Binder jet additive manufacturing (BJ) offers unique advantages, including the ability to produce complex geometries and utilize a wide range of materials, but faces challenges related to part quality and defect formation. This thesis investigates the effects of process parameters on droplet-powder interaction, powder relocation, and line formation in BJ printing. A custom BJ test platform was developed to enable precise control over key process parameters and in-situ monitoring. High-speed synchrotron X-ray imaging revealed modes of powder relocation above and below the powder bed surface. Testing revealed that parameters that increase moisture in the powder bed, such as lower droplet spacings, printing adjacent to previously printed geometry, and pre-wetting, reduce powder disturbance. Powder ejection above the powder bed surface was found to be affected by powder material, density, pre-wetting, previously printed geometry, and droplet spacing. Powder relocation below the powder bed surface was found to be largely independent of binder infiltration behavior, suggesting that powder relocation below the powder bed surface is driven by the kinetic impact of the droplet. A novel approach for analyzing printed lines demonstrated the sensitivity of line formation to various parameters, including droplet spacing, inter-arrival time, volume, and velocity. Lines were found to ball more readily at lower droplet spacings when printing at lower droplet velocities, although other coupled droplet parameters such as droplet volume and formation of satellite droplets also play a role. In printing conditions susceptible to balling, the droplets at the beginning of printed lines were observed to agglomerate, relocating powder and introducing error to the starting position of the line. Pre-wetting the powder bed with a water/TEG mixture significantly reduced balling and increased the range of droplet spacings and inter-arrival times resulting in successful line formation. Printing with low droplet velocities on moisture treated powder beds further increased the range of inter-arrival times that successfully formed lines. Reducing the kinetic energy of droplet impact by reducing droplet velocity and reducing the impact of balling by pre-wetting presents a set of process print process parameters that show promise to reduce powder relocation during the printing process. These findings provide valuable insights into the fundamental mechanisms of droplet-powder interaction, modes of powder relocation during printing that may contribute to porosity defects seen in final parts, and print process parameters that mitigate powder relocation due to droplet-powder interaction.

This dissertation presents the development of a modeling scheme that is developed to model the membrane potentials and ion currents through a bilayer network system. The modeling platform builds off of work performed by Hodgkin and Huxley in modeling cell membrane potentials and ion currents with electrical circuits. This modeling platform is built specifically for cell mimics where individual aqueous volumes are separated by single bilayers like the droplet-interface-bilayer. Applied potentials in one of the aqueous volumes will propagate through the system creating membrane potentials across the bilayers of the system and ion currents through the membranes when proteins are incorporated to form pores or channels within the bilayers. The model design allows the system to be divided into individual nodes of single bilayers. The conductance properties of the proteins embedded within these bilayers are modeled and a finite element analysis scheme is used to form the system equations for all of the nodes. The system equation can be solved for the membrane potentials through the network and then solve for the ion currents through individual membranes in the system. A major part of this work is modeling the conductance of the proteins embedded within the bilayers. Some proteins embedded in bilayers open pores and channels through the bilayer in response to specific stimuli and allow ion currents to flow from one aqueous volume to an adjacent volume. Modeling examples of the conductance behavior of specific proteins are presented. The examples demonstrate aggregate conductance behavior of multiple embedded proteins in a single bilayer, and at examples where few proteins are embedded in the bilayer and the conductance comes from a single-channel or pore. The effect of ion gradients on the single channel conductance example is explored and those effects are included in the single-channel conductance model. Ultimately these conductance models are used with the system model to predict ion currents through a bilayer or through part of a bilayer network system. These modeling efforts provide a modeling tool that will assist engineers in designing bilayer network systems. / Ph. D.

Fog harvesting is emerging as a promising means to ease the water shortage crisis in arid regions of the world with ample fog. The current state-of-the-art for fog harvesting is mesh netting, which is accessible yet struggles from a dual constraint: a course mesh lets most microscopic fog droplets pass through it, while a fine mesh clogs. In recent years, fog harps have been gaining attention as a superior alternative to meshes, bypassing these inherent constraints. In this work, we expand upon previous fog harp research with a focus on optimization. First, we analyze wire tangling in a harp due to capillary forces, resulting in a mathematical model that is able to predict when wire tangling will occur. Second, we systematically vary three key parameters of a fog harp (wire material, center-to-center wire pitch, and wire length), arriving at an optimal combination. Finally, we develop a numerical model to describe the dynamics of a fog droplet sliding down a harp wire while coalescing with others littered along it. By applying all knowledge acquired through these studies, the next generation of fog harps will push the performance ceiling of practical fog harvesters higher than ever. / Master of Science / The human population continues to grow, and with it the demand for fresh water. This need has caused many to turn to unconventional sources of water, including fog (the suspension of microscopic liquid water droplets in the air). Fog harvesters already exist in arid regions of the world as mesh nets, but suffer dual constraints from their grid-like structure: course meshes fail to capture most fog droplets passing through, while fine meshes get clogged. To bypass these inherent limits, we turn to nature for a solution. It has been observed that California redwood trees are able to effectively collect fog on their straight leaf needles, dripping droplets to the roots below. Inspired by this, we fabricate a device called a fog harp, which removes the impeding horizontal wires of meshes to effectively capture and slide droplets down its vertical wires. In this work, we expand upon previous fog harp research by investigating ways to optimize its water collection efficiency. First, we develop a mathematical model to describe the tangling of harp wires due to merging droplets on adjacent wires pulling them together. Second, we systematically vary three key parameters of the fog harp (wire material, center-to-center wire spacing, and wire length) to arrive at the optimal combination. Finally, we develop a model to describe the dynamics of droplets sliding down harp wires while merging with others littered along it. These studies will raise the performance ceiling of fog harps and push them to real-world applications.

A computational model is developed to simulate the impact dynamics of a droplet on solid substrates with the purpose of predicting the droplet spreading characteristics over time. Previous studies focused on finding relations between the impact parameters and outcome dynamics. A modified approach like the one used in this project revolves around modeling the moving contact lines at the interface in a multiphase flow environment. Focusing on research from an aircraft de-icing point of view, this study is considered a prerequisite in understanding the physics of droplet impact. The primary focus is on extending the application to incorporate super-cooled environments. Development of the model involved the use of the Volume-of-Fluid function coupled with the High-Resolution Interface Capturing scheme to model the moving contact line. The evolution of the moving contact line is modeled with contact angles as their inputs to understand the effect of the surface tension forces. Contact angle modeling is based on the Blended-Kistler method, which captures the contact angle evolution based on the surface tension and capillary number. Preliminary validation performed on the model proves its effectiveness in accurately simulating the impact behavior when compared to the literature, where the spread diameter and height agree well with experiments. The validated model is also compared to the in-house experiments performed at the Cavitation and Multiphase flow laboratory using different substrate materials. The substrates each show unique behavior - Impact on Glass results in the droplet depositing on the surface. Aluminum results in a full rebound and PET-G, results in a drop ejection. Based on inputs from the experiments - contact angles, spread diameter, and the maximum spread $beta$, show good agreement in comparison to the literature. / Master of Science / Computational model developed to simulate the impact dynamics of the droplet on solid surfaces, which predicts the evolution of the droplet over time in order to analyze the effect of the surface and properties of the fluid on the behavior of the droplet on impact. Focusing on research from an aircraft de-icing point of view, this study is considered a pre-requisite in understanding the physics of droplet impact, with potential scope in extending the simulation to applications at temperatures lower than $0^{circ}$ C. A model developed with the help of basic governing equations in fluid mechanics helps capture the effect of interactions between different physical states. The angle at which the droplet interacts with the surface (Contact Angle) and the diameter evolution (d/D) help us understand the effectiveness of the model to simulate droplet impact. Preliminary validation of the model is performed with respect to the literature where the droplet diameter evolution and the height variation match well with the experiments, which was the major criterion in determining the accuracy of the model. This model is compared to the in-house experiments performed at the Cavitation and Multiphase flow laboratory on different surfaces such as Glass, Aluminum, and Plastic (PET-G). The surfaces each show unique behavior with impact on Glass having the droplet deposit on the surface, aluminum resulting in the droplet bouncing after hitting the surface, and PET-G resulting in a small droplet being ejected from the bigger droplet which eventually deposits on the surface. Conclusions from the comparison between the experiments and the numerical simulation show how the model is effective in capturing the impact behavior on surfaces like glass in comparison to surfaces like Aluminum in this case that repels water.

The formation of lipid bilayer membranes between droplets of ionic liquid is presented as a means of forming functional bimolecular networks for use in sensor applications. Ionic liquids are salts that have a number of useful properties, such as low melting points making them liquid at room temperature and exceedingly low vapor pressure. Ionic liquids have seen recent popularity as environmentally friendly industrial solvent alternatives. Our research demonstrates that it is possible to consistently form lipid bilayers between droplets of ionic liquid solutions. Analysis shows that the ionic liquids have negligible effects on the physical stability and electrical properties of the bilayer. It is also shown that the magnitude of the conductance levels of Alamethicin peptide are altered by some ionic liquids. / Master of Science

During this investigation, vibration induced droplet generation from a liquid layer was examined as a means for achieving high heat flux evaporative cooling. Experiments were performed in which droplets were generated from a liquid layer using a submerged vibrating piezoelectric driver. Parameters determined during this investigation of droplet generation were droplet mass flow rate, droplet size, driver frequency, driver voltage, and liquid layer thickness. The results showed that as the liquid layer thickness was increased, the frequencies and frequency ranges at which droplet generation occurred decreased. Droplet mass flow rates were varied by adjustment of the liquid layer thickness, driver frequency, and driver voltage. The dependence of the drivers displacement, velocity, and acceleration on frequency and voltage was determined, and the drivers frequency response was related to the occurrence of droplet generation. As a result, a frequency-dependent dimensionless parameter was proposed as a method for predicting droplet generation from the surface of the liquid layer. The dimensionless parameter is a combination of the Froude number and the dimensionless driver acceleration. The measurements have shown that droplet generation occurs when the parameter is between distinct upper and lower bounds. An analytical heat transfer model of a droplet cooling heat transfer cell was developed to simulate the performance of such a cell for thermal management applications. Using droplet flow rates determined as functions of driver voltage, driver frequency, liquid layer thickness, and interception distance, the heat transfer rate of a droplet cooling heat transfer cell was predicted for varied heat source temperatures and cell conditions. The heat transfer model was formulated in such a way as to accommodate a number of parameter variations that can be used for the design of a simple heat transfer cell. The model was used to determine the effect of droplet cooling on the heat transfer rate from a heated surface, but it can also be used to determine the influence of any of the other embodied parameters that may be of interest for thermal management applications.

Spray drying is a widely used unit operation for producing particulate products directly from a liquid feed. Important processes that occur inside the spray dryer are droplet formation, droplet drying and interactions between droplets and recycled fines. Various studies have looked at the first two processes, but the latter phenomenon has received less attention. Literature on droplet-particle interaction which aims at quantitatively describing agglomeration in spray drying is scarce and mainly qualitative. For product quality the formation of agglomerates is often desirable. This thesis models and investigates the collisions of individual particles with single droplets. The surface tack of drying droplets has been identified as an important variable for the formation of agglomerates. In this thesis a novel method for measuring tack from the liquid phase has been further improved. The improvements are a more accurate load measurement, an automated control of the tack probe and an improved layout of the sample holder and probe. The key feature of the device is its ability to measure tack of drying droplets, whereas other devices measure tack by wetting a powder. Using our method the tack of a commonly spray dried product, yeast extract, has been measured. From these experiments it was found that with decreasing average moisture content the surface tack increases to a maximum. Below a critical average moisture content the surface of the droplet is dry and the tack rapidly decreases upon further drying. Another important parameter in determining the degree of agglomeration is the degree of penetration. If the particle penetrates the droplet too deeply, the agglomerate structure becomes too dense. To predict the penetration depth, a non-dimensional model has been developed. It describes the penetration of a particle into a liquid droplet during a head-on collision. It is based on a force balance and incorporates surface tension force, viscous force and capillary pressure force. The important parameters determining the collision outcome are the contact angle, the size of the droplet relative to the particle, the Reynolds and Weber numbers. For each contact angle an equilibrium penetration position exists, at this point the surface tension force vector is perpendicular to the penetration direction. Five different penetrations regimes are identified. At low Reynolds numbers, viscous forces dominate and the particle asymptotically travels towards the equilibrium position. Reducing the viscous drag force by increasing the Reynolds number results in initially overshooting the equilibrium position, but the surface tension force pulls the particle back, to attain the equilibrium in an oscillating motion. At even higher Reynolds numbers the particle fully penetrates the droplet, and reaches the centre of the droplet for even higher values for the Reynolds number. The ejection regime is found at high Reynolds number and low Weber numbers and the liquid should be non-wetting. Using the regime maps one is able to identify in which region a spray dryer is operating. Although the full penetration regimes are useful for capturing fines, it should be avoided when agglomeration is desired. The ejection regime should be avoided as well. To validate the model, impact experiments were carried out by dropping glass spheres on the surface of different liquids. These validation experiments were the first attempt to experimentally validate the collision of a single particle with a liquid surface. Besides yeast extract, which has non-Newtonian rheological properties, silicone oils with constant viscosities of 100 mPa•s and 1 Pa•s have been tested. The penetration over time for different impact velocities was determined by analysing high speed camera recordings. The typical penetration times ranged from 0.2 s to 2 s. To obtain accurate location data was recorded at frame rates up to 38 000 frames per second. Glass spheres, with a size of 2 mm were used to allow the visual tracking. Modelling the impacts showed that the model consistently predicted faster penetration times than were observed experimentally. The relative difference increased with increasing viscosity. A parameter fitting exercise showed that better agreement could be obtained by using a higher viscosity and a higher contact angle in the model. With this knowledge the most likely factor influencing the model-experiment mismatch was identified as being the dynamics of wetting of the particle surface. It was also found that using the dynamic contact angle in the model would improve its results. The non-Newtonian characteristics of the yeast extract resulted in the particle rebound and the formation of an air cavity upon impact. The tack measurement technique and penetration model presented in this thesis will be useful tools for the design of spray dryers. Recommendations are made for further model improvement. The experimental validation is the first attempt to validate the presented model. Future improvements are recommended and suggestions are presented.

The use of ultrasound contrast agents increases the information available for reconstruction during ultrasound imaging. Previously studied microbubbles, consisting of a gas core, are subject to limitations such as a short lifetime and a large size. Droplets with a liquid perfluorocarbon core that is stabilized by cellulose nanofibers eliminate these drawbacks, but require further investigation. By studying the frequency-dependent attenuation profile of the cellulose nanofiber coated perfluorocarbon droplets within an ultrasound field, information about the droplets as oscillators can be retrieved, enabling characterization of their physical properties. In this study, the frequency-dependent attenuation profile was experimentally acquired and compared between two concentrations, using flat transducers covering the frequency range of 1-15 MHz. The data collected in the time domain was processed and transformed into the frequency domain and the attenuation was calculated across the entire frequency range. Among the frequencies studied, the attenuation increases with frequency and covers the range of approximately 0.25-8.3 dB/cm and 0.01-3.3 dB/cm at the concentrations of 50 million droplets/ml and 10 million droplets/ml respectively. The attenuation reaches a minimum at 3 MHz within the highest concentration, compared to 4.43 MHz within the lowest. The increase of the attenuation with frequency is explained by the droplets not exhibiting large oscillations within the range covered. It is probable that the droplets do exhibit oscillations, due to a viscosity lower than that of water, but a resonance frequency is not found within the spectrum studied. This could be explained by a shell elasticity or a small droplet radius placing the resonance frequency outside of the spectrum studied, or high levels of damping broadening the resonance peak. Localizing the resonance frequency would enable characterization of these physical properties of the cellulose nanofiber shell as well as the perfluorocarbon liquid core of the droplets. The increase of the attenuation with frequency demonstrates that the droplets do not produce a maximized amount of scattering at a specific frequency within the range studied, which is observed among other oscillating particles implemented as ultrasound contrast agents. The attenuation is, however, larger than that of blood across all frequencies except for those among which the attenuation reaches its minimum. Potential errors that are affecting the results include droplet vaporization, the formation of flocs after the mechanical agitation has ceased, the experimental setup, the settings on the pulse generator, the sensitivity of the transducers and the processing code.

No description available.

<p>Understanding the drying of droplets is of interest for processes such as spray drying, where particulate materials are produced by evaporating moisture. Even though spray-drying is a widely used method, there are still challenges, such as undesired agglomeration or controlling the morphology and size of the final dried product. This dissertation develops a physics based model that is used to examine the droplet dynamics and drying kinetics at large and small scales.  In addition, the model simulates the internal motion of insoluble particles and  is used to better understand particle formation during spray drying type processes.</p> <p><br></p> <p>The first part of this work examines the effect of droplet-droplet collisions on evaporation and the size distribution at a large scale. Droplet collision dynamics are implemented into an Eulerian-Lagrangian framework, where droplets are tracked in the Lagrangian frame, and the background gas is modeled as a continuum. The modeling framework includes fully coupled interphase heat and momentum transfer between the droplet and gas phases. Binary collision of droplets could result in coalescence, reduction in surface area, or separation of droplets, resulting in the generation of satellite droplets and an increase in total surface area. By capturing the change in size distribution due to the collision of particles, our results show a linear relationship between the Weber number and the evaporation rate at low droplet number densities. Further, it is shown that droplet number density is a critical factor influencing the evaporation rate. At high droplet number densities, the relationship between the evaporation rate and the Weber number becomes non-linear, and at extremely high droplet number densities, the evaporation rate decreases even at high Weber numbers.</p> <p><br></p> <p>In the next part of this dissertation, the drying of a single droplet containing insoluble solid particles is investigated. Using a volume-of-fluid framework coupled with the Lagrangian phase, we study the particle transport within a droplet, and how it is affected by airflow, phase properties (e.g., viscosity and density of each phase), surface tension, and evaporation. Unlike the traditional one-dimensional modeling approach, our multi-dimensional model can capture the generation of internal flow patterns due to shear flow and the accumulation of solid particles on the surface of the drying droplet. Our results show that the surface tension effect is more pronounced at larger droplet diameters and low airflow velocities. Our approach also provides a quantitative method for modeling crust growth and formation. </p> <p>Our results show that increasing solids mass fraction, and decreasing particle diameter, slow down the internal transport of solid particles, leading to a more quick accumulation near the surface of the droplet. Further, despite the droplet undergoing a constant-rate drying stage, the accumulation of solids near the surface is non-linear. In addition, the inclusion of solids within the droplet drastically reshapes the formation of internal vortices compared to the uncoupled case, which determines solids distribution.</p>