Colloidal diffusion and hydrodynamic screening near boundaries

The hydrodynamic interactions between colloidal particles in small ensembles are measured at varying distances from a no-slip surface over a range of inter-particle separations. The diffusion tensor for motion parallel to the wall of each ensemble is calculated by analyzing thousands of particle trajectories generated by blinking holographic optical tweezers and by dynamic simulation. The Stokesian Dynamics simulations predict similar particle dynamics. By separating the dynamics into three classes of modes: self, relative and collective diffusion, we observe qualitatively different behavior depending on the relative magnitudes of the distance of the ensemble from the wall and the inter-particle separation. A simple picture of the pair-hydrodynamic interactions is developed, while many-body-hydrodynamic interactions give rise to more complicated behavior. The results demonstrate that the effect of many-body hydrodynamic interactions in the presence of a wall is much richer than the single particle behavior and that the multiple-particle behavior cannot be simply predicted by a superposition of pair interactions

The Spark of Symmetric Matrices Described by a Graph

We investigate the sparsity of null vectors of real symmetric matrices whose off-diagonal pattern of zero and nonzero entries is described by the adjacencies of a graph. We use the definition of the spark of a matrix, the smallest number of nonzero coordinates of any null vector, to define the spark of a graph as the smallest possible spark of a corresponding matrix. We study connections of graph spark to well-known concepts including minimum rank, forts, orthogonal representations, Parter and Fiedler vertices, and vertex connectivity.Comment: 20 pages, 2 figure

Outcomes of altered teaching strategy in first year physics instruction

The first year Physics teaching effort at the University of Newcastle has embraced on-line quizzes from 1998 with moderate success at fostering weekly engagement. Since 2012 we have altered the nature of these quizzes and supplied electronic support material to varying degrees to the student. We note a discernable improvement in topical grades associated with both the material and teaching techniques for the better student but the poorer student does not discernably engage with the platform or approach. The greater number of less prepared students under instruction mask the results of statistically significant improvement in overall outcomes

Coherent Excitation of the Singlet-triplet Mixed 1s4f State of Helium

In this paper, we present a detailed theoretical description for the coherent electron-impact excitation, the subsequent time evolution, and the cascading decay process of the singlet-triplet mixed 1s4f state of helium. The excitation amplitude and phase of each sublevel of this state are related to measurable coincidence intensities and polarizations of the emitted photons. It is found that the intensity and polarization of the emitted photons are time modulated due to the singlet and triplet mixing in the 1s4f state

Do students need face to face teaching?

At the University of Newcastle, we have two first year physics courses that comprise the first of many core courses for science and engineering students. These first year physics courses contain a variety of topics that have been staggered to outlay a story enveloping all aspects of basic physics. Over the years we have implemented a variety of teaching methods and technology, including active demonstrations, clickers, in-house mobile app technology, and computer aided learning as well as online quizzes and e-book usage. With all these changes there is no statistical difference in the final grades over the last 8 years. This presentation however is aimed at discovering if there is any statistical difference between attendance in lectures and exam results. There is however a clear statistical difference in the mid semester tests and final exam marks for those students attending lectures (or engaged with the course) over those who are either relying on videos of the lectures simply the notes supplied. Our first year classes typically have around 170 students with approximately 55% attending lectures and only 40% attending tutorials. These numbers are an average across the semester as they progressively drop over time. The individual topics in the courses also showed a variety of outcomes. In general, those students who engaged averaged a pass mark while those that did not engage failed. Topics such as thermal physics for example show almost two grades of separation (16% difference, p-value = 0.007) in the average marks for those attending lectures compared to those who did not

First year physics at Newcastle: A longitudinal study

At the University of Newcastle, we offer a pair of Advanced Physics courses for physics majors, science teachers and electrical engineers spanning two semesters of instruction and in which student numbers (after completion) have increased from 138 in 2010 to 180 in 2015. We commenced a longitudinal study in 2012 to measure the effect of altered teaching strategies on our first year outcomes and we report on the latest development in this stud

Relationship between methylation status of Vitamin D-related genes, Vitamin D levels, and methyl-donor biochemistry

© 2016 The Authors. Published by Elsevier Inc. Vitamin D is known for its role in the regulation of gene expression via the Vitamin D receptor, a nuclear transcription factor. More recently, a role for Vitamin D in regulating DNA methylation has been identified as an additional mechanism of modulation of gene expression. How methylation status influences Vitamin D metabolism and response pathways is not yet clear. Therefore, we aimed to assess the relationship between plasma 25-hydroxycholecalciferol (25(OH)D) and the methylation status of Vitamin D metabolism enzyme genes (CYP2R1, CYP27B1 and CYP24A1) and the Vitamin D receptor gene (VDR). This analysis was conducted in the context of dietary Vitamin D, and background methyl donor related biochemistry, with adjustment for several dietary and lifestyle variables. Percentage methylation at CpG sites was assessed in peripheral blood cells using methylation sensitive and dependent enzymes and qPCR. Standard analytical techniques were used to determine plasma 25(OH)D and homocysteine, and serum folate and B12, with the relationship to methylation status assessed using multi-variable regression analysis. CYP2R1 and VDR methylation were found to be independent predictors of plasma 25(OH)D, when adjusted for Vitamin D intake and other lifestyle variables. CYP24A1 was related to plasma 25(OH)D directly, but not in the context of Vitamin D intake. Methyl-group donor biochemistry was associated with the methylation status of some genes, but did not alter the relationship between methylation and plasma 25(OH)D. Modulation of methylation status of CYP2R1, CYP24A1 and VDR in response to plasma 25(OH)D may be part of feedback loops involved in maintaining Vitamin D homeostasis, and may explain a portion of the variance in plasma 25(OH)D levels in response to intake and sun exposure. Methyl-group donor biochemistry, while a potential independent modulator, did not alter this effect

NuSTAR and Swift observations of the black hole candidate XTE J1908+094 during its 2013 outburst

The black hole candidate XTE J1908+094 went into outburst for the first time since 2003 in October 2013. We report on an observation with the Nuclear Spectroscopic Telescope Array (NuSTAR) and monitoring observations with Swift during the outburst. NuSTAR caught the source in the soft state: the spectra show a broad relativistic iron line, and the light curves reveal a ~40 ks flare with the count rate peaking about 40% above the non-flare level and with significant spectral variation. A model combining a multi-temperature thermal component, a power-law, and a reflection component with an iron line provides a good description of the NuSTAR spectrum. Although relativistic broadening of the iron line is observed, it is not possible to constrain the black hole spin with these data. The variability of the power-law component, which can also be modeled as a Comptonization component, is responsible for the flux and spectral change during the flare, suggesting that changes in the corona (or possibly continued jet activity) are the likely cause of the flare.Comment: 9 pages, 6 figures, 3 tables, accepted for publication in Ap

Optical and near-infrared spectroscopy of the black hole swift J1753.5-0127

We report on a multiwavelength observational campaign of the black hole (BH) X-ray binary Swift J1753.5-0127 that consists of an ESO/X-shooter spectrum supported by contemporaneous Swift/X-ray Telescope+Ultra-Violet/Optical Telescope (UVOT) and Australia Telescope Compact Array data. Interstellar medium absorption lines in the X-shooter spectrum allow us to determine E(B-V)=0.45+/- 0.02 along the line of sight to the source. We also report detection of emission signatures of He ii λ 4686, Hα, and, for the first time, H i λ 10906 and Paβ. The double-peaked morphology of these four lines is typical of the chromosphere of a rotating accretion disk. Nonetheless, the paucity of disk features points toward a low level of irradiation in the system. This is confirmed through spectral energy distribution modeling, and we find that the UVOT+X-shooter continuum mostly stems from the thermal emission of a viscous disk. We speculate that the absence of reprocessing is due to the compactness of an illumination-induced envelope that fails to reflect enough incoming hard X-ray photons back to the outer regions. The disk also marginally contributes to the Compton-dominated X-ray emission and is strongly truncated, with an inner radius about 1000 times larger than the BH's gravitational radius. A near-infrared excess is present, and we associate it with synchrotron radiation from a compact jet. However, the measured X-ray flux is significantly higher than what can be explained by the optically thin synchrotron jet component. We discuss these findings in the framework of the radio-quiet versus X-ray-bright hypothesis, favoring the presence of a residual disk, predicted by evaporation models, that contributes to the X-ray emission without enhancing the radio flux