfaces, and increase the contact time on hydrophobic surfaces or even avoid bouncing.

Capillarity and dynamic wetting of non-Newtonian fluids are important in many natural and industrial processes, examples cover from a daily phenomenon as splashing of a cup of yogurt to advanced technologies such as additive manufacturing. The applicable non-Newtonian fluids are usually viscoelastic compounds of polymers and solvents. Previous experiments observed diverse interesting behaviors of a polymeric droplet on a wetted substrate or in a microfluidic device. However, our understanding of how viscoelasticity affects droplet dynamics remains very limited. This work intends to shed light on viscoelastic effect on two small scale processes, i.e., the motion of a wetting contact line and droplet splitting at a bifurcation tip.   Numerical simulation is employed to reveal detailed information such as elastic stresses and interfacial flow field. A numerical model is built, combining the phase field method, computational rheology techniques and computational fluid dynamics. The system is capable for calculation of realistic circumstances such as a droplet made of aqueous solution of polymers with moderate relaxation time, impacting a partially wetting surface in ambient air.   The work is divided into three flow cases. For the flow case of bifurcation tube, the evolution of the interface and droplet dynamics are compared between viscoelastic fluids and Newtonian fluids. The splitting or non-splitting behavior influenced by elastic stresses is analyzed. For the flow case of dynamic wetting, the flow field and rheological details such as effective viscosity and normal stress difference near a moving contact line are presented. The effects of shear-thinning and elasticity on droplet spreading and receding are analyzed, under inertial and inertialess circumstances. In the last part, droplet impact of both Newtonian and viscoelastic fluids are demonstrated. For Newtonian droplets, a phase diagram is drawn to visualize different impact regions for spreading, splashing and gas entrapment. For viscoelastic droplets, the viscoelastic effects on droplet deformation, spreading radius and contact line motion are revealed and discussed. / <p>QC 20160329</p>

This work introduces technology for rapid pathogen detection using handheld optical immunoassay and wire-guided droplet PCR systems. There have been a number of cases of foodborne or waterborne illness among humans that are caused by pathogens such as Escherichia coli O157:H7, Salmonella typhimurium, Influenza A H1N1, and the norovirus. The current practices to detect such pathogenic agents are: cell/viral culturing, immunoassays, or polymerase chain reactions (PCRs). These methods are essentially laboratory-based methods that are not at all real-time and thus unavailable for early-monitoring of such pathogens. They are also very difficult to be implemented in field, preventing early detection opportunities. This dissertation is divided into three papers that present methodologies towards the expeditious detections of infectious pathogens and the miniaturization of these detection systems towards field-deployable and point-of-care applications. Specifically, the work presented focuses on two methodologies: (1) light scatter detection using immunoagglutination assays with optimized Mie light scatter parameters in a real biological matrix consisting of plant tissue, and (2) wire-guided droplet manipulations for rapid and improved sample analysis, preparation, and PCR thermocycling. Both of these methods carry a collective objective towards providing high impact technologies for addressing the issues of food-related outbreaks and overall public safety. In the first paper, the direct and sensitive detection of foodborne pathogens from fresh produce samples was accomplished using a handheld lab-on-a-chip device, requiring little to no sample processing and enrichment steps for a near-real-time detection and truly field-deployable device. The detection of Escherichia coli K12 and O157:H7 in iceberg lettuce was achieved utilizing optimized Mie light scatter parameters with a latex particle immunoagglutination assay. The system exhibited good sensitivity, with a limit of detection of 10 CFU mL⁻¹ and an assay time of <6 min. Minimal pretreatment with no detrimental effects on assay sensitivity and reproducibility was accomplished with a simple and cost-effective KimWipes filter and disposable syringe. Mie simulations were used to determine the optimal parameters (particle size d, wavelength λ, and scatter angle θ) for the assay that maximize light scatter intensity of agglutinated latex microparticles and minimize light scatter intensity of the tissue fragments of iceberg lettuce, which were experimentally validated. This introduces a powerful method for detecting foodborne pathogens in fresh produce and other potential sample matrices. The integration of a multi-channel microfluidic chip allowed for differential detection of the agglutinated particles in the presence of the antigen, revealing a true field-deployable detection system with decreased assay time and improved robustness over comparable benchtop systems. In the second paper, we demonstrate a novel method of wire-guided droplet manipulations towards very quick RT-PCR. Because typical RT-PCR assays take about 1–2 h for thermocycling, there is a growing need to further speed up the thermocycling to less than 30 min. Additionally, the PCR assay system should be made portable as a point- of-care detection tool. Rapid movements of droplets (immersed in oil) over three different temperature zones make very quick PCR possible, as heating/cooling will be made by convective heat transfer, whose heat transfer coefficients are much higher than that of conduction, the latter of which is used in most conventional PCR systems. A 30-cycle PCR of a 160 bp gene sequence amplified from 2009 H1N1 influenza A (human origin) was successfully demonstrates in 6 min and 50 sec for a very large 10 μL droplet (with additional 4 min for reverse transcription). The proposed system has a potential to become a rapid, portable, point-of-care tool for detecting influenza A. In the third paper, a wire-guided CNC apparatus was used to perform droplet centrifugation, DNA extraction, and VQ-PCR thermocycling on a single superhydrophobic surface measuring 25 mm by 55 mm and a multi-chambered PCB heater. This methodology exhibited no limitations on the complexity and configuration of procedures that it can perform, making it versatile and far-reaching in its applications. The only modification required for adding or implementing changes for a new protocol is through simple pre-defined programming. The highly adaptive and flexible system was used to execute easily pre-programmed droplet movements and manipulations for the rapid detection of Escherichia coli from PCR detection. Serial dilutions were performed to simulate a diluted field sample with a high level of accuracy. Centrifugation of the diluted sample containing E. coli demonstrated a novel approach to sample pre-treatment. Furthermore, the extraction of DNA from the sample droplet containing E. coli was also performed on the same superhydrophobic surface as the previous 2 steps, requiring less than 10 min. Following extraction, the genetic material was amplified using wire-guided droplet PCR thermocycling, successfully completing 30 cycles of Peptidase D (a long 1500 bp sequence) in 10 min. The droplet centrifugation process was determined to greatly improve the positive band intensity over the non-centrifuged sample. Thus, this work demonstrates the adaptability of the system to replace many common laboratory tasks on a single platform (through re-programmability), in rapid succession (using droplets), and with a high level of accuracy and automation.

<p>Conventional rocket propellants such as monomethyl hydrazine (MMH) and hydrazine have been used for decades due to their high specific impulse and performance. However, interest in greener alternatives, including HAN or HAN-based propellants, has grown due to high levels of toxicity and difficulties in the handling and storage of conventional fuels. Included among potential propellants are monopropellants nitromethane (NM) and isopropyl nitrate (IPN) and their blends. Though large-scale investigations on the ignition and combustion of these fuels have been done, the ignition and combustion processes of these monopropellant fuels are still not well understood. Droplet studies have been traditionally and extensively employed to decipher the influence of ambient conditions and fuel properties on ignition and combustion of different fuels. These fundamental studies allow for the isolation of different factors such as ambient temperature and initial droplet size among others, to provide a deeper understanding of their effects in overall spray combustion.</p> <p> </p> <p>The research described here seeks to add to the knowledge on the ignition and combustion processes of NM and IPN through single droplet ignition and combustion studies. To this end, the first effort has been to establish a suitable method of studying the ignition and combustion of droplets in conditions similar to those in practical systems. Droplet ignition delay measurements for NM and IPN droplets have also been conducted, and the influence of ambient temperature and droplet size has been studied. The double flame structures of NM and IPN, representative of hybrid combustion, have also been observed. In addition, the applicability of the hybrid combustion model, developed to predict mass burning rates for hypergolic fuels exhibiting hybrid burning including MMH, UDMH and hydrazine, has been assessed. Lastly, the ability of the quasi-steady droplet ignition model to predict ignition delays of IPN and NM monopropellant droplets is also discussed.</p>

The effect of superheat on the disruption of liquid droplets in a compressible gas flow was investigated experimentally in a small-scale, supersonic wind tunnel. Aerodynamically generated ethanol droplets of an average diameter of 0.1 mm were injected via a normal sonic jet into a Mach 1.8 freestream. Both nonsuperheated and superheated droplets were injected with initial Weber numbers of approximately 700. The droplets and flow structure were photographed using the shadowgraph method. The relatively high momentum of the liquid droplets typically caused them to pass out of the sonic jet structure. Nonsuperheated droplets showed no signs of disrupting after traveling over 200 mm downstream from the injection point. Only droplets with injection temperatures above the predicted boiling point at tunnel freestream static pressure (48°C) showed signs of disruption, typically after they left the sonic jet structure (30 to 100 mm downstream of the injection point). Droplets in this range of temperatures appeared to begin to boil from the downstream side of the droplet, shedding a vapor cloud before disrupting completely in the chaotic mode. Droplets with temperatures above the boiling point at the exit plane of the sonic jet began to disrupt in the chaotic mode almost instantly (within 1 exit nozzle diameter).

Two new mechanisms of droplet splashing and breakup during impact have been identified and analyzed. One is the internal rupture of spreading droplet film through formation of holes, and the other is the splashing of droplet due to its freezing during spreading. The mechanism of film rupture was investigated by two different methods. In the first method, circular water films were produced by directing a 1 mm diameter water jet onto a flat, horizontal plate for 10 ms. In the second method, films were produced by making 0.6 mm water droplets impact a solid surface mounted on the rim of a rotating flywheel. Substrate wettability was varied over a wide range, including superhydrophobic. In both cases, the tendency to film rupture first increased and then decreased with contact angle. A thermodynamic stability analysis predicted this behavior by showing that films would be stable at very small or very large contact angle, but unstable in between. Film rupture was also found to be promoted by increasing surface roughness or decreasing film thickness. To study the effect of solidification, the impact of molten tin droplets (0.6 mm diameter) on solid surfaces was observed for a range of impact velocities (10 to 30 m/s), substrate temperatures (25 to 200°C) and substrate materials (stainless steel, aluminum and glass) using the rotating flywheel apparatus. Droplets splashed extensively on a cold surface but on a hot surface there was no splashing. Splashing could be completely suppressed by either increasing the substrate temperature or reducing its thermal diffusivity. An analytical model was developed to predict this splashing behavior. The above two theories of freezing-induced splashing and film rupture were combined to predict the morphology of splats typically observed in a thermal spray process. A dimensionless solidification parameter, which takes into account factors such as the droplet diameter and velocity, substrate temperature, splat and substrate thermophysical properties, and thermal contact resistance between the two, was developed. Predictions from the model were compared with a wide range of experimental data and found to agree well.

Two new mechanisms of droplet splashing and breakup during impact have been identified and analyzed. One is the internal rupture of spreading droplet film through formation of holes, and the other is the splashing of droplet due to its freezing during spreading. The mechanism of film rupture was investigated by two different methods. In the first method, circular water films were produced by directing a 1 mm diameter water jet onto a flat, horizontal plate for 10 ms. In the second method, films were produced by making 0.6 mm water droplets impact a solid surface mounted on the rim of a rotating flywheel. Substrate wettability was varied over a wide range, including superhydrophobic. In both cases, the tendency to film rupture first increased and then decreased with contact angle. A thermodynamic stability analysis predicted this behavior by showing that films would be stable at very small or very large contact angle, but unstable in between. Film rupture was also found to be promoted by increasing surface roughness or decreasing film thickness. To study the effect of solidification, the impact of molten tin droplets (0.6 mm diameter) on solid surfaces was observed for a range of impact velocities (10 to 30 m/s), substrate temperatures (25 to 200°C) and substrate materials (stainless steel, aluminum and glass) using the rotating flywheel apparatus. Droplets splashed extensively on a cold surface but on a hot surface there was no splashing. Splashing could be completely suppressed by either increasing the substrate temperature or reducing its thermal diffusivity. An analytical model was developed to predict this splashing behavior. The above two theories of freezing-induced splashing and film rupture were combined to predict the morphology of splats typically observed in a thermal spray process. A dimensionless solidification parameter, which takes into account factors such as the droplet diameter and velocity, substrate temperature, splat and substrate thermophysical properties, and thermal contact resistance between the two, was developed. Predictions from the model were compared with a wide range of experimental data and found to agree well.

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