Browsing by Subject "droplet"

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    Interactions between Plasma and Liquid Micro-Droplets
    (2021-02) Nayak, Gaurav
    The interaction of plasmas with a liquid phase results into various complex multiphase phenomena that is beneficial for a wide range of applications, such as water treatment, nanoparticle synthesis, material processing, combustion, decontamination, food safety and human health care. These applications have been made possible due to the transferof highly reactive species from the gas phase plasma to the bulk liquid phase. Upon interaction with a liquid phase, atmospheric pressure non-equilibrium plasmas produce numerous chemically reactive species, including ions, radicals, electrons and (V)UV photons. The resulting short-lived highly reactive species can exhibit huge density gradients in the liquid phase as their finite lifetime does not allow them to penetrate into the bulk liquid. This makes the multiphase reactivity transfer highly transport limited. Additionally, the presence of electric field-induced effects, charging, evaporation and heat and mass transfer makes plasma-liquid interaction studies more challenging due to the lack of a detailed understanding of the inter-coupling of these phenomena and their direct impact on the plasma-produced reactive species fluxes to the liquid. To understand and overcome the challenge of transport limitations, a novel plasma-liquid configuration was developed, which involves plasma activation of small dispersed liquid droplets or aerosols. The key advantage of such a configuration is that the large surface-to-volume ratio of these micrometer-sized liquid droplets enhances the reactivity transfer from the gas phase plasma to the liquid phase. Since the droplets are immersed in the plasma, the short-lived reactive species (electrons, ions and radicals) produced in the gas phase plasma due to electron impact processes on average only need to cover smaller length scales to reach and subsequently penetrate the droplet. The foremost goal of the research reported in this thesis is the development of a controlled and well-defined plasma-microdroplet reactor, which is easy to model and experimentally accessible with different diagnostic techniques enabling to obtain quantitative measurements of reactive species densities. Due to the efficient generation of reactive species, a radiofrequency (RF)-driven capacitively coupled diffuse plasma generated in parallel-plate configuration at atmospheric pressure is used in this work. Complete characterization of the RF plasma is helpful for the assessment of the role of different gas-phase reactive species generated by the plasma and their respective fluxes to the liquid microdroplets in transit through the plasma. The reactive species potentially playing a key role in plasma-liquid interactions include electrons, OH radicals, H and O atoms, singlet oxygen, O3, He and Ar metastable atoms and molecules. The absolute densities of electrons and metastable atoms and molecules along with gas and electron temperatures that play a major role in plasma-induced chemistry in He and Ar plasmas were measured using optical emission and broadband absorption spectroscopy. We found that the densities of He and Ar metastables peaked near the sheath edges away from the droplet trajectory, while the densities in the bulk were below their respective detection limits. The Ar and He metastable fluxes to the droplet were found to be 2 orders of magnitude and 5 times smaller than the electron flux to droplet, respectively. Hence, the effect of Ar and He metastable atoms on the droplet chemistry can be considered negligible compared to charged species fluxes. However, these metastable species are an important source of ionization in such low electron density plasmas and play, nonetheless, a major role in the plasma dynamics including ionization processes and the generation of radicals in the gas phase. An understanding of the dynamics of liquid microdroplets in the plasma is important as it not only relays information about the residence time of the droplets in the plasma (i.e. treatment time of the solution by short-lived plasma-produced species) but also droplet charging and the presence of electric fields when entering and exiting the plasma. This residence time dictates the fate of the droplet chemistry and its interactions with the plasma. We characterized the droplet and its trajectory by fast frame imaging in diffuse He glow discharge. From the droplet velocity and acceleration data, the various forces acting on the droplet were evaluated. Using the equilibrium of forces and the droplet charge estimated from an analytical model, the electric field at the plasma edges was determined. We also report on the effect of the initial droplet acceleration during droplet ejection from the piezoelectric nozzle, the plasma composition (He with admixtures of Ar, H2O, H2 and O2), gas flow rate, and the plasma power on the droplet dynamics. Plasmas in or in contact with water have been extensively investigated in the context of plasma-aided decomposition and mineralization of recalcitrant organic pollutants in water for environmental remediation. However, plasma, while being highly effective, sometimes lacks energy efficiency. The water treatment is often due to the transfer of long-lived species into the liquid bulk and secondary reactions. Using the approach of microdroplets treated by plasma, the efficiency of plasma treatment of organics in liquid water microdroplets can be improved by increasing the species fluxes to the droplets. Using detailed plasma diagnostics, droplet characterization and ex situ chemical analysis of the treated droplets, we assessed the relative importance of short-lived radicals, such as O and H atoms, singlet oxygen, solvated electrons and ions, besides OH radicals, responsible for the decomposition of formate, a model organic compound inwater treatment studies, dissolved in droplets. We ascertained the role of OH and O radicals in electronegative plasmas. We also showed for electropositive plasmas that solvated electrons and/or ions injected into the droplet were dominantly responsible for plasma-induced chemistry in the droplet. Results suggested that the charged species lead to the formation of H or OH radicals near the droplet interface via secondary reactions, enabling further decomposition of formate in the droplets. The obtained results allowed us to estimate minimum values of transport properties of O in solution and reaction rates of O radical with formate using a one-dimensional reaction-diffusion model. Gold and silver nanoparticles (AuNP and AgNP) exhibit unique optical, electrical, and thermal properties, and can be synthesized by plasmas in a green and environmentally friendly approach without the use of hazardous chemicals, and without producing harmful byproducts. Previously, researchers have established unprecedented gold ions reduction synthesis rates in liquid droplets treated by an RF plasma to synthesize AuNPs. The reported reduction rates are several orders of magnitude larger than for any other reported electron-initiated methods. The mechanism for the synthesis of nanoparticles using this approach is still largely unknown. Using the known gas-phase plasma properties, absorption spectroscopy, and transmission electron microscopy (TEM), we identified the role of hydrogen peroxide in the reduction of gold ions. On the other hand, the reported results for AgNP formation from Ag ions in the same reactor, suggested that the effect of solvated electrons and H radicals were dominant for the reduction of silver ions. We also demonstrate the possibility to use plasma-droplet interactions for the synthesis of surfactant-free, spherical and crystalline Au and AgNPs without the use of any stabilizer(s) within a few milliseconds. In view of the recent COVID-19 pandemic caused by the airborne transmission of SARS-CoV-2 virus aerosols, many excellent surveillance and control measures are being implemented in conned spaces, where the effect of the virus transmission is the highest. The application of plasma-aided virus disinfection is quite nascent, and the interaction of plasma with the virus aerosols in air streams has not been dealt with in detail. The actual mechanism for the virus inactivation is often ascribed to ozone, however, in short time scales of milliseconds, more reactive species might be needed to obtain an effective inactivation. We report on the use of a dielectric barrier discharge (DBD) for in-flight inactivation of airborne aerosolized porcine reproductive and respiratory syndrome (PRRS) virus. The measurements were performed in a laboratory-scale wind test tunnel. Using infectivity tests and reverse transcriptase quantitative polymerase chain reaction (RT-qPCR), we showed a 3.5 log10 reduction in the viable virus titer during in-flight treatment by the DBD within a few milliseconds. We identified both short-lived species such as OH radicals and singlet oxygen and peroxynitrous acid chemistry at low pH in the virus-laden droplets responsible for the observed inactivation of virus aerosols by plasma. The fundamental understanding of the interactions of plasma and liquid microdroplets, gained in this work, have the potential to increase significantly the efficacy of plasma processes. There is no doubt that improved reactor design and plasma generation informed by our findings will further advance the development of such unique interactions for the novel applications of water treatment, nanoparticle synthesis and virus aerosol inactivation.
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    Modeling of Multicomponent Coatings
    (2023-05) Larsson, Christopher
    Thin liquid films play a central role in coating processes and other industrial and natural applications. Efficient optimization of these processes requires an understanding of capillary leveling, Marangoni flow, evaporation, and related phenomena. Although mathematical models are useful for gaining such understanding, it can be difficult to extract physical insight as the number of phenomena considered increases, so simplifying assumptions such as the vertical-averaging (VA) approximation for solute concentration are often employed. In the first part of this work, we examine the performance of the VA approximation for three common evaporation models: constant, one-sided, and diffusion-limited. We find that the formal regime of validity of the VA approximation is inaccurate and strongly depends on the evaporation rate. Furthermore, applying the VA approximation outside of its regime of validity results in drastically different film-height and solute-distribution predictions depending on the evaporation model. Many applications often demand multilayer films where each layer has distinct properties, and this gives rise to additional challenges. It has been experimentally demonstrated that two-layer films in which the layers are miscible can undergo dewetting, but theoretical understanding of this phenomenon is lacking. The second part of this work addresses the mechanisms that may initiate dewetting in miscible two-layer two-component films. It is found that a disparity in initial solute concentration between the film layers drives flows that lead to significant film-height nonuniformities. The third part of this talk focuses on evaporating sessile droplets which are critical to many industrial applications and are also ubiquitous in nature. Two predominant evaporation models have emerged in the literature, one-sided and diffusion-limited, with differing assumptions on the evaporation process. While both models are widely used and their predictions can differ greatly from each other, the physical mechanisms underlying these differences are not yet well understood. For the one-sided model, we derive expressions for the droplet lifetime, show that the evaporation rate is proportional to the droplet surface area, and demonstrate that the contact line is always warmer than the bulk of the droplet. Furthermore, we show that differences in the structure of the evaporation models near the contact line lead to qualitatively different behavior of the apparent contact angles and interface temperature profiles.