Bulletin of the American Physical Society
2007 Joint New England Sections of the APS and AAPT Spring Meeting
Volume 52, Number 4
Friday–Saturday, April 20–21, 2007; Orono, Maine
Session B1: Poster Session |
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Chair: David Clark, University of Maine Room: Black Bear Inn and Conference Center, 5:15 pm-6:30 pm |
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B1.00001: Quantitative Shape Analysis of Giant Unilamellar Vesicles as a Function of Cholesterol Content Michael Mlodzianoski, Manasa Gudheti, Samuel Hess Giant unilamellar vesicles (GUVs) created from a ternary mixture of saturated lipids, unsaturated lipids, and cholesterol are a way to effectively model cell membranes. For specific concentrations of the constituent components, the model membranes separate into two distinct phases, liquid ordered and liquid disordered. The liquid ordered phase is sometimes referred to as a lipid raft, a region enriched in cholesterol and saturated lipid. This experiment offers an insight into the effects of cholesterol concentration on the physical properties of the membrane, which could alter protein distribution and cell function. We use confocal fluorescence microscopy to image GUVs created from egg sphingomyelin, dioleoylphosphatidylcholine, and cholesterol as well as trace amounts of two fluorescent probes, Bodipy-FL C12-sphingomyelin and lissamine rhodamine-B-DOPE. Shape tracing programs analyze the confocal images to determine dye partitioning, phase area fractions, meridional curvature, and line tensions at the phase boundary of GUVs. The differences resulting from changes in cholesterol concentration could significantly affect function in cellular membranes. [Preview Abstract] |
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B1.00002: Alteration of Biomembrane Properties by Trans-Unsaturated Lipids Manasa Gudheti, Michael Mlodzianoski, Samuel Hess Trans-unsaturated lipids have been associated with a higher incidence of heart disease, but not enough is known about the effect of trans-lipids on membrane properties. We use giant unilamellar vesicles (GUVs) as a model membrane system to explore the effect of the trans-lipid, trans-DOPC on biophysical membrane parameters. GUVs are made by electroformation, imaged by confocal microscopy and analyzed for changes in membrane morphology and properties. Trans-DOPC induces some membrane domains to form unusual morphologies that differ from the typical circular and truncated spherical shapes observed in its absence. Trans-DOPC also alters the membrane curvature distribution, especially in the l$_{o}$ phase near the phase boundary where significantly negative curvatures ($<$-0.5 $\mu $m$^{-1})$ are observed. A narrower distribution of meridional curvatures in GUVs with trans-DOPC is suggestive of higher membrane bending rigidity. Though the l$_{o}$-l$_{d}$ area fractions are similar in the presence and absence of trans-DOPC, the ratio of fluorescent intensities in the l$_{d}$/l$_{o}$ phases indicates a greater concentration or brightness of the probes in the l$_{o}$ phase in the presence of trans-DOPC. The presence of trans-lipids could significantly impact cell function. [Preview Abstract] |
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B1.00003: Energy Transfer Processes in Pr-doped Y$_{2}$SiO$_{5}$ John Collins, Jessica Tolson, B. Di Bartolo Insulating materials containing trivalent Pr are candidates for lasers in the blue and red spectral regions, as well as efficient phosphors. The viability of Pr -based compounds in these applications depends on the efficiency of luminescence from various energy levels of the Pr ion. This work reports on the concentration and temperature effects of the luminescence from Pr$^{3+}$ in yttrium silicate (Y$_{2}$SiO$_{5})$ in samples with Pr concentrations of 0.05{\%} and 1.0{\%} in two sites. Our work shows that the decay rate of the $^{1}$D$_{2}$ level of Pr$^{3+}$ at both sites decreases considerably as the concentration of Pr increases. The (primarily red) luminescence from the $^{1}$D$_{2}$ level is strongly quenched, with the decay rate changing by more than an order of magnitude from the .05{\%} sample to the 1.0{\%} sample. The decay curve of the $^{1}$D$_{2}$ level is non-exponential, showing an initial fast decay and a long tail whose lifetime approaches that of the $^{1}$D$_{2}$ level in the 0.05{\%} sample. The $^{1}$D$_{2}$ decay time also decreases as the temperature increases. The mechanism responsible for the quenching of the $^{1}$D$_{2}$ emission is cross relaxation, aided by diffusion at low temperature and the presence of phonons at high temperature. [Preview Abstract] |
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B1.00004: UV Induced Degradation of Polycarbonate-Based Lens Materials and Implications for the Heath Care Field J. Russell Harkay, Jerry Henry Experimental undergraduate research at Keene State College has utilized facilities in physics and chemistry and at Polyonics, a local firm to study the effects of mono- and polychromatic UV radiation from various sources, including a Deuterium lamp, a solarization unit, a monochromator, and natural sunlight to study the photodegradation of polycarbonate-based lens materials used to produce eyewear using spectrophotometry and FTIR analysis. Ophthalmologic literature indicates a correlation between exposure to the UVB band of sunlight and the onset of cataract formation and macular degeneration. It is well known that polycarbonate plastic ``yellows'' when exposed to intense sunlight and, particularly, UV light either via photo-Fries rearrangement or by a photo oxidative process, forming polyconjugated systems and is a concern primarily for cosmetic reasons. Our data indicates that the ``yellowing'' is an indication of a more sinister problem in the case of eyeglasses in that spectrophotometric comparison shows it is accompanied by an increase in transmissivity in the UVB band where the wearer expects and needs protection. FTIR results indicate a degradation of molecular stabilizers and the appearance of free radicals that indicate a breakdown of the resin's chemical structure. [Preview Abstract] |
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B1.00005: Group Theoretical Techniques in Analyzing Vibronic Spectra from Doped Crystals. Nicholas apRoberts-Warren, John Collins In this work we consider the use of vibronic spectra of rare earth and transition metal ions in ionic crystals to gain information on the phonon density of states. The impurity ion destroys the translational symmetry, leading to vibronic sidebands to emission lines that resemble the density of states. We focus on the application of selection rules for vibronic transitions to select crystals. Using group theoretical techniques, the symmetry of a crystal's unit cell and Brillouin Zone lead to ``irreducible representations'' of the space group of the crystal. Each of these representations corresponds to specific phonon modes of the perfect crystal. To determine which phonon modes can interact with the ion, the space group representations are reduced in terms of the site symmetry of the impurity, after which the selection rules can be applied. The specific case of Vn$^{2+}$ in MgO is examined, and the results are compared with neutron scattering, Raman, and infrared data. Selection rules are also found for the cases of Cr$^{3+}$ in strontium titanate and in yttrium aluminum garnet. [Preview Abstract] |
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B1.00006: Computer Modeling of Energy Transfer in Crystal Christopher Stuetzle, John Collins, Mike Gousie The luminescence from impurity ions in ionic crystals is important in terms of its applications to lasers and lamp phosphors, among others. One important process that affects the efficiency of photon emission in these materials is the non-radiative energy transfer among the impurity ions. We have developed a computer program that allows us to investigate the energy transfer process among ions in crystals, and to view the luminescence output from the crystal as a function of time. The user has the ability to model most any crystal structure, and to replace certain ions in the crystal with impurity ions placed either randomly or non-randomly. The probabilities of photon emission and of energy transfer are based on user input. Excitation of the impurities is mimicked by having a portion of the impurity ions ``absorb'' photons. The program also includes a visualization feature which allows the user to view the periodic nature of the crystal and creates an animation of the energy transfer. We have applied the program to the case of Nd:YAG under several pumping levels, and the results are compared with experimental data. [Preview Abstract] |
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B1.00007: Lecture Demonstrations and Laboratory Experiments as Learning Experiences: A Comparison of Perceived Effectiveness to Assessed Effectiveness Heidar A. Kashkooli, Charles W. Smith The perceived effectiveness of lecture demonstrations and laboratory experiments as learning experiences was measured using a student opinion survey. The assessed effectiveness of these learning experiences was measured by problem solving performance on examinations and quizzes covering specific demonstrations and laboratory topics. Results of the comparison of the perceived effectiveness to the assessed effectiveness will be presented. [Preview Abstract] |
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B1.00008: In The Light of The Dark Matter Sunil Thakur Dark matter and Higgs field are two of the biggest mysteries the world of physics. However, dark matter and Higgs field are not the only mysteries that have troubled the physicists. Mechanism of creation of universe, expansion of universe, loss of energy, wave-particle duality, concept of gravity and gravitational waves, pioneer anomaly are some of the problems that need to be resolved. Higgs field is a theoretical necessity but there are no evidences of its existence and yet almost all of our theories of physics are directly or indirectly based on its predicted existence. Presence of dark matter on the other hand can be theoretically derived from the observed phenomenon and yet none of our theories take into account its existence. Since total mass and total energy in the universe has already been accounted for, existence of Higgs field is automatically ruled out. This paper attempts to resolve these issues by explaining that the dark matter forms the Higgs field. Higgs field absorbs most of the energy and converts this energy into Higgs particle that results in the expansion of the Higgs field. Expanse of the Higgs field is the expanse of our universe. Almost homogenous distribution of dark matter cancels the gravitational pull exerted by it except at the places where it gets distorted due to presence of substance and energy. This theory explains in detail the structure of the Higgs field and how the information flows in the Higgs field. [Preview Abstract] |
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