1,720,975 research outputs found
Role of Polarity on the Interfaces of Binary Combinations
Full text linkOwing to a large number of applications, starting from pharmaceutical packaging to advancement in nanotechnology and microfluidic devices, wetting characteristics has always been regarded as an essential prerequisite for many phenomenal processes. In the present time, the term ‘wetting’ is not limited to spreading of a liquid on a solid but also portrays displacement ability of a gas over a liquid. To quantify wetting of a solid surface by a liquid, numerous established theories argue that interactions between polar–polar, polar–non-polar and non-polar–non-polar components of surface tension or equivalently, surface energy dictate the final equilibrium contact angle.
In this work, the extent to which individual phases of binary liquid−vapor and solid−liquid system interacts, and how such interactions are influenced by the polar and the dispersive components of the surface tension is examined. For liquid−fluid systems, the effect of the polarity of the surrounding (saturated) vapor medium on the equilibrium surface tension is critically investigated. Such measurements being prone to inaccuracy for highly volatile liquids, a standard protocol to obtain equilibrium surface tension with reasonable accuracy has been developed. A wide range of fluids covering polar−polar, polar−non-polar and non-polar−non-polar liquid-vapor combinations are studied and results confirm that the influence of molecular weight of both of the phases (drop and surrounding saturated vapor) must be accounted for in addition to the interactions (polar-polar, polar-non-polar etc.) that occur therein. For the case of polar-polar and non-polar-non-polar combination, observations suggest that the liquid drop interface becomes active only if the molecular weight of vapor is lower than the liquid phase. Further, it is observed that polar component of surface tension of drop (for polar liquid) influences the interaction between equilibrium surface tension and Fowkes’ dispersive interaction. Similar influence of the polar component of the surface tension has been observed for the solid−liquid systems where the percentage polarity of the (gold) substrates are varied by means of functionalization with mixed self assembled monolayer, SAM. Moreover, the surface characterization using X-ray photoelectron spectroscopy (XPS) proved that the mole fraction of the chemicals on the surface is different than that of the immersion solution
Dynamics of Compound Droplets: Rolling and Evaporation
Full text linkSuperior control of multiphase micro-drops owns much of the future in microfluidic technology. Understanding the dynamics of such compound systems is the key to its large-scale applications. Interfacial interaction of a droplet at a liquid-fluid interface dictates its successful generation and stability. The knowledge of the interface dynamics creates a rich profusion of domains that were previously unexplored. The century-old power law, which was believed to be universal in governing temporal drop spreading on solid substrates, is seen to fail in predicting spreading on liquid-fluid interface. Rather a coalescence like behavior becomes imminent. The study of the fundamental physics of evaporation of double emulsion droplets and under liquid rolling dynamics are extensions of the successful generation technique. In contrast to the rigid body motion, dissipation inside and outside of a deformable drop always results in convoluted physics. While rolling on an incline, single-phase drops travel slower with increase in size. But a concealed direct dependency between the drop size and traveling velocity can be exposed by merely altering the medium resistance. Rolling motion of double emulsion droplets even affirms the presence of both of these dependencies and a control over the transition from one to the other is achievable. A threshold size limit for such a transition has been identified demonstrating that the dependency between drop size and its velocity is not unidirectional. This thesis further explores the evaporation of double emulsion droplets and identifies two new regimes of evaporation. Resurfacing of a daughter droplet from an evanescing drop preceded by sudden spreading are uncommon observations in the literature. Detailed comprehension of the resurfacing of micro-droplets provides a possibility to control the evaporation mode, which was considered to be a random occurrence in the past
Dynamic Response of Droplets to Single Wave Electrowetting Perturbation
No full textActive control over the spreading of a liquid can be the avenue of endless opportunities. This spreading is dictated by the surface energy of a solid surface in which the wetting of a given liquid is quantified by the Young's contact angle. This quantification is the outcome of the interactions and the magnitudes of the interfacial energies at the solid-liquid, the solid-medium, and the drop-medium interfaces. Electrowetting-on-dielectric (EWOD) is a technique that can permit the control over spreading and its rate without permanently altering the surface energies of these three interfaces. This is the key aspect and the most important advantage of electrowetting. The new equilibrium configuration of the drop, as a response to EWOD by means of an externally applied electric field, can be predicted by the Young-Lippmann's equation. Although electrowetting has been widely studied, there is limited knowledge on the initial response of the drop, mainly after exerting different kinds of electric fields. During the implementation of the electric field a spreading behaviour is triggered, and this initial reaction of the drop to electrowetting was investigated thoroughly in this study. The transient evolution in the drop's shape was analyzed meticulously while the drop acquired the new equilibrium position. If the applied electrical actuation is oscillatory (AC), the number of cycles of this actuation dictated the equilibrium configuration and, in some cases, it deviates from the Young-Lippmann's equation prediction. Moreover, the system properties, i.e., the thermophysical properties of the droplet and the surrounding medium, along with the voltage and the frequency of the applied wave, completely altered the initial drop dynamics that eventually resulted in the corresponding equilibrium of the droplet. It is also observed that the first actuation cycle of AC EWOD had the greatest influence on the drop to achieve the maximum possible spreading. Hence, as an extension of this research, the transient dynamics of a drop submitted to a single electrokinetic excitation is scrutinized. The response time of the drop, defined as the time from the instant that the excitation is introduced until the maximum spreading reached during the single actuation cycle, is directly and inversely proportional to the viscosity of the liquids and the frequency of the actuation force, respectively. Based on the extensive parametric study, a unique definition of time scale is proposed to predict the time to reach the maximum spreading of AC electrowetting. This new time scale was based on the imposed actuation time by the frequency of the wave and on the viscous time scale modified by the electrowetting properties. The aftereffects of the actuation on the drop's motion, particularly at the drop-medium interface, are also investigated to find out the role of this induced perturbation on the equilibration process. The oscillatory motion at the drop-medium interface resembled an inertial capillarity response. An equivalent mass-spring-dampener model is also proposed to analyze these oscillations. Finally, with increased knowledge about the response of a single drop to electrowetting, a new technique for electrocoalescence is proposed. The in-house developed technique circumvented the repulsion at the point of merging between two drops, i.e., at the three-phase contact lines. The coalescence of sessile drops is characterized by the evolution of a film bridge between the drops that represents a mass transfer phenomenon. If a redefined time scale for maximum spreading is considered for the coalescence analysis, initial bridge growth suggested a universal growth irrespective of the system properties
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Fabrication and Wettability Characterization of Electrospun Fibrous Nylon 6/Silica Nanocomposites
Full text linkElectrospinning is a simple and versatile method to fabricate polymeric nanofibers and nanocomposites fibers for a wide range of applications。 Electrospun Nylon 6 nanofiber mats have been used in tissue engineering, filtration and protective clothing. Silica nanoparticles are a type of popular reinforcements for electrospun fibrous nanocomposites due to its tunable size and biocompatibility. Therefore, Nylon 6/silica nanocomposite fibers are studied in this thesis.
Firstly, the fabrication process is developed. More specifically, the effect of Nylon 6/Formic Acid concentration and silica weight fraction on the solution viscosity and the properties of the end product is studied. In microscopic view, Scanning Electron Microscope was used to characterize the morphology and dimensions of the nanofibers. It was found that the average fiber diameter increased significantly with the increase of Nylon 6 concentration and slightly with the rise of silica weight fraction. Also, when silica weight fraction exceeds a critical value, silica nanoparticles start to agglomerate and silica beads appear from time to time. In macroscopic view, surface roughness and porosity of the mats were measured. The porosity remains unchanged while surface roughness increased by increasing silica weight fraction and decreasing Nylon 6 concentration.
In the applications such as filtration and tissue engineering, wettability is a very important surface property. Therefore, after the development of the fabrication process, the change of the mats’ wettability with respect to the morphology of the electrospun nanocomposites is investigated. Both the equilibrium contact angle and the dynamic water contact angle was studied. For pristine Nylon 6 nanofiber mats, the equilibrium contact angle was increased with the increase of Nylon 6 concentration. The reason is that higher Nylon 6 concentration gives higher average fiber diameter, which results in lower surface roughness. On the other hand, the dynamic water contact angle curves are similar for the mats made from solutions with different Nylon 6 concentration. When silica nanoparticles are reinforced, the variation of equilibrium contact angle is complicated since silica nanoparticles is hydrophobic, but the particles also increased the surface roughness of the nanofiber mats. It can be concluded that the surface roughness played a more important role than the particles’ hydrophobicity. Dynamic water contact angle is significantly changed by the silica addition. the water drops on the 9% silica reinforced mats were absorbed much quicker than those on pristine Nylon 6 nanofiber mats
Dimensional assessment and process optimization of additively manufactured structured 3D porosity via primitive triply periodic minimal surface lattice structure and laser powder bed fusion technique
Full text linkThis thesis reports the challenges that need to be addressed before any heat transfer analysis of a proposed novel cellular-walled pipe heat sink system manufactured by LPBF technique. The proper cellular structure type selection for enhanced heat transfer performance, as well as providing a detailed analysis of its dimensional trends and CAD to manufactured deviations, are investigated. Triply Periodic Minimal Surface (TPMS) lattices have been heavily investigated lately due to their superior thermo-mechanical performance compared with their lattice counterparts. The advancement of additive manufacturing, i.e., laser powder bed fusion (LPBF), has easily enabled the manufacturing of such complex lattices. Recent studies have investigated the heat transfer performance of multiple TPMS lattice types such as Gyroid, Dimond, IWP, and Primitive structures. The Primitive TPMS (PTPMS) showed enhanced heat transfer performance mainly due to its cell shape and thickness (i.e., lattice topology). Hence, it was selected for the proposed cellular-walled pipe heat sink. Micro X-ray computed tomography (μCT) and optical microscopy (OM) were utilized to conduct the lattice dimensional analysis. Increasing the PTPMS lattice cell size from 2.9 to 10 mm showed an increase in the lattice wall thickness and pore size but a decrease in the SA:Vol ratio. However, increasing the lattice porosity from 45 to 90% resulted in a decrease in the lattice wall thickness but an increase in both the SA:Vol ratio and pore size. Comparing CAD to manufactured PTPMS lattices, the resulting lattice samples showed lower wall thicknesses and higher surface area to volume (SA:Vol) ratios than designed, which is attributed to shrinkage during the building process. The printed lattice pore size and porosity values were observed to be higher than the CAD values. Moreover, the minimum PTPMS lattice wall thickness and pore size that can successfully be printed were investigated and found to be 152 μm and 317 μm, respectively.
The type of powder material used in manufacturing the cellular-walled pipe heat sink is another challenge. In the LPBF printing process, the printing parameters for any selected material need to be optimized to manufacture fully dense parts. 2507 super duplex stainless steel (2507 SDSS) is a promising material that was selected for manufacturing the proposed heat sink system. The printing parameters for 2507 SDSS, namely: laser power, scan speed, and hatch distance, were optimized. The response surface methodology was used in generating a detailed design of experiment to investigate the different pore formation types over a wide energy density range (22.22 - 428.87 J/mm3), examine the effects of each process parameter and their interactions on the resulting porosity, and identify an optimized parameter set for producing highly dense parts. Different process parameters showed different pore formation mechanisms, with lack-of-fusion, metallurgical or gas, and keyhole regimes being the most prevalent pore types identified. The lack-of-fusion pores are observed to decrease significantly with increasing the energy density at low values. However, a gradual increase in the keyhole pores was observed at higher energy densities. An optimal energy density process window from 68.24 J/mm3 to 126.67 J/mm3 is identified for manufacturing highly dense (≥99.6%) 2507 SDSS parts. Furthermore, an optimized printing parameter set at a laser power of 217.4 W, a scan speed of 1735.7 mm/s, and a hatch distance of 51.3 µm was identified, which was able to produce samples with 99.961% relative density. Using the optimized parameter set, the as-built 2507 SDSS sample had a ferrite phase fraction of 89.3% with a yield and ultimate tensile strength of 1115.4 ± 120.7 MPa and 1256.7 ± 181.9 MPa, respectively
Enhancing Heat Pipe Efficacy: Thermosyphon Analysis, New Generation Refrigerant Transition, and Design Optimization
Full text linkThe efficacy of heat pipes is often constrained by their ability to effectively return condensed liquid from the condenser to the evaporator section through capillary pumping. However, when the heat pipe wick is absent or flooded due to overfilling, capillary pumping becomes irrelevant, resulting in a thermosyphon system. In a close-to-horizontal orientation, the driving force for liquid return in a thermosyphon is derived from the difference in liquid pool depth between the evaporator and condenser. Increasing the depth of condensed liquid augments the driving force for flow. However, an excessively deep liquid pool in the condenser can limit radial heat transfer and hinder heat rejection. Therefore, it is crucial to strike a balance that favors intermediate-depth liquid pools. Increasing the fill ratio beyond the optimized value leads to escalated manufacturing costs and adverse effects on performance. This study employs a theoretical approach based on the lubrication approximation to the Navier-Stokes equations to determine the fill ratio that maximizes thermosyphon performance. Additionally, we explore the variations of this ratio in relation to factors such as the axial temperature difference along the thermosyphon. Our analysis overcomes a common simplification observed in conventional thermosyphon descriptions by considering the incremental flow resistance resulting from axial variations in the liquid film thickness. Neglecting this aspect can lead to inaccuracies in estimating the axial heat flux. The modeling of heat pipe and thermosyphon results aids in the selection between a thermosyphon and a heat pipe, while also utilizing the hydrostatic-driven flow limit in the thermosyphon to replace the capillary limit in the heat pipe ``fundamental diagram.'' \\\\
Furthermore, we compare the efficacy of R513a, an HFC/HFO refrigerant blend with lower global warming potential, to the commonly used R134a refrigerant in the context of heat pipe applications. Considering the adverse environmental impact of refrigerants on global warming, it is imperative to identify and implement eco-friendly alternatives with reduced global warming potential. Tests were conducted on both smooth and grooved heat pipes under uniform environmental conditions to evaluate the principal variations in performance and operation between the two refrigerants. Our study suggests that, in most cases, R513a can serve as a one-to-one substitute for R134a, demonstrating superior performance and enhanced heat transfer capacity. In certain situations, an integrated approach that involves adjusting only the fill mass is necessary to achieve comparable outcomes. This study illuminates a critical transition strategy towards alternative refrigerants, highlighting the potential for eco-friendly substitutes that rival or outperform conventional refrigerants.\\\\
Additionally, we present a Matlab-based, GUI-driven algorithm developed for heat pipe design and optimization. The algorithm predicts the thermodynamic performance of a heat pipe by considering various limiting conditions imposed by viscosity, capillary action, entrainment, boiling, and compressibility. This standalone tool assists in selecting the appropriate working fluid for a heat pipe and constructs a fundamental diagram over a wide temperature range. Furthermore, it facilitates the identification of optimal design parameters in a given setting. We utilized this tool to achieve an optimized design of axial heat transfer with enhanced performance, leveraging experimental data from the previous section specifically for a helical grooved heat pipe.\\\\
To sum up, our study contributes to the enhancement of heat pipe efficacy through an analysis of thermosyphon behavior, a comparison between eco-friendly refrigerants, and the development of a Matlab-based algorithm for heat pipe design and optimization. These findings offer valuable insights into achieving optimal heat transfer and provide a foundation for selecting suitable heat pipe configurations and working fluids for various applications
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Experimental investigation of surface chemistry on the performance of heat pipes
No full textMuch research has been performed over the past few decades on heat pipe performance enhancement. A major thrust of investigation has been to examine factors impacting condensation. Measurements suggest that drop-wise condensation achieves a comparatively larger rate of condensate discharge on smooth and textured surfaces. To promote drop-wise condensation, surface chemical treatments can be used. In some cases (e.g. carbon nanotube coating), the chemical treatments in question increase the overall thermal conductivity. More typically, however, the effect of the chemical treatment is to increase the resistance to heat transfer by conduction. In such instances, there is a trade-off associated with the application of surface chemical treatments because they decrease conduction but facilitate enhanced convection. The current research focuses on the effect of surface energy on the condenser section of a representative flat plate heat pipe. Our experimental design includes a transparent top, and so allows for visual confirmation of the mode of condensation. Observations and measurements can, therefore, be compared against those collected in a small-scale condensation chamber that includes metal coupons having different surface chemical properties.
In this study small coupons of 25.4 mm * 25.4 mm * 3 mm (Length * Breadth * Thickness) are chemically treated to create hydrophobic and superhydrophobic surfaces and examined in terms of condensation behaviour inside a condensation chamber. Visual data has been analyzed with the help of Image Pro Premier software. In particular, we quantify the degree to which a superhydrophobic coating increases the degree of water droplet shedding. By extension, we quantify the degree to which applying a low surface energy coating in the condenser section of a heat pipe decreases overall thermal resistance and thereby increases heat pipe performance. Additionally, we also explore the effect of the angle of inclination on the performance of the heat pipe in terms of the temperature profile along the external wall of the heat pipe
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