Dissertations / Theses on the topic 'Levitated droplets'

Ultraschalllevitation kombiniert die Vorteile von Mikrofluidik, wie beispielsweise die sehr geringe benötigte Probenmenge, mit einer wandlosen Probenhandhabung. Obwohl die Kopplung zwischen le- vitierten Tröpchen und verschiedenster analytischer Methoden wie optischer Spektroskopie und Röntgenbeugung sehr genau untersucht ist, fehlt es immer noch an einer etablierten Kopplung mit einer massenspektrometrischen Methode für die Analyse auf molekularer Ebene. Die vorliegende Arbeit beschreibt die Prinzipien, auf denen eine kontaktlose massenspektrometrische Analyse von levitierten flüssi- gen Proben beruht. Zuerst wurde der neu entworfene akustische Levitator bezüglich des Einflusses seiner Geometrie auf die Levi- tationseigenschaften experimentell und mittels numerischer Simul- tationen untersucht. Die anschließend durch geführten Experimen- te demonstrieren das Potential von Infrarot-Lasern als kombinierte Desorptions- und Ionisationsquelle für organische Substanzen aus einer Mischung aus Wasser und Glycerin als Cromophor. Um einen tieferen Einblick in die hierbei ablaufenden Ionisationsmechanismen zu erhalten, wurde als Modell ein “Sonic-Spray” Konus räumlich per Massenspektrometrie und Laser-induzierter Fluoreszenz untersucht. Levitator-Geometrie auf die Levitationseigenschaften stimmen sehr gut mit numerischen Simulationen überein. Als komplementäre Ionisationsmethode wurde eine Niedertemperatur-Plasmaquelle ein- gesetzt. Nach einer zeitaufgelösten Untersuchung der grundlegenden Ionisationsmechanismen wurde diese Quelle für die Untersuchung flüchtiger Spezies aus der levitierten Probe in deren direkten Umgebung ohne störende Interferenzen ge- nutzt.
Ultrasonic levitation combines advantages of microfluidics like the required small sample volumes with a wall-less sample handling. While the coupling of analytical methods like optical spectroscopy as well as x-ray scattering are very well elaborated, an established mass spectrometric method to obtain molecular analytical information is still lacking. The herein presented work describes the fundamental processes for a contactless mass spectrometric analysis of levitated droplets. First, the influences of the specially designed levitator geometry on the levitation capabilities is described. During further experiments, the use of infrared lasers has proven useful as a combined desorption and ionization source for organic molecules from a mixture of water and glycerol as chromophore. Subsequently, sonic-spray ionization was used to gain a deeper understanding of the ionization processes occurring within the spray plume. Mass spectrometric mapping as well as laser-induced fluorescence were performed to investigate the ionization during an aerodynamic breakup of the micro droplets in the spray process. As a complementary sampling method, the ionization with a low- temperature plasma source is described. First, a time-resolved mass spectrometric investigation of the ionization process is shown. Sub- sequent to this fundamental study, the application of such a plasma source for the direct analysis of volatile compounds from within the droplets in the surrounding environment without interferences from the droplets bulk phase is described.

Evaporation of pure and binary liquid droplets is of interest in thermal sprays and spray drying of food, ceramics and pharmaceutical products. Understanding the rate of heat and mass transfer in any drying process is important not only to enhance evaporation rate or vapor-gas mixing, but also to predict and control the final morphology and microstructure of the precipitates. Acoustic levitation is an alternative method to study micron-sized droplets without wall effects, which eliminates chemical and thermal contamination with surfaces. This work uses an ultrasonic levitation technique to investigate the vaporization dynamics under radiative heating, with focus on evaporation characteristics, precipitation kinetics, particle agglomeration, structure formation and droplet stability. Timescale and temperature scales are developed to compare convective heating in actual sprays and radiative heating in the current experiments. These relationships show that simple experiments can be conducted in a levitator to extrapolate information in realistic convective environments in spray drying. The effect of acoustic streaming, droplet size and liquid properties on internal flow is important to understand as the heat and mass transfer and particle motion within the droplet is significantly controlled by internal motion. Therefore, the droplet internal flow is characterized by Particle Image Velocimetry for different dropsize and viscosity. Nanosuspension droplets suspended under levitation show preferential accumulation and agglomeration kinetics. Under certain conditions, they form bowl shaped structures upon complete evaporation. At higher concentrations, this initial bowl shaped structure morphs into a ring structure. Nanoparticle migration due to internal recirculation forms a density stratification, the location of which depends on initial particle concentration. The time scale of density stratification is similar to that of perikinetic-driven agglomeration of particle flocculation. The density stratification ultimately leads to force imbalance leading to a unique bowl-shaped structure. Chemically active precursor droplet under acoustic levitation shows events such as vaporization, precipitation and chemical reaction leading to nanoceria formation with a porous morphology. The cerium nitrate droplet undergoes phase and shape changes throughout the vaporization process followed by formation of precipitate. Ex-situ analyses using TEM and SEM reveal highly porous morphology with trapped gas pockets and nanoceria crystalline structures at 70 degree C. Inhomogeneity in acoustic pressure around the heated droplet can induce thermal instability. Short wavelength (Kelvin-Helmholtz) instability for diesel and bio-diesel droplets triggers this secondary atomization, which occurs due to relative velocity between liquid and gas phase at the droplet equator. On the other hand, liquids such as Kerosene and FC43 show uncontrollable stretching followed by a catastrophic break-up due to reduction in surface tension and viscosity coupled with inhomogeneity of pressure around the droplet. Finally, a scaling analysis has been established between vaporizing droplets in a convective and radiative environment. The transient temperature normalized by the respective scales exhibits a unified profile for both modes of heating. The analysis allows for the prediction of required laser flux in the levitator experiments to show its equivalence in a corresponding heated gas stream. The theoretical equivalence shows good agreement with experiments for a range of droplet sizes.
ID: 031001564; System requirements: World Wide Web browser and PDF reader.; Mode of access: World Wide Web.; Adviser: Ranganathan Kumar.; Co-adviser: Saptarshi Basu.; Title from PDF title page (viewed August 26, 2013).; Thesis (Ph.D.)--University of Central Florida, 2012.; Includes bibliographical references (p. 234-250).
Ph.D.
Doctorate
Mechanical and Aerospace Engineering
Engineering and Computer Science
Mechanical Engineering

Acoustic levitation permits the study of droplet dynamics without the effects of surface interactions present in other techniques such as pendant droplet methods. Despite the complexities of the interactions of the acoustic field with the suspended droplet, acoustic levitation provides distinct advantages of controlling morphology of droplets with nanosuspensions post precipitation. Droplet morphology is controlled by vaporization, deformation and agglomeration of nanoparticles, and therefore their respective timescales are important to control the final shape. The balance of forces acting on the droplet, such as the acoustic pressure and surface tension, determine the geometry of the levitated droplet. Thus, the morphology of the resultant structure can be controlled by manipulating the amplitude of the levitator and the fluid properties of the precursor nanosuspensions. The interface area in colloidal nanosuspensions is very large even at low particle concentrations. The effects of the presence of this interface have large influence in the properties of the solution even at low concentrations. This thesis focuses on the dynamics of particle agglomeration in acoustically levitated evaporating nanofluid droplets leading to shell structure formation. These experiments were performed by suspending 500&"181;m droplets in a pressure node of a standing acoustic wave in a levitator and heating them using a carbon dioxide laser. These radiatively heated functional droplets exhibit three distinct stages, namely, pure evaporation, agglomeration and structure formation. The temporal history of the droplet surface temperature shows two inflection points. Morphology and final precipitation structures of levitated droplets are due to competing mechanisms of particle agglomeration, evaporation and shape deformation. This thesis provides a detailed analysis for each process and proposes two important timescales for evaporation and agglomeration that determine the final diameter of the structure formed. It is seen that both agglomeration and evaporation timescales are similar functions of acoustic amplitude (sound pressure level), droplet size, viscosity and density. However it is shown that while the agglomeration timescale decreases with initial particle concentration, the evaporation timescale shows the opposite trend. The final normalized diameter hence can be shown to be dependent solely on the ratio of agglomeration to evaporation timescales for all concentrations and acoustic amplitudes. The experiments were conducted with 10nm silica, 20nm silica, 20nm alumina and 50nm alumina solutions. The structures exhibit various aspect ratios (bowls, rings, spheroids) which depend on the ratio of the deformation timescale (tdef) and the agglomeration timescale (tg).
M.S.A.E.
Masters
Mechanical and Aerospace Engineering
Engineering and Computer Science
Aerospace Engineering; Thermofluid Aerodynamic Systems

An induction setup for levitation studies of molten metals was built. The setup was used to levitate and heat stainless steel samples of 2.00 g to 1600 °C and subject them to different atmospheres. Changes in shape and temperature were recorded by video and infrared thermocouple. Oxide films forming on the droplets during levitation were observed. It was possible to notice an immediate surface reaction when the reaction gas was introduced. This reaction is concluded to influence the surface and bulk composition, and therefore have an effect on the shape evolution of the droplet. A more oxidizing atmosphere resulted in a more conical droplet shape; this is thought to be an effect of lowered surface tension and the conically shaped volumetric force caused by the magnetic field. Changes in temperature after the sample is molten are thought to be an effect of changes in emissivity, caused by surface oxidization. Post mortem analysis show a difference in surface morphology for samples subjected to different gases, as well as a difference in amount of oxidization.

The electrodynamic balance is a very unique and versatile device to study single particles. In-situ measurements of particles in a well characterized and controlled environment are possible. Supersaturated solutions can also be studied. In this research, its applications in studying light scattering, water activities and evaporation kinetics of single droplets are demonstrated. In particular, we studied the elastic and Raman scattering of an evaporating NaNO3 droplet. Different types of size dependent optical resonance structures were identified. The strongest Raman signal received was due to internal resonance of the excitation beam, giving similar enhancements to all Raman emissions. The intensity ratio of Raman nitrate to Raman water peaks can be used as a probe to semi-quantitatively characterize the droplet compositions. Water activities of mixed NH4NO3/(NH4)2SO4 aqueous solutions were also studied using the Spherical Void Electrodynamic Levitator. The compositional water activity data were used to evaluate the performance of three commonly used mixed electrolyte models: the Zdanovskii-Strokes-Robinson model, the Kusik and Meissner model, and the Pitzer model. They all predict droplet concentrations in mass fractions to a few percents error within the range where water activity data of single electrolytes are available. Evaporation of a few ceramic precursor solution droplets were investigated. While some precursor solutions crystallized, some others formed gels. Gel formation hindered further evaporation of water and the droplets exhibited a sharp decrease in evaporation rates. An approach to study rapid evaporation of droplets in the time scales of a few seconds was also demonstrated.

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.

The condition of fluid flow has been proven to have a significant influence on a wide variety of material processes. In electromagnetic levitation (EML) experiments, the internal flow is driven primarily by electromagnetic forces. In 1-g, the positioning forces are very strong and the internal flows are turbulent. To reduce the flows driven by the levitation field, experiments may be performed in reduced gravity and parabolic flights experiments have been adopted as the support in advance. Tracer particles on the surface of levitated droplets in EML experiment performed by SUPOS have been used to investigate the transition from laminar to turbulent flow. A sample of NiAl3 was electromagnetically levitated in parabolic flight and the laminar-turbulent transition observed from the case was studied in this work. For the sample with clearly visible tracer patterns, the fluid flow has been numerical evaluated with magnetohydrodynamic models and the laminar-turbulent transition happened during the acceleration of the flow, instead of steady state. The Reynolds number at transition was estimated approximately as 860 by the experiment record. The predicted time to transition obtained from the results of simulation showed significant difference (~ up to 300 times) compared with the time obtained from the experiment—0.37s. The discrepancy between numerical and experimental results could not be explained by the proposed hypotheses: geometry, boundary conditions or solid core. The simulations predict that the flow would become turbulent almost instantaneously after the droplet was fully molten. There are important physics shown by the simulation which were not captured.

Spray drying is widely used in many industries, e.g. chemical, food and pharmaceutical processing, in which the spray generation is followed by solvent/liquid evaporation leading to a final powder product. As the liquid component evaporates, the droplet weight decreases. This evaporation process is accompanied by phase change of the volatile component. For a multiphase droplet, solid material is suspended or dissolved in the liquid phase. In such cases, it is important to have knowledge of transport phenomena for single droplets as it pertains to their evaporation and component distribution. The investigation of the evaporation of single droplets is particularly applicable to the study of spray drying and atomization. There are several different measurement techniques that can be used to analyze droplets of various components such as pure liquids, mixtures, solutions, and colloidal suspensions. The principle of ultrasonic levitation is unique in that it allows the isolation of a single droplet in space rather than a myriad of droplets. Having a multiphase droplet suspended in the acoustic field facilitates the study of the drying rate at different operational parameters, and tracking the morphological changes during the course of the drying process. The experiments conducted in this work were performed for a single droplet or separation of two single droplets. Single droplet analysis allows for characterization of the droplets drying rate. The multi-droplet analysis is pertinent to sound pressure level calibration. Understanding the sub-models for these processes is largely beneficial to the advance of modeling the overall spray drying process. In the present work, the drying of further example systems has been studied, and a characterization of the outer crust formed at the end of the first drying stage is presented. These are accompanied by "quasi" 2-D model formulations for the simulation of individual drop drying to describe the solid concentration profiles and the creation of the crust at the end of the constant rate period, when the solids concentration at the surface exceed the saturation concentration.

by Chan Chiu Wah.
Thesis (M.Phil.)--Chinese University of Hong Kong, 1996.
Includes bibliographical references (leaves [68-69]).
by Chan Chiu Wah.
List of Tables --- p.i
List of Figures --- p.ii
Acknowledgments --- p.iv
Abstract --- p.v
Chapter Chapter 1 --- Introduction --- p.1
Chapter Chapter 2 --- Theory --- p.4
Chapter 2.1 --- Radiation pressure on a droplet --- p.4
Chapter 2.2 --- Laser levitation of a droplet --- p.7
Chapter 2.3 --- Dynamic of a laser levitated spherical droplet --- p.9
Chapter Chapter 3 --- Experiment --- p.11
Chapter 3.1 --- Principle and calibration of the position sensor --- p.14
Chapter 3.2 --- Measurement of the levitated laser beam waist --- p.16
Chapter Chapter 4 --- Results and discussion --- p.18
Chapter 4.1 --- Vertical motion due to MDRs --- p.22
Chapter 4.2 --- Artifacts in the displacement --- p.26
Chapter 4.3 --- Elastic scattering light intensity --- p.28
Chapter 4.4 --- Effect of the size of beam waist --- p.31
Chapter 4.5 --- Small size parameter droplet --- p.33
Chapter Chapter 5 --- Conclusion and future outlook --- p.35