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Droplets and bubbles are important for understanding natural phenomena such as falling raindrops, airborne disease transmission, and plant respiration systems, and also for engineering contexts such as semiconductor fabrication, nuclear power plants, and electronics cooling. However, still, more understanding is needed of these complex dynamics problems. This dissertation will talk about the droplet impact and bubble departure dynamics that are happening on various surfaces. In Chapters 2 and 3, we will explore how raindrops can transmit plant pathogens. When the raindrop impacts the infected wheat leaf, the micron-sized dry spore can liberate from the surface in two different ways: dry dispersal and wet dispersal. The dry spore can liberate from the surface by the inertia of the drop, after that, the air vortex generated by the drop impact can carry the dry spores above the laminar boundary layer, with the potential for long-distance transport. For the wet dispersal, spore-laden droplets can be generated after raindrop impact, but how these spore-laden droplets can make neighboring plant diseases is still a mystery. We have shown that the splashed droplets can stick to the adjacent healthy leaf depending on the inertia of the impacting droplet, anisotropic leaf orientation, and whether it is treated with fungicide or not. In Chapter 4, We design a micropillar aluminum substrate that preferentially grows frost on top of the pillars. When deposited droplets impact the frost-tipped pillars, the dynamic pressure causes the water to wick within the frost faster than it can impale the gaps between the pillars. Upon freezing, this safely suspends the resulting ice sheet in the air-trapping Cassie state, without any surface coatings required. For the last part (Chapter 5), we investigated the bubble coalescence dynamics that can depart the bubble with a micrometer size. We made the micro-structured surfaces tailored to nucleation sites to enable the coalescence-induced departure of micro-bubbles. A scaling model reveals two different modes of bubble departure following the coalescence-induced depinning: capillary-inertial jumping for micrometric bubbles and a buoyant-inertial departure for millimetric ones. Eventually, this small bubble departure can delay film boiling which can be the barrier to the boiling heat transfer. / Doctor of Philosophy / Dynamic interaction of droplets and bubbles with different surfaces is ubiquitous: an impacting rain droplet on a plant leaf is responsible for transmitting thousands of plant pathogens, or decreasing the departure size of bubbles on the surface of heat exchangers would increase their efficiency. It is now well-understood that the departure of condensed droplets on water repellent surfaces exhibits superior heat transfer compared to all other modes of condensation and also enables self-cleaning, delayed frosting, and anti-fogging surface technology.In Chapters 2 and 3, we are studying the dynamic interaction of raindrops and wheat leaves. By depositing water droplets on diseased leaves, we found out a raindrop can transmit wheat pathogens. This simple but important phenomenon would adversely affect the quality of our wheat which is the most widely grown crop in the world, contributing to a large amount of portion the global food supply. Chapter 4 sheds light on another example of the dynamic interaction of raindrops and an icy surface. We designed a pillared aluminum substrate that preferentially grows condensation frosting on top of the pillars. With this passive anti-frosting technology, we are able to trap water droplets and ice in the suspending water droplets in the air-trapping Cassie state without using a fragile nanotextured structure or a complex re-entrant structure. Upon freezing, this safely suspends the resulting ice sheet in the air-trapping Cassie state, without any surface coatings required. Under a cold and humid environment, Cassie water freezes into Cassie ice which is advantageous for its low surface adhesion. In Chapter 5, we show that rationally micro-structured surfaces tailor nucleation sites to enable the coalescence-induced departure of micro-bubbles. With this technique, we are able to remove surface bubbles at smaller sizes that would result in enhancing the critical heat flux of nucleate boiling. We have used a blend of experiments and scaling to understand the underlying physics of this phase-change problem.

Bubbles are ubiquitous influencing a multitude of biological processes in natural and industrial environments; this influence is especially relevant during and after bubble rupture. Indeed, the influence of a bubble can extend well beyond its lifetime via the droplets produced when it ruptures. These droplets are known to effectively transport nearby particulates including bacteria and viruses into the surroundings, which in addition to affecting human health can influence global climate by acting as cloud condensation nuclei. Further, the bubble's rupture is a violent event that has been linked to decreased cell viability in bioreactors. However, in all these applications many of the studies have taken an empirical approach, making the results difficult to generalize. Here we combine theory and experiment to investigate the static and dynamic interactions between bubbles and the surrounding microorganisms at a free interface. Our first study focuses on the equilibrium shape a bubble forms after reaching the surface of a liquid. Existing literature is limited to a bubble resting on a flat interface; for example, the surface of a pool or calm lake. However, there are instances where this assumption no longer applies -- a bubble bursting on a raindrop, for example. By relaxing this assumption, we show how a curved boundary alters the final shape of the bubble. Our next study focuses on the enrichment of particulates in the cap of a bursting bubble. As a bubble rises to a free surface, particulates in the bulk liquid are frequently transported to the surface by attaching to the bubble's interface. When the bubble ruptures, a fraction of these particulates are often ejected into the surroundings in film droplets with particulate concentrations higher than the liquid from which originate. However, the precise mechanisms responsible for this enrichment are unclear. By simultaneously recording the drainage and rupture events with high-speed and standard photography, we directly measure the concentrations in a thin bubble film. Based on our results, we develop a physical model and provide evidence that the enrichment is due to a combination of scavenging and film drainage. Our next study focuses on the conditions necessary for a jet droplet to be produced. Past research shows that droplet production is halted when either gravitational or viscous effects are significant. Through systematic experimentation we uncover an intermediate region where both effects are significant, leading to an early end of droplet production. By numerically decoupling the gravitational effects into before and after rupture, we find that the equilibrium shape is responsible for the existence of this intermediate region. Our last study focuses on quantifying the localized stresses produced during spontaneous bubble bursting. Directly simulating each bubble and its effect on the suspended cells in a bioreactor is currently infeasible. Here we illustrate how the results of past works, which disagree by several orders of magnitude for similarly sized bubbles, are primarily a result of the chosen numerical mesh, not the underlying physics. By implementing a particle tracking method, we eliminate this mesh dependence and quantify the extent or volume effected by a single bubble bursting event. Based on our results, we develop a generalizable framework that could be integrated into existing models as a parameterization, removing the need to simulate both phases. / 2019-07-09T00:00:00Z

The complex multiscale physics of nanoparticle laden functional droplets in a reacting environment is of fundamental and applied significance for a wide variety of applications ranging from thermal sprays to pharmaceutics to modern day combustors using new brands of bio-fuels. Understanding the combustion characteristics of these novel fuels (laden with energetic nanoparticle NP) is pivotal for lowering ignition delay, reducing pollutant emissions and increasing the combustion efficiency in next generation combustors. On the way to understanding the complex dynamics of sprays is to first study the behaviour of an isolated droplet. A single droplet represents a sub-grid unit of spray. In vaporizing functional droplets under high heat flux conditions, the bubble formation inside the droplet represents an unstable system. This may be either through homogenous nucleation at the superheat limit or by dispersed nanoparticle acting as heterogeneous nucleation sites. First it is shown that such self-induced boiling in burning functional pendant droplets can induce severe volumetric shape oscillations in the droplet. Internal pressure build-up due to ebullition activity force ejects bubbles from the droplet domain causing undulations on the droplet surface and oscillations in bulk thereby leading to secondary break-up of the primary droplet. Through experiments, it is established that the degree of droplet deformation depends on the frequency and intensity of these bubble expulsion events. However, in a distinct regime of single isolated bubble growing inside the droplet, pre-ejection transient time is identified by Darrieus-Landau (DL) instability at the evaporative bubble-droplet interface. In this regime the bubble-droplet system behaves as a synchronized driver-driven system with bulk bubble-shape oscillations being imposed on the droplet. However, the agglomeration of suspended anaphase additives modulates the flow structures within the droplet and also influences the bubble inception and growth leading to distinct atomization characteristics. Secondly, the secondary atomization characteristics of burning bi-component (ethanol-water) droplets containing titania nanoparticle (NPs) at both dilute (0.5% and 1% by weight) and dense particle loading rates (PLR: 5% and 7.5 wt. %) are studied experimentally at atmospheric pressure under normal gravity. It is observed that both types of nanofuel droplets undergo distinct modes of secondary break-up that are primarily responsible for transporting particles from the droplet domain to the flame zone. For dilute nanosuspensions, disruptive response is characterized by low intensity atomization modes that cause small-scale localized flame distortion. In contrast, the disruption behavior at dense concentrations is governed by high intensity bubble ejections which result in severe disruption of the flame envelope. The atomization events occur locally at the droplet surface while their cumulative effect is observed globally at the droplet scale. Apart from this, a feedback coupling between two key interacting mechanisms, namely, atomization frequency and particle agglomeration also influence the droplet deformation characteristics by regulating the effective mass fraction of NPs within the droplet. Thus, third part of the study elucidates how the initial NP concentration modulates the relative dominance of these two mechanisms thereby leading to a master-slave configuration. Secondary atomization of novel nanofuels is a crucial process since it enables an effective transport of dispersed NPs to the flame (a pre-requisite condition for NPs to burn). Contrarily, NP agglomeration at the droplet surface leads to shell formation thereby retaining NPs inside the droplet. In particular, it is shown that at dense concentrations shell formation (master process) dominates over secondary atomization (slave) while at dilute particle loading it is the high frequency bubble ejections (master) that disrupt shell formation (slave) through its rupture and continuous out flux of NPs. These results in distinct combustion residues at dilute and dense concentrations, thus, providing a method of manufacturing flame synthesized microstructures with distinct morphologies. Next, it is shown that by using external stimuli (preferential acoustic excitation) the secondary atomization of the droplet can be suppressed i.e. the external flame-acoustic interaction with bubbles inside the droplet results in controlled droplet deformation. Particularly, by exciting the droplet flame in a critical, responsive frequency range i.e. 80 Hz ≤ fP ≤ 120 Hz, the droplet deformation cycle is altered through suppression of self-excited instabilities and intensity/frequency of bubble ejection events. The acoustic tuning also enables the control of bubble dynamics, bulk droplet-shape distortion and final precipitate morphology even in burning nanoparticle laden droplets. Droplets in a non-reacting environment (heated radioactively) are also subject to instabilities. One such instability observed in drying colloidal droplets is the buckling of thin viscoelastic shell formed through consolidation of NPs. In the final part of the thesis, buckling instability driven morphology transition (sphere to ring structure) in an acoustically levitated heated nanosilica dispersion droplet is elucidated using dynamic energy balance. Droplet deformation featuring formation of symmetric cavities is initiated by the capillary pressure that is two to three orders of magnitude greater than acoustic radiation pressure, thus indicating that the standing pressure field has no influence on the buckling front kinetics. With increase in heat flux, the growth rate of surface cavities and their post-buckled volume increases while the buckling time period reduces, thereby altering the buckling pathway and resulting in distinct precipitate structures. Thus, the cavity growth is primarily driven by evaporation. However, irrespective of the heating rate, volumetric droplet deformation exhibits linear time dependence and droplet vaporization is observed to deviate from the classical D2-law. Understanding such transients of buckling phenomenon in drying colloidal suspensions is pivotal for producing new functional microstructures with tenable morphology and is particularly critical for spray drying applications that produce powders through vaporization of colloidal droplets.

Invadolysin (INV) is a member of the M8 family of metzincin metalloproteases. The gene was discovered in the Heck laboratory. Based on studies in Drosophila, INV is important for mitotic progression, nuclear envelope protein dynamics, and germ cell migration. INV-like immunoreactivity has shown its association with lipid droplets (LDs), which are intracellular organelles for lipid and protein storage. INV is the first metalloprotease found on LDs. Thus, INV’s role and LD-associated pathways are the puzzles we would like to investigate. The formation of LDs is dependent on the nutritional status of cells and starvation can disrupt the generation of LDs. Based on this concept, I established a starvation / re-feeding system. When nutrition is sufficient, LDs were surrounded by INV, whereas no INV or LDs were found in the majority of starved cells. With a supply of oleic acid (OA), LDs re-appeared and so did INV localized to LDs. In this system, inhibition of protein kinase C (PKC) disrupts INV’s re-localization to LDs. As I found INV to be phosphorylated by PKC in vitro (residues within the N-terminus might be phosphorylated by PKC), I conclude that PKC might regulate INV’s re-localization in the starvation / re-feeding system. 3T3-L1 mouse fibroblasts can differentiate into adipocytes in vitro; this is termed adipogenesis. Since INV is a LD associated protein, the role of INV in adipogenesis is of interest. INV localized on LDs in the early stage of differentiation but disassociated from LDs in mature adipocytes. The levels of INV mRNA and protein were significantly increased upon differentiation to adipocytes. On the other hand, INV decreased when adipocyte differentiation was inhibited by PKC and PI3K inhibitors, suggesting that the increase of INV is required for adipocyte differentiation. I was interested to examine the possible role of INV in InR/PI3K/Akt signalling, and therefore compared wild type with mutant INV (Drosophila INV4Y7). Decreased levels of phospho-Akt and phospho-S6K, and increased mRNA levels of d4E-BP were observed in INV4Y7 mutant larvae, suggesting that INV may be required for InR/PI3K/Akt signalling. In addition, a decreased level of Lsd2 (LD binding protein) was found in INV4Y7 mutants. These correlations between INV and molecules important for signaling suggest that INV might be a mediator of nutritional metabolism. In light of these data, I speculate that INV plays a homeostatic role, possibly by affecting the InR/PI3K/Akt signaling pathway. In conclusion, the localization of INV to LDs is dependent on the activity of PKC. An increase in invadolysin accompanies adipogenesis, in which PKC and PI3K may be mediators. Examining mutant Drosophila, I found INV to be involved in InR/PI3K/Akt signalling. Collectively, I conclude that INV may serve as a regulator in adipogenesis and the InR/PI3K/Akt signaling pathway.

We investigate the behavior of moving droplets and rivulets, driven by a combination of gravity and surface shear (wind). The problem is motivated by a desire to model the behavior of raindrops on aircraft wings. We begin with the Stokes equations and use the approximations of lubrication theory to derive the specific thin film equation relevant to our situation. This fourth-order partial differential equation describing the height of the fluid is then solved numerically from varying initial conditions, using a fully implicit discretization for time stepping, and a precursor film to avoid singularities at the drop contact line. Results describing general features of droplet deformation, limited parameter studies, and the applicability of our implementation to the long-term goal of modeling wings in rain are discussed.

Focused ultrasound therapy can be enhanced with microbubbles by thermal and cavitation effects. However, localization of treatment becomes difficult as bioeffects can occur outside of the target region. Spatial control of gas bubbles can be achieved with acoustic vaporization of perfluorocarbon droplets. This study was undertaken to determine the acoustic parameters for bubble production by droplet vaporization and how it depends on the acoustic conditions and droplet physical parameters. Droplets of varying sizes were sonicated in vitro with a focused ultrasound transducer and varying frequency and exposure. Simultaneous measurements of the vaporization and inertial cavitation thresholds were performed. The results show that droplets cannot be vaporized at low frequency without inertial cavitation occurring. However, the vaporization threshold decreased with increasing frequency, exposure and droplet size. In summary, we have demonstrated that droplet vaporization is feasible for clinically-relevant sized droplets and acoustic exposures.

Focused ultrasound therapy can be enhanced with microbubbles by thermal and cavitation effects. However, localization of treatment becomes difficult as bioeffects can occur outside of the target region. Spatial control of gas bubbles can be achieved with acoustic vaporization of perfluorocarbon droplets. This study was undertaken to determine the acoustic parameters for bubble production by droplet vaporization and how it depends on the acoustic conditions and droplet physical parameters. Droplets of varying sizes were sonicated in vitro with a focused ultrasound transducer and varying frequency and exposure. Simultaneous measurements of the vaporization and inertial cavitation thresholds were performed. The results show that droplets cannot be vaporized at low frequency without inertial cavitation occurring. However, the vaporization threshold decreased with increasing frequency, exposure and droplet size. In summary, we have demonstrated that droplet vaporization is feasible for clinically-relevant sized droplets and acoustic exposures.

The work contained in this thesis presents four research manuscripts concerning the flow and motion of drops and bubbles in different geometries. The first project explores the geometry of a totally wetting droplet on a conical fiber. A droplet on a fiber undergoes spontaneous motion toward the base of the fiber due to capillary forces, and viscous dissipation opposes the motion. In the first paper (Chapter 3), it was found that balancing the viscous shear force with the driving capillary force describes the motion of the droplet along the fiber. However, in nature, if fibers are coated with a liquid, there is rarely one droplet present; the second paper (Chapter 4) studies a conical fiber coated with multiple droplets. A liquid film coating a fiber will break up into droplets and it is found that the spacing of droplets depends on the shape of the fiber. The merging of droplets was studied and the dynamics well matches numerical simulations. The third paper (Chapter 5) studies the fluid film that a droplet will leave behind as it moves along the fiber. Using asymptotic matching to film deposition theory, this study found that the film thickness is affected by the curvature of the droplet. These studies show that the conical geometry and droplet curvature play an important role in droplet motion and film deposition. The last project (Chapter 6) in this thesis concerns a chain of uniform sticky bubbles that rise through an aqueous bath. It is found that the chain of bubbles will buckle regularly as it moves through a liquid bath, much like a solid rope will buckle when impacting a surface. As the bubble chain rises through the bath, a compressive force develops due to an imbalance between the buoyancy of the chain and the viscous drag of the liquid surrounding it. Unlike solid ropes, there is no bending to stabilize the bubble chain and the regular buckling pattern is unex- pected. Using scaling arguments, it is found that the viscous bath both stabilizes the chain and introduces the compressive force. The geometry of the buckling can be described from a force balance between the compressive and stabilizing forces. Drops and bubbles prove to be useful experimental tools to probe driven flow in different geometries and provide valuable insight into fundamental and applied physics systems. / Thesis / Doctor of Science (PhD)