A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Impact of Polymeric Drops on Drops and Films of a Different but Miscible Polymer(2024) Bera, Arka; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The fluid mechanics of a liquid drop impacting on another stationery (or spreading) liquid drop or on a liquid film (of thickness comparable, or smaller, or larger than the impacting drop) has attracted significant attention over the past several years. Such problems represent interesting deviations from the more widely studied problems of liquid drops impacting on solid surfaces having different wettabilities with respect to the impacting drops. These deviations stem from the fact that the resting liquid (in the form of the drop or the film) itself undergoes deformation on account of the drop impact and can significantly affect the overall combined drop-drop or drop-film dynamics. The problem becomes even more intriguing depending on the rheology of the drop(s) and the film as well as the (im)miscibility of the impacting drop with the underlying drop or the film. Interestingly, the majority of such drop-impact-on-drop or drop-impact-on-film studies have considered Newtonian drop(s) and films, with little attention to polymeric drop(s) and films. This thesis aims to bridge that void by studying, using Direct Numerical Simulation (DNS) based computational methods, the impact-driven dynamics of one polymeric drop on another (different but miscible) polymeric drop or film. As specific examples, we consider two separate problems. In the first problem, we consider the impact of a PMMA (poly-methyl methacrylate) drop on a resting PVAc (polyvinyl acetate) drop as well as the impact of a PVAc drop on a resting PMMA drop. In the second problem, we consider the impact of a PMMA drop on a PVAc film as well as the impact of a PVAc drop on a PMMA film. For the first problem, the wettability of the resting drop (on the resting surface), the Weber number of the impacting drop, the relative surface tension values of the two polymeric liquids (PVAc and PMMA), and the miscibility (or how fast the two liquids mix) dictate the overall dynamics. PVAc has a large wettability on silicon (considered as the underlying solid substrate); as a result, during the problem of the PMMA drop impacting on the PVAc drop, the PVAc drop spreads significantly and the slow mixing of the two liquids ensures that the PMMA drop spreads as a thin film on top of the PVAc film (formed as the PVAc drop spreads quickly on silicon). Depending on the Weber number, such a scenario leads to the formation of transient liquid films (of multitudes of shapes) with stratified layers of PMMA (on top) and PVAc (on bottom) liquids. On the other hand, for the case of the PVAc drop impacting on the PMMA drop, a combination of the weaker spreading of the PMMA drop on silicon and the “engulfing” of the PMMA drop by the PVAc drop (stemming from the PVAc having a smaller surface tension than PMMA) ensures that the impacting PVAc drop covers the entire PMMA drop and itself interacts with the substrate giving rise to highly intriguing transient and stratified multi-polymeric liquid-liquid structures (such as core-shell structure with PMMA core and PVAc shell). For both these cases, we thoroughly discuss the dynamics by studying the velocity field, the concentration profiles (characterizing the mixing), the progression of the mixing front, and the capillary waves (resulting from the impact-driven imposition of the disturbance). In the second problem, we consider a drop of the PMMA (PVAc) impacting on a film of the PVAc (PMMA). In addition to the factors dictating the previous problem, the film thickness (considered to be either identical or smaller than the drop diameter) also governs the overall droplet-impact-driven dynamics. Here, the impact, being on the film, the dynamics is governed by the formation of crown (signifying the pre-splashing stage) and a deep cavity (the depth of which is dictated by the film thickness) on the resting film. In addition to quantifying these facets, we further quantify the problem by studying the velocity and the concentration fields, the capillary waves, and the progression of the mixing front. For the PMMA drop impacting on the thin film, a noticeable effect is the quick thinning of the PMMA drop on the PVAc film (or the impact-driven cavity formed on the PVAc film), which gives rise to a situation similar to the previous study (development of transient multi-polymeric-liquid structures with stratified polymeric liquid layers). For the case of the PVAc drop impacting on the PMMA film, the PVAc liquid “engulfs” the deforming PMMA film, and this in turn, reduces the depth of the cavity formed, the extent of thinning, and the amplitude of the generated capillary waves. All these fascinating phenomena get captured through the detailed DNS results that are provided. The specific problems considered in this thesis have been motivated by the situations often experienced during the droplet-based 3D printing processes (e.g., Aerosol jet printing or inkjet printing). In such printing applications, it is commonplace to find one polymeric drop interacting with an already deposited polymeric drop or a polymeric film (e.g., through the co-deposition of multiple materials during multi-material printing). The scientific background for explaining these specific scenarios routinely encountered in 3D printing problems, unfortunately, has been very limited. Our study aims to fill this gap. Also, the prospect of rapidly solidifying these polymeric systems (via methods such as in-situ curing) can enable us to visualize the formation of solidified multi-polymeric structures of different shapes (by rapidly solidifying the different transient multi-polymeric-liquid structures described above). Specifically, both PMMA and PVAc are polymers well-known to be curable using in-situ ultraviolet curing, thereby establishing the case where the present thesis also raises the potential of developing PMMA-PVAc multi-polymeric solid structures of various shapes and morphologies.Item EXPERIMENTAL INVESTIGATION OF BOUNDARY LAYER TRANSITION ON CONE-FLARE GEOMETRIES AT MACH 4(2024) Norris, Gavin; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study investigates supersonic boundary layer transition on a cone-flarewith a 5° half-angle straight cone and flared bases of +5°, +10°, and +15°. The experiments used the University of Maryland's Multiphase Flow Investigations Tunnel (MIST), a Mach 4 Ludweig tube. Experiments were performed “dry”, without aerosols or droplets, and focus on the first-mode (Tollmien-Schlichting) boundary layer instability waves and their interaction with the compression corner. Using high-speed Schlieren imaging, the boundary layer dynamics on the cone-flare's top surface were analyzed. The data were processed through Power Spectral Density (PSD) and Spectral Proper Orthogonal Decomposition (SPOD) techniques to study the behavior of the first-mode waves and the transition location changes. The findings reveal coherent wave packets within the boundary layer at frequencies characteristic of the first-mode. The wave packets power increased along the cone and peaked near the compression corner before dissipation on the flare. These findings contribute to the understanding of first-mode boundary layer transition mechanisms in hypersonic flows for the cone-flare geometry.Item AEROSOL EFFECTS IN HIGH SUPERSONIC FLOWS(2024) Schoneich, Antonio Giovanni; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The understanding of high-speed aerodynamics is becoming evermore pertinent with thegrowth of space tourism, continued interest in space exploration, and pursuit of advanced highspeed aircraft for both military and commercial use. For initial investigations, ground test facilities are preferred to flight tests as they are far cheaper and carry significantly less risk, although wind tunnels can only replicate a subset of the conditions experienced in actual flight. One of these conditions that has not been adequately captured in wind tunnels is the effect of particulates in the atmosphere. Typical wind tunnels use a pure, clean gas (air, nitrogen, etc.) for testing, but this does notcapture the aerosolized nature of the atmosphere, where humidity and condensation can produce a distribution of liquid droplet sizes ranging from the average rain drop of 2mm to sub-micron diameter particles. Similarly, volcanic eruptions and ever-present wildfires result in solid particles exhibiting a variety of species and sizes that are transported to every layer of the atmosphere. At supersonic speeds, encounters with particulates have been shown to lead to detrimental effects, such as material erosion and boundary layer transition. Previous attempts to study this problem in wind tunnels have focused mainly on sub-micronsized solid particles, since aerosol settling time is a major limiting factor. On the other hand, most high-speed experiments involving large liquid droplet impacts have been carried out in gas guns or ballistic ranges due to the difficulty of trying to accelerate a droplet to high speeds without causing it to break up. While these facilities can be used to study impacts, the moving model means that detailed aerodynamic studies are nearly impossible, leading to a large gap in knowledge. To perform high-speed wind tunnel testing with liquid aerosols representative of cloud-likeenvironments (5-20 μm), a Mach-4 facility, referred to as the Multi-phase Investigations Supersonic Tunnel (MIST) has been designed and developed at the University of Maryland (capable of producing supersonic, particle-laden flows). This range of aerosol sizes makes MIST a unique facility with significant potential for expanding the state of the art in high-speed multi-phase flows. The present work discusses the design and characterization of MIST as well as two major experimental investigations carried out using this new facility. The first investigation examines the force augmentation on a free-flying sphere exposed to supersonic, particle-laden flows. Freeflight measurements are performed with five different particle size and concentration combinations. When comparing the results for particle-free flow in the same facility, the drag coefficient of the sphere was shown to be 1.75-4.5% greater for all multi-phase cases; this is significantly higher than simple estimates based on the increased momentum flux in the freestream would indicate. In addition to force measurements, an experimental investigation into the effect of particle-ladenflows on boundary-layer transition was conducted. It is important to characterize the disturbance environment in wind tunnels since they typically do not represent the levels in atmospheric flight and can lead to earlier onset of boundary-layer transition. In performing such measurements using a single-point Focused Laser Differential Interferometer, it was discovered that the presence of particles in the flow could significantly attenuate the acoustic disturbances generated by the wind tunnel. This finding was further reinforced when investigating the boundary-layer transition on a 5◦ half-angle, sharp cone using high-speed schlieren visualization. For each case presented in this work, the boundary-layer disturbance amplitudes were reduced and transition Reynolds numbers increased in the particle-laden flow cases. This was contrary to expectations, given that prior numerical studies have indicated that particles can induce early transition. These findings potentially open a path to substantially reduce freestream disturbance levels in conventional hypersonic wind tunnels.Item PARTICLE INDUCED TRANSITION IN HIGH-SPEED BOUNDARY-LAYER FLOWS(2024) Abdullah Al Hasnine, Sayed Mohammad; Brehm, Christoph; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Boundary-layer transition to turbulence presents a critical challenge in aerospace engineering due to its impact on thermal load, especially for hypersonic vehicles. This transition, influenced by various disturbances such as acoustic waves, entropy waves, and particle impingement, follows complex and non-unique pathways to turbulence. It significantly affects the surface heat flux and thus will impact the design of thermal protection systems. This dissertation focuses on the transition process initiated by particle impingement, which introduces small-scale disturbances through a complex receptivity process that typically initiates a natural transition path. Using direct numerical simulations, this study explores the particle-induced transition process. The disturbance spectrum, consisting of both stable and unstable modes along with continuous acoustic contributions, is meticulously reconstructed near the particle impingement site using biorthogonal decomposition to assess the contributions of different eigenmodes to the initial disturbance spectrum. A large number of discrete and continuous eigenmodes are seeded, but the dominant eigenmodes capture only a small fraction of the disturbance energy, with the majority reflected into the freestream through the continuous modes associated with the continuous acoustic branches. The modeling fidelity is also investigated, particularly the particle-source-in-cell (PSIC) approach, commonly used due to its efficiency in capturing particle-flow interactions. Comparisons with the Immersed-Boundary-Method (IBM), however, reveal that PSIC inadequately captures particle-wall interactions and needs correction for accurate disturbance modeling. Finally, a reduced-order model is developed for the prediction of particle-induced transition. This model integrates data from high-fidelity simulations, linear stability theory, and a saturation amplitude model while also considering particle characteristics like size, density and concentration. The model’s capability is demonstrated for a wide range of transition scenarios, including data from the HIFiRE-1 flight test, offering a robust tool for rapid transition prediction in hypersonicvehicle design.Item GLOBAL ANALYSIS OF TRANSITIONAL HYPERSONIC FLOW OVER CONE AND CONE-FLARE GEOMETRIES(2024) Sousa, Cole Edward; Laurence, Stuart; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Accurately predicting the laminar-to-turbulent boundary-layer transition on hypersonic vehiclesremains one of the principal challenges in characterizing the expected heat loads and skin friction the vehicle will experience in flight. Ground facilities, while incapable of replicating the complete set of flow conditions found at hypersonic flight, play a critical role in providing physical measurements of the transition process. The experimental characterization of hypersonic boundary-layer disturbances, however, has traditionally faced limitations in its ability to provide spatiotemporally dense data sets comparable to those of computational fluid dynamics (CFD) investigations. The present work aims to provide global off-body measurements of hypersonic boundary-layer disturbances at frequencies much greater than that of the fundamental instability, enabling the exploration of nonlinear phenomena and more extensive comparisons between experimental and computational studies. The current methodology utilizes the fact that hypersonic-boundary layer disturbances havebeen observed to propagate at measurable and statistically predictable velocities. Particularly for the second-mode instability, the density gradient fields acquired by a calibrated schlieren system provide an avenue for resolving dense high-frequency spatiotemporal data. Disturbance propagation velocities extracted from the schlieren images are used to conduct a time-interpolation of the disturbances, which transforms spatially-available descriptions of the travelling waveforms into up-sampled temporal signals at specific pixel locations. When performed across the entire schlieren field of view, the resulting time-resolved signals have a new sampling frequency much greater than the original camera frame rate and a spatial density equal to the camera resolution. This enables the spectral analysis of high-frequency disturbances, including superharmonics of the fundamental instability, which are not originally resolvable from raw time series of the video data. The methodology is employed here in three different experimental data sets, comprising a7° half-angle sharp cone at zero incidence in Mach 6 flow, a 7° half-angle sharp cone at variable incidence in Mach 14 flow, and a cone-flare geometry composed of a 5° frustum with compression angles of +5°, +10°, and +15° at zero incidence in Mach 14 flow. A comprehensive global analysis is conducted on the linear and nonlinear development of the second-mode instability waves in each case. Pointwise measures of the autobicoherence are used to identify specific triadic interactions and the locations of their highest levels of quadratic phase coupling. Significant resonance interactions between the second-mode fundamental and harmonic instabilities are found along with interactions between these and the mean flow. Bispectral mode decomposition is employed to educe the flow structures associated with these interactions. A similar analysis is performed for the power spectrum, with power spectral densities computed for each pixel’s timeseries and spectral proper orthogonal decomposition employed to derive the modal structure and energy of the flow at specific frequencies. The instability measurements taken on the cone-flare geometry are the first of their kind atMach 14. The analysis reveals that incoming second-mode waves undergo extended interactions with the shock waves present at the corner, consistently leading to amplification of the waves and accelerating their nonlinear activity. The disturbance energy is also found to strongly radiate along the shock waves, a behavior that appears to be intensified at high Mach numbers. In the case of separated flow at the corner, additional low-frequency disturbances arise along the shear layer. Self-resonance of these disturbances leads to the radiation of elongated structures upstream of reattachment, which extend outward from the shear layer and terminate at the separation shock. This shear-layer disturbance is determined to be dominantly unstable between separation and reattachment but is significantly damped after reattachment.Item Physics and Modelling of Compressible Turbulent Boundary Layer(2023) Lee, Hanju; Martin, Pino; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Key findings from a research study that focuses on understanding the effect of Mach number, Reynolds number and wall temperature on compressible turbulent boundary layers (CTBL) in the hypersonic regime are presented in this dissertation. The study utilizes a comprehensive CTBL database developed using an in-house direct numerical simulation (DNS) code at the CRoCCo laboratory. The database encompasses a range of semi-local Reynolds numbers (800 to 34,000) and Mach numbers up to 12, incorporating wall-cooling. The effects of density and viscosity fluctuations on the total stress balance are identified and used to create a new mean velocity transformation for compressible boundary layers. The role, significance and physical mechanisms connecting density and viscosity fluctuations to the momentum balance and to the viscous, turbulent and total stresses are presented, allowing the creation of generalized formulations. We identify the significant properties that thus-far have been neglected in the derivation of velocity transformations: (1) the Mach-invariance of the near-wall momentum balance for the generalized total stress, and (2) the Mach-invariance of the relative contributions from the generalized viscous and Reynolds stresses to the total stress. The proposed velocity transformation integrates both properties into a single transformation equation and successfully demonstrates a collapsing of all currently considered compressible cases onto the incompressible law of the wall, within the bounds of reported slope and intercept for incompressible data. Based on the physics embedded in the two scaling properties, the success of the newly proposed transformation is attributed to considering the effects of the viscous stress and turbulent stresses as well as mean and fluctuating density viscosity in a single transformation form. The Reynolds number trends of large turbulent structures in compressible turbulent boundary layers are investigated using the pre-multiplied energy spectra based on the density corrected fluctuating streamwise velocity signal. Results demonstrate the existence of friction as well as semi-local Reynolds number trend associated with large-scale structures, similar to trends observable in incompressible turbulent boundary layers (ITBL). In particular, the behavior of turbulence in the inner layer is seen to exhibit dependence based on both definitions of Reynolds numbers. On the contrary, the strength of large turbulent structures is seen to be only dependent on friction Reynolds number. This result directly contrasts with the observation of the near-wall turbulent intensity peak increasing with semi-local Reynolds number. The discrepancy is mended with a suggestion that the large turbulent scales in the log layer of which the strength increases with friction Reynolds number, are modified through the changes in local fluid properties such that the scale interaction near the wall increases as semi-local Reynolds number. In another words, closer to the wall, the CTBL flow behaves like a semi-local Reynolds number flow, while closer to the freestream, it behaves like a friction Reynolds number flow. Furthermore, the present study examines the Reynolds number dependence of the length scale between small and large turbulent scales. The analysis highlights the inadequacy of using a univariable wavelength based on viscous, semi-local or outer length scales to differentiate small and large scales. Based on this, the use of Reynolds number-dependent length scales is recommended. Overall, the study provides valuable insights into the Reynolds number trends of large turbulent structures in CTBL, emphasizing the influence of both semi-local Reynolds number and friction Reynolds number on turbulence characteristics.Item MULTI-FIDELITY PARAMETRIC SENSITIVITY FOR LARGE EDDY SIMULATION(2023) Oberoi, Nikhil; Larsson, Johan Prof.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Designing engineering systems involving fluid flow under uncertainty or for optimality often requires performing many computational fluid dynamics (CFD) calculations. For low-fidelity turbulence modeling simulations such as Reynolds-averaged Navier-Stokes (RANS), such a framework has been established and is in use. However, for high-fidelity turbulence-resolving simulations such as large eddy simulations (LES), the relatively high computational cost of even a single calculation hinders the development of such a framework. The overarching goal of this work is to aid LES in becoming a usable engineering design tool. In this thesis, a computationally affordable approach to estimate parametric sensitivities of engineering relevant quantities of interest in an LES is explored. The method is based on defining a RANS problem that is constrained to reproduce the LES mean flow field. The proposed method is described and assessed for a shock/boundary layer interaction problem, where the shock angle and wall temperature are considered variable or uncertain. In the current work, a proof-of-concept of the proposed method is demonstrated. The method offers qualitative improvements to the sensitivity prediction of certain flow features as compared to standalone RANS simulations, while using a fraction of the LES cost. Different cost functions to infer auxiliary RANS variables are also examined and their influence on the sensitivity estimation is assessed. Overall, the results serve as an important proof-of-concept of the method and suggests the most promising path for future developments.Item Simulation of polymeric drop dynamics: Effect of photopolymerization, impact velocity, and multi-material coalescence(2023) Sivasankar, Vishal Sankar; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Over the past couple of decades, additive manufacturing has emerged as one of the most promising manufacturing tools and has rightfully garnered the attention of researchers across various fields ranging from biochemistry and medicine to energy and infrastructure. Especially, direct-ink-writing methods (e.g., inkjet printing, aerosol jet printing, or AJ printing, etc.) have been widely studied because of their ability to print highly complex geometries with finer resolution. In order to design a more efficient droplet-based direct ink writing system, it is essential to understand the deposition process and the post-deposition dynamics of the drop. The post-deposition drop dynamics dictate the spreading radius of the drop and hence the print resolution. Such an understanding is even more critical when there are multiple drops interacting with each other, given the fact that such interactions determine the presence/absence of surface defects in addition to determining the print resolution. Moreover, to have a holistic understanding of the post-deposition process, it is essential to further account for the droplet solidification mechanisms (for example, through effects such as in-situ curing) that might interplay with multiple drop dynamics events (such as drop spreading, drop coalescence, drop impact, etc.). In this dissertation, computation fluid dynamics (CFD) frameworks have been developed to investigate the facets dictating the post-deposition dynamics of one (or several) solidifying polymer drops, with these dynamics show-casing the different post-deposition events that are intrinsic to the droplet-based additive manufacturing processes. First, we considered a situation where the polymeric drop undergoes simultaneous spreading and photopolymerization, with the timescales of the spreading and photopolymerization events being τ? and τ? respectively. The findings from this work confirmed the significant impact of the ratio of timescales (τ? and τ?) on the thermo-fluidic-solutal dynamics of the polymeric drops. Moreover, the evolution of the curing front showed distinct behaviors as a function of the timescale ratio.Subsequently, the effect of the interaction of multiple polymeric drops during the post-deposition event, as seen in the typical printing process, was investigated. Specifically, we studied the effect of drop impact on the coalescence dynamics of two polymeric drops of identical and different sizes. The study revealed the presence of two distinct stages of coalescence. The early-stage coalescence was found to be enhanced with an increase in the impact velocity; however, the late-stage coalescence behavior remained unaffected by the impact velocity. Further, the coalescence dynamics of polymeric drops of different materials, as witnessed in multi-jet printing, was probed. This study shed light on the mechanisms that drive the mixing process at different stages of drop coalescence. Finally, we evaluated the effects of the in-situ photopolymerization on the coalescence dynamics of multiple polymeric drops deposited on a substrate. Here too the comparative values of the drop dynamics timescale and the photopolymerization became important. Our results show three-distinct regimes characterizing the bridge growth which was further validated through physics-based theoretical scaling. This study would provide key insights into the direct-ink writing process and would aid in designing parameters for polymer-based additive manufacturing and product repair.Item Mitigation of transverse gusts via open- and closed-loop pitching maneuvers(2022) Sedky, Girguis; Jones, Anya R.; Lagor, Francis D.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Unsteady flow conditions present significant challenges to stable flight, and gust rejectionremains a concern for flight control in many modern flight environments. Examples of gustdominated flight conditions include flight in stormy conditions, aircraft takeoff and landing in strong crosswinds or ship air wakes, and micro air vehicles in strong shear flow engendered by urban settings and complex terrain. Improving flight stability during gust encounters relies on an improved understanding of the flow physics and the development of effective mitigation control strategies. To this end, the present work seeks to (1) improve our understanding of the unsteady flow physics of a pitching wing encountering a transverse gust and (2) develop and characterize successful open- and closed-loop control strategies to mitigate aerodynamic lift transients induced by the gust using wing pitching input. Classic unsteady aerodynamic theory was used to construct the open-loop pitch maneuvers and tune the closed-loop controller for closed-loop control. The dynamical systems treatment of the problem during control design revealed several important physical features important to vehicle control. Two sets of wing-gust encounter experiments were conducted using a flat-plate wing model in a water towing tank. The transverse gust was generated in the center of the towing tank using a recirculating water jet. Data was acquired using a combination of Particle Image Velocimetry (PIV), force, and torque measurements. In the first set of experiments, the constructed openloop pitch maneuvers were implemented as open-loop kinematics in the water towing tank. This study revealed several findings regarding the change in the flow topology due to pitch actuation, the necessity of modeling added mass for open-loop pitch maneuver construction, and the increase in the pitching moment transients due pitch control. This study also demonstrated how lift-mitigating pitching maneuvers minimized the disturbance to the gust’s flow field, thereby reducing the momentum exchange between the gust and the wing. The second set of experiments implemented a proportional control strategy based on classic unsteady aerodynamic theory using a pitch acceleration input and real-time force measurements. The closed-loop control experiments spanned upwards and downwards gusts of various strengths and lift tracking at pre- and post-stall angles of attack. The controller yielded an average rejection performance of 80% without a priori knowledge of gust strength or onset time and for various aerodynamic conditions. Reasons for the controller’s success include using lift measurements directly in control feedback, aerodynamic models that capture the salient physics in the control design process, and wing pitching as input. Simultaneous time-resolved PIV and force measurements were used to discover and understand the flow physics underlying the lift transients and how applying closed-loop control mitigated those transients.Item Explorations of Carbon-Nanotube-Graphene-Oxide Inks: Printability, Radio-Frequency and Sensor Applications, and Reliability(2022) Zhao, Beihan; Das, Siddhartha SD; Dasgupta, Abhijit AD; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Carbon-Nanotube (CNT) is a novel functional material with outstanding electrical and mechanical properties, with excellent potential for various kinds of industrial applications. Additive manufacturing or 3D printing of CNT-based materials or inks has been studied extensively, and it is vital to have a thorough understanding of the fluid mechanics and colloidal science of CNT-based inks for ensuring optimum printability and the desired functionality of such CNT-based materials.In this dissertation, a custom-developed syringe-printable CNT-GO ink (GO: Graphene Oxide) is introduced and the fluid mechanics and colloidal science of this ink as well as the different devices (e.g., temperature sensor, humidity sensor, and RF antenna) fabricated with this ink are studied. The following topics are discussed in this dissertation: (1) the application and printability (in terms of the appropriate fluid mechanics and colloidal science) of CNT-based inks; (2) development of temperature sensors with CNT-GO inks; (3) development of humidity sensors with CNT-GO inks; (4) development of RF patch antenna with CNT-GO inks; and (5) evaporation-driven size-dependent nano-microparticulate three-dimensional deposits (CNTs serve as one type of nanoparticle examined in this part of the study). In Chapter 1 of this dissertation, a literature review is conducted on the application of CNT-based inks and the fluid mechanics and colloidal science issues dictating the printability and performance of such CNT-based inks. The problem statement and overall research plan are also introduced in this chapter. In Chapter 2, the development of our custom CNT-GO ink is introduced. Detailed material selection and the mechanism of shape-dependent arrest of coffee-stain effect, which ensured that the printable ink led to uniform deposition, are discussed in this chapter. Temperature sensor prototypes printed with the CNT-GO inks are also presented in Chapter 2. From Chapter 3 to Chapter 5, the performances of our CNT-GO based flexible temperature sensor, humidity sensor, and patch antenna prototypes are discussed. The ink printability on flexible thin PET films is studied, and a straightforward ‘peel-and-stick’ approach to use the CNT-trace (or patch)-bearing PET films on surfaces of widely varying wettabilities and curvatures as different prototypes is introduced. Excellent temperature and humidity sensitivity of our CNT-GO based sensors are presented in Chapter 3 and Chapter 4, and the potential of this CNT-GO material for fabrication of ultra-wideband (UWB) patch antennas is discussed in Chapter 5. Furthermore, the stability and reliability of these printed CNT-GO-based prototypes are also explored. In previous Chapters, the printed CNT-GO patterns were cured by evaporation-mediated deposition on flat substrates (i.e., 2D deposition spanning in x and y directions). This motivated the extension of the physics to the 3rd dimension and probing of particle deposition on a 3D substrate and particle deposition in all x, y, and z directions. Therefore, in Chapter 6, we perform an experiment to demonstrate this kind of possibility using three kinds of micro-nanoparticle-laden water-based droplets (i.e. coffee particles, silver nanoparticles, and CNTs). These droplets were first deposited at the bottom of an un-cured PDMS film; these droplets were lighter than the PDMS and hence, they rose to the top of the PDMS where they could have either attained a Neuman like state or simply remained as an undeformed spherical drop with the top of the drop breaching the air-liquid-PDMS interface. The calculations based on air-water, water-PDMS, and air-PDMS surface tension values confirmed that the Neuman like state was not possible, and the droplets were likely to retain their undeformed shapes as they breached the air-PDMS interface. The timescale differences between the fast PDMS curing and the slower droplet evaporation, led to the formation of spherical shape cavities inside the PDMS after completion of the curing, and allowed evaporation-driven deposition to occur in all x, y, and z directions inside the cavity, with the exact nature of the deposition being dictated by the sizes of the particles (as confirmed by the experiments conducted with coffee particles, silver nanoparticles, and CNTs). Finally, in Chapter 7, the major contributions of this dissertation and proposed future studies related to this dissertation work are listed.