1,721,068 research outputs found
Structure of Liquids and Solid-Liquid Interfaces
No full textSpaepen Frans. Structure of Liquids and Solid-Liquid Interfaces. In: Bulletin de la Classe des sciences, tome 14, n°1-6, 2003. pp. 195-196
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
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Nanocalorimetry Study of Phase Transformations in Thin-Film Shape Memory Alloys
No full textThe nanocalorimetry technique enables measurement of novel thin film materials at non-equilibrium conditions. This study utilizes the synergy between nanocalorimetry, transmission electron microscopy, density functional theory simulations, and theoretical models to explore the Cu-Zr and Ni-Ti shape memory alloy systems, revealing interesting phenomena and their underlying mechanisms.
We have investigated the phase evolution of sputter-deposited equiatomic Cu-Zr thin films. The nanocalorimetry heat treatment to form the shape memory phases are determined. We found that in CuZr and some of its ternary alloys, the austenitic phase can be supercooled below the normal temperature range of martensitic transformation, resulting in an explosive-type transformation that converts the entire sample in microseconds. This phenomenon is due to a lack of nucleation sites and can be controlled by annealing of defects.
We carried out a computational-experimental study on the ternary alloying effect of CuZr-based shape memory alloy. The computed energy difference between martensite and austenite suggests that both Co and Ni increases the transformation temperature. However, experiments show that the effect of Co goes in the opposite direction. We attribute this discrepancy to the microstructure-related twin boundary energy and strain energy terms.
We found that the crystallization of NiTi and NiTiFe splits into two steps at ultrafast heating rates. The first step forms large grains of supersaturated austenite, and the second step forms secondary phases within the large grains. When a small percentage of ternary element Fe is added, the apparent activation energies of both steps drop significantly, and the resulting grain size is much smaller. Further increase of heating rate to above 10,000 K/s results in less time for nucleation and rapid grain growth of NiTiFe
Variations on the Author
Get PDF“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
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Clogging Mechanisms in Converging Microchannels
Get PDFMany technological and biomedical applications ranging from water filtration and oil extraction to arteriosclerosis and vein thrombosis rely upon the transport of solids in liquids. Particulate matter suspended in liquid flowing through channels that are often microscopic or millimeters in size which leads to clogging. This dissertation examines the clogging behavior of microscopic channels by microscopic particles suspended in liquid. We physically model clogging in microchannels by flowing microparticles through microfluidic channels. Unlike previous studies, we choose non-uniform microchannels; specifically, we study clogging in microchannels whose width narrows over the length of the channel. Converging channels are inspired by the pore size variations in real porous media like membrane filters and sandstone.
Initially we study the clogging behavior of microparticles in arrays of parallel microchannels as we vary the microchannel entrance (mouth) width and microchannel length. We measure the time until each channel clogs and we calculate the number of particles that pass prior to clogging. Contrary to expectation, we show that the number of particles passing through a pore increases exponentially with increasing mouth width but decreases linearly as the channel length increases. Changing the dimensions of the channels changes the particulate suspension’s flow rate which in turn changes the shear stresses that particles experience near the channel wall. When particles experience higher near-wall shear stress, the particles are less likely to adhere to channel walls and engender clogging. We confirm the effect of flow rate on channel clogging by demonstrating that the number of particles needed to clog a tapered channel increases as the pressure applied to the particulate suspension increases.
The connection between flow rate and clogging highlights the interplay between hydrodynamic forces and intermolecular forces that govern particle attachment and ultimately clogging. We further explore this relationship by modulating the interaction between the particle and channel wall in a single tapered channel. While observing single channels clogging, we also resolve individual particles gradually building up on channel walls and forming clogs. Interestingly, particles also cluster on upstream channel walls only to later detach and clog at the downstream constriction. At low pressures, the channel clogs when particles accumulate individually near the constriction. At high pressures, the channel clogs when particle clusters detach from channel walls upstream and flow into the constriction. Finally, we compare the clogging behavior of particles with long, electrosteric stabilizing molecules on the surface to the clogging behavior of particles with shorter electrostatic stabilizing molecules on the surface. We also compare the clogging behavior of both particle types in the presence of varying concentrations of a monovalent salt. We show that clogging is mitigated when Debye length is comparable to the length of the stabilizing molecule on the particle’s surface.Engineering and Applied Sciences - Applied PhysicsClogging, Microfluidics, Colloid
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Elasticity of Compressed Emulsions
Get PDFThe interfaces of bubbles and droplets imbue foams and emulsions with extraordinary mechanical and chemical properties. The remarkably large interfacial area of these structures controls their thermodynamics and makes them practical and functional materials. When these interfaces are forced to touch, they can turn a dispersion of one fluid in another into a solid. These solid-like properties are evident in common household products such as shaving foam and mayonnaise, and our ability to control the fluid and solid properties of these materials is essential to their function.Physic
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Mechanical Hysteresis of Fumed Silica Dispersions
No full textSoft materials often exhibit mechanical properties that depend on the past deformation history the material has undergone. Such mechanical hysteresis can arise from the material's composition and structure, which can rearrange and / or yield under deformation. Colloidal dispersions of solid, usually spherical particles, in a continuous fluid medium, have been explored widely over past decades; by contrast, the behavior of dispersions of branched nanoparticle aggregates, such as transparent fumed silica (used to thicken fluids and reinforce rubber tires) and opaque carbon black, have received comparatively less attention, in spite of their significant practical importance. Nevertheless, the highly non-spherical structure of these aggregates increases substantially the mechanical hysteresis of the dispersions that incorporate them. Therefore, to investigate the structural origins of mechanical hysteresis in branched nanoparticle aggregates, I created a model system comprising transparent dispersions of fumed silica, dispersed in either liquid PDMS or mineral oil, which match the particles' refractive index. I explored their rheological behavior with a strain-controlled rheometer as a function of silica volume fraction, oscillatory strain amplitude, and oscillation frequency. I found that repeated oscillatory shear deformation at low strain amplitudes in the linear response regime leads to a gradual increase in the plateau storage modulus, whereas deformations at higher strain amplitudes in the nonlinear response regime cause a pronounced decrease in the plateau storage modulus, as well as a local dip in the loss modulus near the strain of repeated deformation: a ``strain hole." I measured the stress as a function of strain and find that the cycle-to-cycle evolution depends on the maximum strain amplitude imposed on the samples, with the most striking changes occurring in the first cycle of deformation for strain amplitudes in the nonlinear response regime. These hallmarks persist across a range of silica volume fractions and likely originate from the heterogeneous breakup of the connected particle network. Finally, I characterized the 3D structure of these fumed silica dispersions with confocal fluorescence microscopy, which enabled direct visualization of nanopartlcle aggregates within these systems
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Thin Film Complex Oxide Proton Conductors: Synthesis and Applications
No full textThe performance of ultra-thin film solid oxide fuel cells (μ-SOFC) is highly dependent on the structural, microstructural and transport properties of the electrolyte. The focus of this thesis is on understanding the effect of synthesis and processing parameters of BaY0.2Zr0.8O3 (BYZ), a complex oxide proton-conducting electrolyte, on thin-film solid oxide fuel cell (SOFC) performance.
The properties of BYZ thin films are highly dependent on film growth techniques and parameters. The relationship between electrolyte thickness and fuel cell performance is investigated in the ultra-thin film thickness range of ~ 70 nm to ~ 200 nm for BYZ films grown by RF sputtering. The microstructure, crystal structure, and electrical behavior of BYZ films were examined as a function of thickness to attain high power density in SOFCs. The optimal thickness that allows for a balance between the leakage current and Ohmic resistance for these devices was determined to be t0 ~150 nm. XRD examination showed a thickness dependent stress behavior in BYZ thin films, with the most compressive state occurring for films of thickness t0. A Volmer-Weber thin film growth mode is proposed for the observed thickness dependent evolution in film properties. The findings of this examination can allow for an increase in the limits of SOFC power density in the ultra-thin regime for proton conducting electrolytes.
The presence of a large number of grain boundaries in BYZ films processed at intermediate temperatures leads to diminished conductivity. To mitigate this reduced conductivity while maintaining reasonable processing temperatures, it is essential to increase the effective surface area or TPB of the device. A study of the insertion of ion-selective interfacial layers between the electrode-electrolyte interfaces in μ-SOFCs performance is presented. A nearly two-fold increase in power density of μ-SOFCs in the intermediate temperature range is demonstrated by the addition of ultra-thin palladium interlayers. In addition to enhancing performance, this approach may yield important insight into the proton conduction behavior of BYZ and other proton conducting materials.
Finally, to address some of the shortcomings in the current synthesis techniques for BYZ, a novel intermediate temperature thin film synthesis route is demonstrated. This new technique (SP-GNP) is a combination of a thin film deposition technique, Spray Pyrolysis (SP), with a low temperature oxide powder synthesis technique, Glycine Nitrate Process (GNP). A proposed working mechanism and a discussion of the principal parameters that dictate film properties is presented. By using this technique, single-phase perovskite BYZ films were successfully grown at a temperature of 200 °C followed by annealing at 750 °C. The compositional and microstructural evolution of BYZ thin films obtained by SP-GNP is investigated as a function of several technique parameters such as precursor concentration, solvent properties and substrate properties. A microstructural evolution from porous to dense in BYZ thin films by changing precursor composition is demonstrated. This intermediate temperature technique may allow for a deeper insight into the properties of refractory complex oxides through incorporation of novel dopants and may lead to the emergence of new applications for these materials.Engineering and Applied Sciences - Applied PhysicsComplex oxide, Proton conductor, BYZ, solid oxide fuel cel
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Mechanics and Physics of Soft Materials
No full textMaterials where thermal energy is comparable to the interaction energy between molecules are called soft materials. Soft materials are everywhere in our life: food, rubber, polymer diaper, our own body, etc. The thermal fluctuation endows soft materials with fundamentally different behavior comparing to hard materials like metals and ceramics. This dissertation studies three aspects of the mechanics and physics of soft materials, as is reviewed below.
First, soft materials are generally swellable and viscous. The combination of diffusion and viscous flow gives rise to a length scale we called poroviscous length. The emergence of a length scale results in size dependent relaxation. We show that the coupling between diffusion and viscous flow explains the Brownian motion in supercooled liquids, where the classical result of Stokes-Einstein relation generally fails. The concurrent diffusion and viscous flow cannot be described by the classical hydrodynamics, where all the material transport is lumped into velocity field. We formulated a continuum theory to modify the classical hydrodynamics. In particular, the new theory predicts a new bulk viscosity that could exist in incompressible material. We generalize this idea of bulk viscosity to binary systems and study the mixing of materials that is limited by local structural rearrangement instead of diffusion. This model develops formulation of non-equilibrium thermodynamics by removing the common assumption of local equilibrium.
Second, capillarity has strong influence on the morphology of soft materials. The competition between capillarity and elasticity gives rise to the elastocapillary length, which is defined as surface tension over the shear modulus. We show that elastocapillary effect explains the complex nucleation of crease, a widely observed surface instability in soft elastic materials. We also explore the possible competition between capillarity and osmosis in gels, which defines the osmocapillary length, the surface tension divided by osmotic pressure. We show that at small enough length scale or for a gel that is nearly fully swollen, surface tension can pull liquid solvent out from the gel phase, a phenomenon we termed osmocapillary phase separation.
Third, soft materials are nearly incompressible. The incompressibility and softness makes elastomers ideal for the design of seals. Although the failure of seals has been studies for decades, existing studies mainly focus on the damage and degradation of materials. Here we study the leak of a seal due to elastic deformation without any damage. We call such a failure mode the elastic leak. We point out that elastic leak is involved in any leak event no matter whether material is damaged or not. We also show that the reversible nature of the elastic leak enable seal series to achieve higher sealing capability.Engineering and Applied Sciences - Engineering SciencesSoft materials; Solid mechanics; Glass transition; Brownian motion; Surface tension; Crease; Nucleation; Elastocapillary; Osmosis; Capillarity; Phase separation; Leak; Seal; Oil packer
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