• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 20422
  • 6238
  • 2550
  • 1914
  • 1914
  • 1914
  • 1914
  • 1914
  • 1900
  • 1537
  • 1477
  • 219
  • 200
  • 168
  • 139
  • Tagged with
  • 46080
  • 5562
  • 5056
  • 4075
  • 3955
  • 3590
  • 3390
  • 2615
  • 2587
  • 2552
  • 2500
  • 2413
  • 2353
  • 2317
  • 2219
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
11

Surfactant Dynamics| Spreading and Wave Induced Dynamics of a Monolayer

Strickland, Stephen Lee 17 June 2016 (has links)
<p> Material adsorbed to the surface of a fluid - for instance crude oil in the ocean, biological surfactant on ocular or pulmonary mucous, or emulsions - can form a 2-dimensional mono-molecular layer. These materials, called surfactants, can behave like a compressible viscous 2-dimensional fluid, and can generate surface stresses that influence the sub-fluid&rsquo;s bulk flow. Additionally, the sub-fluid&rsquo;s flow can advect the surfactant and generate gradients in the surfactant distribution and thereby generate gradients in the interfacial properties. Due to the difficulty of non-invasive measurements of the spatial distribution of a molecular monolayer at the surface, little is known about the dynamics that couple the surface motion and the evolving density field. </p><p> In this dissertation, I will present a novel method for measuring the spatiotemporal dynamics of the surfactant surface density through the fluorescence emission of NBD-tagged phosphatidylcholine, a lipid, and we will compare the surfactant dynamics to the dynamics of the surface morphology.With this method, we will consider the inward and outward spreading of a surfactant on a thin fluid film as well as the advection of a surfactant by linear and non-linear gravity-capillary waves. These two types of surfactant coupled fluid flows will allow us to probe well-accepted assumptions about the coupled fluid-surfactant dynamics. In chapter 1, we review the models used for understanding the spreading of a surfactant on a thin fluid film and the motion of surfactant on a linear gravity-capillary wave. In chapter 2, we will present the experimental methods used in this dissertation. In chapter 3, we will study the outward spreading of a localized region of surfactant and show that the spreading of a monolayer is considerably different from the spreading of thicker-layered surfactant. In chapter 4, we will investigate the inward spreading of a surfactant into a circular surfactant-free region and show that hole closure and the rate of hole closure depends upon the mean surfactant concentration. In chapter 5, we will consider the effects of surface gravity-capillary waves on a monolayer of surfactant and identify that surfactant accumulates on the leading edge of a traveling wave and in the troughs of a standing wave. In chapter 6, we quantify the effect of surfactant on the onset of Faraday waves. In all of these chapters, we will show that the current theoretical understanding is unable to fully capture the dynamics of the surfactant distribution.</p>
12

Molecular dynamics and simulations studies of metal surfaces

Raphuthi, Akie M. 01 June 1994 (has links)
Bulk phonon spectra for several noble metals have been investigated, and the results are being reported in this thesis. The parameters appearing in the semi empirical glue model, are fitted to several physical properties of the noble metals. The calculation of the surface structure and dynamics of (100), (110), and (111) surfaces of aluminum using molecular dynamics and an empirical many body interatomic potential is presented. The multilayer relaxation of these surfaces was calculated by use of simulated annealing. Surface phonon spectra at finite, low temperatures was calculated by means of time-dependent correlation functions. Results are compared with experimental measurements and other calculations.
13

Hunting for MHz Gravitational Waves with the Fermilab Holometer

Kamai, Brittany Lehua 24 July 2016 (has links)
A new ground-based detector, the Fermilab Holometer, has extended the accessible gravitational wave frequency range from kHz up to MHz. At these higher frequencies, exotic sources that were produced shortly after the Big Bang could radiate. The existence of nearby remnants is tested using the Holometer, which consists of separate yet identical 39-meter Michelson interferometers operated at Fermi National Accelerator Laboratory. Utilizing a 130-hour dataset collected between July 15, 2015 and August 15, 2015, constraints are made on both the stochastic gravitational wave background and primordial black hole binaries. The first result is a 3-sigma upper limit of the stochastic gravitational wave background at MHz frequencies. This is the only direct gravitational wave measurement at these frequencies. The Holometer 3-sigma upper limit on the energy density, Omega_GW is 5.6e12 at 1 MHz and goes up to 8.4e15 at 13 MHz. This result is much higher than existing indirect measurements. However, this does place constraints on early universe models that predict large bursts of gravitational radiation in a narrow MHz band. The second result is a measurement of in-spiraling primordial black hole binaries from 1 to 1.92 MHz. We report that there are no detectable primordial black hole binaries in the mass range 0.7e21 - 3.5e21 g between the earth and the moon. Utilizing the same dataset, an alternative analysis path can constrain primordial black hole binaries in the mass range from 0.6e25-2.5e25 g which would increase the distance out to Jupiter. Additionally, the sensitivity of the Holometer with a new data acquisition system can constrain merging black hole binary pairs up to ~1e30 g within the Milky Way halo. This instrument opens up a new opportunity to improve measurements on one of the least constrained mass ranges for primordial black holes.
14

Measurements of directed, elliptic, and triangular flow of charged hadrons from Cu+Au collisions at sqrt(sNN) = 200 GeV with PHENIX at RHIC

Schaefer, Brennan 26 July 2016 (has links)
Measurements of anisotropic flow of inclusive charged hadrons and identified +-p, +-K, p and ¯ p produced at midrapidity in asymmetric Cu+Au collisions at sqrt(sNN) = 200 GeV are presented. The data were collected by the PHENIX experiment at the Relativistic Heavy Ion Collider in 2012. The first three Fourier coefficients in the particlesâ azimuthal distributions, v1, v2, and v3 characterizing directed, elliptic, and triangular flow are studied over a broad range of transverse momentum and collision centrality. System comparisons are made with Cu+Cu and Au+Au collisions at the same center-of-mass energy and varying scaling parameters are used to investigate the effects of the initial geometry. The slopes of the measured vn distributions are ordered by particle mass as expected in a system with global flow. The results are well described by viscous hydrodynamics calculations with shear viscosity over entropy density values close to the quantum minimum.
15

Methods for Probing New Physics at High Energies

Denton, Peter Bennert 23 July 2016 (has links)
This thesis covers two distinct topics: integral dispersion relations (IDRs) and ultra high energy cosmic ray (UHECR) anisotropy. Many models of electroweak symmetry breaking predict new physics scales near LHC energies. Even if these particles are too massive to be produced on shell, it may be possible to infer their existence through the use of IDRs. Making use of Cauchy's integral formula and the analyticity of the scattering amplitude, IDRs are sensitive to changes in the cross section at all energies. We find that a sudden order one increase in the cross section can be detected well below the threshold energy. For two more physical models, the reach of the IDR technique is greatly reduced. The peak sensitivity for the IDR technique is shown to occur when the new particle masses are near the machine energy. Thus, IDRs do extend the reach of the LHC, but only to a window around M_X^2~s_LHC. Determining anisotropies in the arrival directions of UHECRs (>5e19 eV) is an important task in astrophysics. Spherical harmonics are a useful measure of anisotropy. The two lowest nontrivial spherical harmonics, the dipole and the quadrupole, are of particular interest, since they encapsulate a single source and a planar source. The best current UHECR experiments are all ground based, with highly nonuniform exposures which increases the complexity and error in inferring anisotropies. The two main advantages of space based observation of UHECRs are the increased field of view and the full sky coverage with uniform systematics. It turns out that there is an optimal latitude, which runs near the two largest experiments, for an experiment at which nonuniform exposure does not diminish the inference of the quadrupole moment. Consequently, assuming a quadrupole distribution, these experiments can reconstruct a quadrupole distribution to a high precision, without concern for their partial sky exposure. We then investigate the reach of a full sky experiment to detect anisotropies compared to these partial sky experiments. Simulations with dipoles and quadrupoles quantify the advantages of space based, all sky coverage.
16

Development of an Energy Determination Algorithm and the Optimized Detector Design for an Ultrahigh-Energy Cosmic Neutrino Experiment

Tang, Zhen 01 May 2014 (has links)
In this thesis, we discuss the optimization procedure for the TAUWER (TAU shoWER) experiment, which is designed to detect showers generated by Earth-skimming neutrinos. Monte Carlo Simulations are done through CORSIKA (COsmic Ray SImulations for KAscade) software to provide us with detailed information about hit patterns on the detector array from these showers. We use this to determine the trigger conditions, rates, and optimal detector layout. We also use machine learning classification methods to generate classifiers to assign the energy scale for observed showers.
17

Iron-based magnetic metallic superlattices.

Boufelfel, Ahmed. January 1988 (has links)
For the first time we prepared and investigated the structural, magnetic, and electrical transport properties of Fe/W, Fe/Mo, and Fe/Pd metallic superlattices. We made a theoretical attempt to explain the induced increase or decrease of the magnetization at the magnetic superlattice interfaces. We used several x-ray diffraction techniques to determine the structural properties of our superlattices. Mossbauer spectroscopy and neutron scattering were used to determine the induced microscopic magnetic effects due to the superlattice structure. Brillouin scattering spectroscopy was used to determine the elastic and magnetic properties of our samples. We investigated the electrical transport properties over a wide range of temperatures of Fe/Pd and Fe/W superlattices.
18

Design and analysis of the Astrometric Telescope Facility.

Huang, Chunsheng. January 1990 (has links)
The Astrometric Telescope Facility (ATF) is designed to be a space-based facility searching for planets and extra solar planetary systems. In order to be able to positively identify other planetary systems such as Uranus/Neptune-class planets, the ATF is required to be capable of surveying approximately 100 stars within about 10 parsecs of the earth, of measuring a change in the relative position of stars to an accuracy of 10 microarcseconds, and of being stable for about 10 to 20 years. The ATF approach to astrometry is to modulate the intensity on the focal plane of the telescope by a moving Ronchi ruling or grating and then to determine the relative star positions from the phases of the modulated signals. This approach reduces boise from background stray light and reduces random noise by averaging over many measurements. The optical performance of the ATF system has been modeled mathematically using the concept of the system transfer function. Each subsystem has been studied analytically. The relationship between the measured parameter and aberrations of the system has been established analytically. Error sources from the system have been identified and calibration for the system is provided. Design and optimization for the astrometric telescope and gratings have been investigated. The key issues to reach the 10 microarcseconds are addressed.
19

Microstructural investigations of optical coatings by backscattering spectrometry, electron diffraction, and spectrophotometry.

Lehan, John Philip. January 1990 (has links)
Backscattering spectrometry with MeV ⁴He ion beams is investigated as a tool for determining composition with applications to optical coatings. Equations for the compositional analysis of thin films are reviewed. The effect of nuclear charge screening on compositional analysis by MeV ⁴He beams is discussed and examples involving the lanthanide trifluorides illustrate the importance of this correction to avoid possibly erroneous conclusions about sample composition. High probe beam energy is also briefly discussed as a method of reducing the overlap of peaks in backscattering spectra which reduces the technique's accuracy. Complications such as non-Rutherford scattering cross sections for light elements are addressed and an example given. The application of backscattering spectrometry to the depth profiling of elemental constituents in thin films is discussed. It is found that the backscattering spectrum itself provides a reasonable depth profile; however, its depth resolution is limited by the energy resolution of the detection system and energy straggling of the probe beam in the solid. In addition, the depth profile suffers from considerable noise. A method is derived using the principle of maximum likelihood which allows hypothetical depth profiles to be tested and the effects of energy straggling and detection system resolution to be separated from the depth profile. Several examples involving two hypothetical depth profile models are presented. Finally, backscattering spectrometry is combined with scanning electron microscopy, transmission electron microscopy, electron diffraction, x-ray photoelectron spectroscopy, and spectrophotometry in a microstructural survey of hafnium dioxide optical coatings deposited by electron beam evaporation and ion-assisted deposition (IAD). It is found that hafnium dioxide films deposited at temperatures below 300°C are amorphous and exhibit a negative optical inhomogeneity. The refractive index as well as the inhomogeneity are strongly influenced by the oxygen present during film deposition. The inhomogeneity can be removed by IAD which also increases the refractive index of the film. In addition, low energy IAD is found to increase the refractive of the films without affecting the inhomogeneity. This is explained by the preferential sputtering of hydroxide from the growing film surface by the bombarding ions.
20

The Pluto-Charon system as revealed during the mutual events.

Marcialis, Robert Louis. January 1990 (has links)
This year is the last of a five-year interval when the Earth passes through the orbital plane of Pluto and its satellite Charon, causing alternate transits and occultations of the satellite as seen from Earth. Spectrophotometric observations of the system made both in and out of eclipse have been obtained in the visual and near-infrared. The Pluto-Charon system is found to be compositionally diverse, a result unanticipated before the mutual events. Water frost has been identified and is ubiquitous on Charon's surface, while Pluto has a methane veneer. The spectral activity of Pluto's methane is seen to vary with rotational phase, i.e., planetary longitude. On Pluto, surface albedo appears to be correlated with composition. Dark regions tend to be redder and depleted in methane relative to bright regions. Dependence of geometric albedo with wavelength has been calculated for both bodies, from 0.4 to 2.4 μm. The albedo model of Marcialis (1983, 1988) has emerged favorably after several severe tests. Accurate radii and system bulk density derived from the mutual events have been used to construct models of phenomena unanticipated a decade ago. Charon's gravity is feeble enough that it could have shed a substantial primordial methane inventory to space and to Pluto, thereby explaining its different surface composition and lower albedo. Recent interior models are used to show that viscous relaxation of topography is expected to be significant on Pluto but not on Charon. Horizontal topographic features on the primary probably are limited in extent to less than a few tens of kilometers (or are geologically young), much as has been found subsequently for Triton. Globally, Pluto's figure is essentially hydrostatic. Astrometric observations of the system are presented, as is evidence that the discovery of Charon just seven years before the initial mutual events was not fortuitous, but most probable. The astrometry will help to refine Pluto's orbit, making prediction of future stellar occultations by the system more reliable.

Page generated in 0.0575 seconds