An acoustic field is used to increase the critical heat flux of a copper boiling heat transfer surface. The increase is a result of the acoustic effects on the vapor bubbles. Experiments are being performed to explore the effects of an acoustic field on vapor bubbles in the vicinity of a rigid heated wall. Work includes the construction of a novel heater used to produce a single vapor bubble of a prescribed size and at a prescribed location on a flat boiling surface for better study of an individual vapor bubble s reaction to the acoustic field. Work also includes application of the results from the single bubble heater to a calibrated copper heater used for quantifying the improvements in critical heat flux.

As digital microfluidics has continued to mature since its advent in the early 1980's, an increase in new and novel applications of this technology have been developed. However, even as this technology has become more common place, a consensus on the physics and force models of the motion of the contact line between the fluid, substrate, and ambient has not been reached. This uncertainty along with the dependence of the droplet geometry on the force to cause its motion has directed much of the research at specific geometries and droplet actuation methods. The goal of this thesis is to help characterize the components of the friction force which opposes droplet motion as a one dimensional system model based upon simple system parameters independent from the actuation method. To this end, the force opposing the motion of a droplet under a thin rectangular glass cover slip was measured for varying cover slip dimensions (widths, length), gap height between the cover slip and substrate, and bulk droplet velocity. The stiffness of the droplet before droplet motion began, the force at which the motion initiated, and the steady-state force opposing the droplet motion were measured. The data was then correlated to hypothesized equations and compared to simple models accounting for the forces due to the contact angle hysteresis, contact line friction, and viscous losses. It was found that the stiffness, breakaway force, and steady-state force of the droplet could be correlated to with an error standard deviation of 8 %, 14%, and 10 % respectively. Much of the error was due to an unexpected height dependence for the breakaway and steady-state forces and testing error associated with the velocity. The models for the stiffness and breakaway force over predicted the results by 36% and 16% respectively. During testing, viii stability issues with the cover slip were observed and simple dye testing was conducted to visualize the droplet flow field.

Many industrial processes such as printing and cleaning, as well as products like adhesives, coatings, and biological testing devices, rely on the wetting of liquids on a surfaces. Wetting is commonly controlled through material selection, coatings, and/or surface texture, but these means are sensitive to environmental conditions. Wetting is influenced by variables like surface tension, density, the surface chemistry, local energy barriers like surface roughness, and how the droplet is placed on the surface. Wetting of droplets can also be influenced externally in many ways such as introducing surfactants, applying electrical fields, or by dynamically excitation. Low-frequency, high amplitude vibration can initiate wetting changes prompted by droplet contact line oscillations that exceed the range of stable contact angles inherent of a droplet on a solid surface. The study of ultrasonic vibration wetting and spreading effects is sparse [1, 2], and is usually only qualitatively analyzed. Therefore, the specific goal of this thesis is somewhat unique, but also has potential as a means to controllably reverse surface adhesion. High frequency vibration effects and the governing mechanisms are relatively uncharacterized due to difficulties posed by the spatial and temporal scales. To investigate, droplets of 10, 20, and 30 µL are imaged as they vibrate on a hydrophobic surface forced via a piezoelectric transducer over different high frequencies (>10 kHz). Wetting transitions occur abruptly over a range of parameters, but coincide with transducer resonance modes. The magnitude of contact angle change is dependent on droplet volume and surface acceleration, and remains after cessation of vibration, however new droplets wet with the original contact angle. A more detailed investigation of this phenomenon was necessary to obtain a better understanding. This required repeatable testing conditions, which relies heavily on surface integrity. However, some “hydrophobic” coatings are sensitive to extended water exposure. To determine which hydrophobic coatings may be appropriate for investigating dynamic wetting phenomena, samples of glass slides coated with a series of fluoropolymer coatings were tested by measuring water contact angle before, during, and after extended submersion in deionized water and compared to the same coatings subjected to ultrasonic vibration while covered in deionized water. Both methods caused changes in advancing and receding contact angle, but degradation rates of vibrated coatings, when apparent, were significantly increased. Prolonged soaking caused significant decreases in the contact angle of most coatings, but experienced significant recovery of hydrophobicity when later heat-treated at 160 C. Dissimilar trends apparent in receding contact angles suggests a unique degradation cause in each case. Roughening and smoothing of coatings was noted for coatings that were submerged and heat-treated respectively, but this did not correlate well with the changing water contact angle. Degradation did not correspond to surface acceleration levels, but may be related to how well coatings adhere to the substrate, indicative of a dissolved coating. Most coatings suffered from contact angle degradation between 20-70% when exposed to water over a long period of time, however the hydrophobic fluoropolymer coating FluoroSyl was found to remain unchanged. For this reason it was found to be the most robust coating for providing long term wetting repeatability of vibrated droplets. Droplets (10 to 70 µL) were imaged on hydrophobic surfaces as they were vibrated with ultrasonic piezoelectric transducers. Droplets were vibrated at a constant frequency with ramped amplitude. Spreading of droplets occurs abruptly when a threshold surface acceleration is exceeded of approximately 20,000 m/s2. Droplet contact area (diameter) can be controlled by varying acceleration levels above the threshold. The threshold acceleration was relatively independent of droplet volume, while initial contact angle impacts the extent of spreading. Wetting changes remain after cessation of vibration as long as the vibrated droplet remained within the equilibrium contact angle range for the surface (> the receding contact angle), however new droplets wet with the original contact angle except for some cases where vibration of liquid can affect the integrity of the coating. Reversible wettability of textured surfaces is a desired effect that has various industry applications where droplet manipulation is used, like biomedical devices, coating technologies, and agriculture [3-5].

<p>The aim of the project is to collect empirical data on contact line motion in vibrating tanks under zero-gravity (zero-g) conditions. This study is particularly focused on the behavior of current green propellants, which have a high contact angle compared to traditional stores like water. As a result, the non-linear contact line and angle is expected to have a significant impact on zero-g behavior. The thesis focuses on the dynamic experiment of developing an experimental payload designed to fly on Blue Origin\textquotesingle s New Shepherd suborbital flight. The data collected from this experiment will provide a benchmark case for developers of zero-g fluid dynamics simulations to compare or improve their simulation results. The results will also be useful for testing non-linear hysteresis contact line simulations.</p> <p><br></p> <p>The design of the experiment mainly focuses on conducting oscillatory motion in zero gravity to observe the contact line at varying speeds. Two different liquids are intended to be tested on the same payload. The liquid is to be filled so that the free surface has a height of 1 inch, and the vibration amplitude is to be 0.1 inches. The liquid chosen closely simulates the current green propellants under development or other poorly-wetting liquids. The purpose of each of the components used in the experiment is justified with respect to the given flight design constraints, along with how the constraints impacted the experiment. The experiment is designed to sustain the forces in case of hard landing during the flight and autonomous control of motion. The experiment is staged to be ready for flight on the New Shepherd, and any future works are mentioned. </p> <p><br></p> <p>To meet these constraints, the experimental payload is designed with a variety of components, each chosen for its ability to perform under the given conditions. The payload includes a custom-built system, which generates the oscillatory motion necessary to observe the contact line behavior. The system is designed to be compact and lightweight, yet robust enough to withstand the forces of launch and landing. In addition, the payload includes a custom-built tank designed to hold the liquids being tested. The study of contact line motion in vibrating tanks under zero-g conditions is important in understanding the behavior of liquids in space. This study will provide crucial data that will help in the development of more accurate fluid dynamic simulations for future space missions.</p>

We employ molecular dynamics simulations to investigate the evaporation process of nanosized droplets adsorbed on a substrate. Beads interacting with each other via Lennard-Jones (LJ) potentials are used to construct the simulation systems. The solid substrate contains 6 layers of beads forming a face-centered-cubic lattice. The bottom 3 layers are held rigid while the rest is kept at a constant temperature with a Langevin thermostat. A liquid droplet, consisting of LJ beads as well, is placed on top of the substrate. An appropriate amount of vapor beads are also supplied to the simulation box to help establish liquid-vapor equilibrium. To ensure adsorption, a stronger attraction is rendered between the droplet and a circular patch of 3 layers of beads at the center of the substrate surface while the rest of the substrate is made non-sticky for the fluid beads. During equilibration, the droplet and vapor are maintained at the same temperature as the thermalized substrate. After relaxation, the droplet adheres to the attractive patch as expected. Then a deletion zone is introduced into the top part of the vapor region. Fluid beads in this zone are removed at a given rate. To ensure that the evaporation dynamics and kinetics are properly captured, only the thermalized substrate is kept at the constant temperature during droplet evaporation. To carry out steady-state evaporation, the removed beads are reintroduced into a channel through the substrate and right below the droplet's center. These beads are then supplied to the droplet, compensating for the evaporation loss at the droplet surface. When the evaporation rate and the insertion rate are balanced, the system enters a steady state with the droplet undergoing continuous evaporation and its contact line pinned at the boundary of the attractive patch on the substrate. A one-to-one correspondence is found between the evaporation rate and the total number of fluid beads in the simulation box, as well as the contact angle of the droplet. Using this steady nonequilibrium system, we have mapped out the flow, temperature, and density fields inside and around the evaporating droplet as well as the local evaporation flux along the droplet surface with unprecedented resolutions. The results are used to test the existing theories on sessile droplet evaporation. / Master of Science / Droplet evaporation is a widespread natural phenomenon with numerous applications across various fields. While there has been extensive research on droplet evaporation, it remains a challenge to characterize the interior of the droplet and the local evaporation behavior on the droplet surface. Here we employ molecular dynamics (MD) simulation to model a nanosized droplet adsorbed on a substrate, which evaporates continuously while maintains a constant shape. This is realized by supplying the evaporated fluid back to the bottom of the droplet through an in-silico approach. Such a steady-evaporation system allows us to accurately map out the internal capillary flow of the evaporating droplet with a pinned contact line, where the droplet, vapor, and substrate meet. We find that local evaporation occurs faster near the contact line than at the apex of the droplet.

Wetting, which addresses either spontaneous or forced spreading of liquids on a solid surface, is a ubiquitous phenomenon in nature and can be observed by us on a daily basis, e.g., rain drops falling on a windshield and lubricants protecting our corneas. The study of wetting phenomena can be traced back to the observation of water rising in a capillary tube by Hauksbee in 1706 and still remains as a hot topic, since it lays the foundation for a wide spectrum of applications, such as fluid mechanics, surface chemistry, micro/nanofluidic devices, and phase change heat transfer enhancement. Generally, wetting is governed by the dynamic and static behaviors of the three-phase contact line. Therefore, a deep insight into the dynamics and statics of three-phase contact line at nanoscale is necessary for the technological advancement in nanotechnology and nanoscience. This dissertation aims to understand the dynamic wetting under a molecular kinetic framework and resolve the reconfiguration of liquid molecules at the molecular region of contact line. Water spreading on polytetrafluoroethylene surfaces is selected as a classical example to study the dynamic behaviors of three-phase contact line. To accommodate the moving contact line paradox, the excess free energy is considered to be dissipated in the form of molecular dissipation. As-formed contact line friction/dissipation coefficient is calculated for water interacting with PTFE surfaces with varying structures and is found to be on the same order of magnitude with dynamic viscosity. From an ab initio perspective, contact line friction is decomposed into contributions from solid-liquid retarding and viscous damping. A mathematical model is established to generalize the overall friction between a droplet and a solid surface, which is able to clarify the static-to-kinetic transition of solid-liquid friction without introducing contact angle hysteresis. Moreover, drag reduction on lotus-leaf-like surface is accounted for as well. For the first time, the concept of contact line friction is used in the rational design of a superhydrophobic condenser surface for continuous dropwise condensation. We focus on the transport and reconfiguration of liquid molecules confined by a solid wall to shed light on the morphology of the molecular region of a three-phase contact line. A governing equation, which originates from the free energy analysis of a nonuniform monocomponent system, is derived to describe the patterned oscillations of liquid density. By comparing to the Reynolds transport theorem, we find that the oscillatory profiles of interfacial liquids are indeed governed in a combined manner by self-diffusion, surface-induced convection and shifted glass transition. Particularly for interfacial water, the solid confining effects give rise to a bifurcating configuration of hydrogen bonds. Such unique configuration consists of repetitive layer-by-layer water sheets with intra-layer hydrogen bonds and inter-layer defects. Molecular dynamics simulations on the interfacial configuration of water on solid surfaces reveal a quadratic dependence of adhesion on solid-liquid affinity, which bridges the gap between macroscopic interfacial properties and microscopic parameters. / Doctor of Philosophy / The study of wetting phenomena can be traced back to the observation of water rising in a capillary tube by Hauksbee in 1706 and still remains as a hot topic, since it lays the foundation for a wide spectrum of applications, such as fluid mechanics, surface chemistry, micro/nanofluidic devices, and phase change heat transfer enhancement. The conventional hydrodynamic analysis with no slip boundary condition predicts a diverging shear stress at the contact line as well as an unbounded shear force exerted on the solid surface. To accommodate this paradox, different mechanism and models have been proposed to clarify the slip between a moving contact line and a solid surface. Although almost all models yield reasonable agreement with experimental observations or numerical simulations, it is still difficult to pick up a specific model using only macroscopic properties and experiment-accessible quantities, because the energy dissipation mechanism during dynamic wetting is not identified and the contact line deforms over different length scales. In this dissertation, we ascribe the energy dissipation in dynamic wetting to contact line friction/dissipation under the framework of molecular kinetic theory, as it is assumed that the contact line is constantly oscillating around its equilibrium position. By decomposing contact line friction into two contributions: solid-liquid retarding and viscous damping, we are able to derive a universal model for the contact line friction. This model predicts a decaying solid-liquid friction on superhydrophobic surfaces, corresponding to the lotus effect. In the meantime, this model is able to clarify the recently-discovered static-to-kinetic transition of frictional force between a sessile drop and a solid surface. Later, we used the concept of contact line friction in the droplet growth process in dropwise condensation so as to promote the rational design of superhydrophobic condenser surfaces for sustainable dropwise condensation. As the morphology of a contact line is dependent on the length scale of interest, we focus on the molecular region of contact line. We study the transport and structural change of liquid molecules that are several molecular layers away from the solid surface. It is found that liquid molecules in this region experience patterned density oscillations, which cannot be simply attributed to the random deviations from continuum limit. By combining free energy analysis and Reynolds transport theorem, it is demonstrated that the omnipresent density oscillations arise from the collective effects of self-diffusion, surface-induced convection and shifted glass transition. For liquid water, we propose a bifurcating hydrogen bonding network in contrast to its tetrahedron configuration in bulk water.

Interaction between a solid body and a liquid-air interface plays an important role in multiphase flows, which includes numerous engineering applications such as mineral flotation, dip coating operations, and air-to-sea and sea-to-air projectiles. It is also crucial in animal behaviors like the locomotion of water-walking animals, the plunge-diving of birds, and the jumping out of water of marine creatures. Depending on the moving direction of a solid, such diverse phenomena can be classified into two categories; liquid-entry and liquid-exit. Liquid-entry, or more widely called water-entry, is the behavior of a solid object entering liquid from air. The opposite case is referred to as liquid-exit. Liquid-entry has been extensively studied, especially focusing on cavity formation and the estimation on capillary and hydrodynamic forces on a solid object. However, as the behavior of a triple contact line has not been understood on a sinking object, previous studies were limited to the special case of hydrophobic object to fix the contact line. Moreover, a more recent study pointed out the important role of contact line behavior to characterize the performance of film flotation, which is one of the direct applications of liquid-entry. However, there are no existing previous studies on the dynamics of the contact line on a sinking object. This subject will be first discussed in Chapter 2. In Chapters 3 and 4, the topics related to liquid-exit will be discussed, where a solid sphere exits out of a liquid toward air with constant velocity, acceleration, or deceleration. Chapter 3 will focus on the penetration and bouncing behaviors of a solid sphere while impacting a liquid-air interface. The solid sphere experiences the resistance of surface tension and gravity while impacting the interface. Thus, liquid-exit spheres should have enough momentum to penetrate the interface to overcome these resistances, which indicates that the critical momentum exits. This understanding would give a mechanistic explanation as to why some aquatic species, especially plankton, are able to jump out of water while the others cannot despite their similar size. This study can help to understand the particle-bubble interaction for froth flotation applications, in which the particle tends to attach to the bubble. In the last Chapter, the formation of a liquid column during the liquid-exit will be discussed. It has been observed that the evolution of a liquid column strongly depends on experimental conditions, especially the acceleration of a solid sphere. The pinch-off dynamics of a liquid column is categorized as two branches: upper and lower pinch-off's. The pinch-off location affects the entrained liquid volume adhered to the solid object, which is directly related to the uniform quality of a dip-coating operation. In addition to the pinch-off location and time in relation to the aforementioned experimental conditions will be discussed. In summary, studies in the present dissertation are designed and performed to provide mechanistic insight to the problems in the liquid-entry and liquid-exit, which are all closely related to animal's daily life as well as engineering applications. / PHD

In this work, we develop an interface-preserving level-set method in the finite-element framework for interfacial flows with moving contact lines. In our method, the contact line is advected naturally by the flow field. Contact angle hysteresis can be easily implemented without explicit calculation of the contact angle or the contact line velocity, and meshindependent results can be obtained following a simple computational strategy. We have implemented the method in three dimensions and provide numerical studies that compare well with analytical solutions to verify our algorithm. We first develop a high-order numerical method for interface-preserving level-set reinitialization. Within the interface cells, the gradient of the level set function is determined by a weighted local projection scheme and the missing additive constant is determined such that the position of the zero level set is preserved. For the non-interface cells, we compute the gradient of the level set function by solving a Hamilton-Jacobi equation as a conservation law system using the discontinuous Galerkin method. This follows the work by Hu and Shu [SIAM J. Sci. Comput. 21 (1999) 660-690]. The missing constant for these cells is recovered using the continuity of the level set function while taking into account the characteristics. To treat highly distorted initial conditions, we develop a hybrid numerical flux that combines the Lax-Friedrichs flux and a penalty flux. Our method is accurate for non-trivial test cases and handles singularities away from the interface very well. When derivative singularities are present on the interface, a second-derivative limiter is designed to suppress the oscillations. At least (N + 1)th order accuracy in the interface cells and Nth order accuracy in the whole domain are observed for smooth solutions when Nth degree polynomials are used. Two dimensional test cases are presented to demonstrate superior properties such as accuracy, long-term stability, interface-preserving capability, and easy treatment of contact lines. We then develop a level-set method in the finite-element framework. The contact line singularity is removed by the slip boundary condition proposed by Ren and E [Phys. Fluids, vol. 19, p. 022101, 2007], which has two friction coefficients: βN that controls the slip between the bulk fluids and the solid wall and βCL that controls the deviation of the microscopic dynamic contact angle from the static one. The predicted contact line dynamics from our method matches the Cox theory very well. We further find that the same slip length in the Cox theory can be reproduced by different combinations of (βN; βCL). This combination leads to a computational strategy for mesh-independent results that can match the experiments. There is no need to impose the contact angle condition geometrically, and the dynamic contact angle automatically emerges as part of the numerical solution. With a little modification, our method can also be used to compute contact angle hysteresis, where the tendency of contact line motion is readily available from the level-set function. Different test cases, including code validation and mesh-convergence study, are provided to demonstrate the efficiency and capability of our method. Lastly, we extend our method to three-dimensional simulations, where an extension equation is solved on the wall boundary to obtain the boundary condition for level-set reinitializaiton with contact lines. Reinitialization of ellipsoidal interfaces is presented to show the accuracy and stability of our method. In addition, simulations of a drop on an inclined wall are presented that are in agreement with theoretical results. / Doctor of Philosophy / When a liquid droplet is sliding along a solid surface, a moving contact line is formed at the intersection of the three phases: liquid, air and solid. This work develops a numerical method to study problems with moving contact lines. The partial differential equations describing the problem are solved by finite element methods. Our numerical method is validated against experiments and theories. Furthermore, we have implemented our method in three-dimensional problems.

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 dissertation explores the physics of droplet impingement on superhydrophobic surfaces. The research is divided in three categories. First, the effect of a slip boundary condition on droplet spreading/retracting is considered. A model is developed based on energy conservation to evaluate spreading rates on surfaces exhibiting isotropic and anisotropic slip. The results show that larger slip causes the droplet to spread out farther owing to reduced friction at the interface for both slip scenarios. Furthermore, effects of slip become magnified for large Weber numbers due to the larger solid-liquid contact area during the process. On surfaces with anisotropic slip, droplets adopt an elliptical shape following the azimuthal contour of the slip on the surface. It is common for liquid to penetrate into the cavities at the superhydrophobic interface following droplet impact. Once penetrated, the flow is said to be in the Wenzel state and many superhydrophobic advantages, such as self-cleaning and drag-reduction, become negated. Transition from the Wenzel to the Cassie state (liquid resides above the texture) is referred to as dewetting and is the focus of the second piece of this dissertation. Micro-pillar pitch, height and temperature play a role on dewetting dynamics. The results show that dewetting rates increase with increasing pillar height and increasing surface temperature. A scaling model is constructed to obtain an explanation for the experimental observations and suggests that increasing pillar height increasing the driving dewetting force, while increasing surface temperature decreases dissipation. The last piece of work of this dissertation entails droplet impingement on superheated surfaces (100°C - 400°C). We find that the Leidenfrost point (LFP) occurs at a lower temperature on a hydrophobic surface than a hydrophilic one, where the LFP refers to the lowest temperature at which secondary atomization ceases to occur. This behavior is attributed to the manner in which vapor bubbles grow at the solid-liquid interface. Also in this work, high-speed photographs reveal that secondary atomization can be significantly suppressed on a superhydrophobic surface owing to the micro-pillar forest which allows vapor to escape hence minimizing bubble formation within the droplet. However, a more in-depth study into different superhydrophobic texture patterns later reveals that atomization intensity can significantly increase for small pitch values given the obstruction to vapor flow presented by the increased frequency of the pillars.