Constraints and vibrations in static packings of ellipsoidal particles

We numerically investigate the mechanical properties of static packings of ellipsoidal particles in 2D and 3D over a range of aspect ratio and compression $\Delta \phi$. While amorphous packings of spherical particles at jamming onset ($\Delta \phi=0$) are isostatic and possess the minimum contact number $z_{\rm iso}$ required for them to be collectively jammed, amorphous packings of ellipsoidal particles generally possess fewer contacts than expected for collective jamming ($z < z_{\rm iso}$) from naive counting arguments, which assume that all contacts give rise to linearly independent constraints on interparticle separations. To understand this behavior, we decompose the dynamical matrix $M=H-S$ for static packings of ellipsoidal particles into two important components: the stiffness $H$ and stress $S$ matrices. We find that the stiffness matrix possesses $N(z_{\rm iso} - z)$ eigenmodes ${\hat e}_0$ with zero eigenvalues even at finite compression, where $N$ is the number of particles. In addition, these modes ${\hat e}_0$ are nearly eigenvectors of the dynamical matrix with eigenvalues that scale as $\Delta \phi$, and thus finite compression stabilizes packings of ellipsoidal particles. At jamming onset, the harmonic response of static packings of ellipsoidal particles vanishes, and the total potential energy scales as $\delta^4$ for perturbations by amplitude $\delta$ along these `quartic' modes, ${\hat e}_0$. These findings illustrate the significant differences between static packings of spherical and ellipsoidal particles.Comment: 18 pages, 21 figure

An Analysis of the Impact of Local Drought Conditions on Gross Sales in the Lake Hartwell Region

Proceedings of the 2011 Georgia Water Resources Conference, April 11, 12, and 13, 2011, Athens, Georgia.Lake Hartwell is a United States Army Corps of Engineers (USACE) impoundment of the Savannah River constructed between 1955 and 1963 as a part of a flood control, navigation and hydropower project on the border of South Carolina and Georgia. In addition to the original reasons for its creation, the lake is increasingly used today for tourism and recreational purposes, as well as issues related to water quality, water supply, and fish and wildlife management. As economic activity on and around the lake has intensified, the perceived importance of the lake as a driver of economic activity has correspondingly strengthened. Increased interest in Lake Hartwell has resulted in the creation of a number of stakeholder associations with the objective of protecting the economic interests of lake property owners and lake-oriented businesses. These stakeholders have increasingly brought their economic concerns into discussions of lake management. The multiyear regional droughts of 1999 to 2003 and 2006 to 2009 escalated stakeholder concerns about the economic impact of prolonged low lake levels on lake-oriented real estate and businesses. As drought conditions worsened throughout 2007 and 2008, stakeholders increasingly called for policy changes that would take greater account of the economic and tourism concerns of lake stakeholders. This research documents the unique relationship between consumer spending activity and Lake Hartwell lake levels for the six county region bordering the lake. While this research is portion of a much larger analysis documenting the economic impacts of drought conditions on the region, this piece of the project is informative for understanding the complexities of the relationship between general economic activity and lake levels. This analysis covers the six counties that border Lake Hartwell: Franklin, Hart, and Stephens counties in Georgia, and Anderson, Oconee, and Pickens counties in South Carolina.Sponsored by: Georgia Environmental Protection Division U.S. Geological Survey, Georgia Water Science Center U.S. Department of Agriculture, Natural Resources Conservation Service Georgia Institute of Technology, Georgia Water Resources Institute The University of Georgia, Water Resources FacultyThis book was published by Warnell School of Forestry and Natural Resources, The University of Georgia, Athens, Georgia 30602-2152. The views and statements advanced in this publication are solely those of the authors and do not represent official views or policies of The University of Georgia, the U.S. Geological Survey, the Georgia Water Research Institute as authorized by the Water Research Institutes Authorization Act of 1990 (P.L. 101-307) or the other conference sponsors

N,N′-Bis­[(E)-2-fluoro­benzyl­idene]-1-(2-fluoro­phen­yl)methane­diamine

In the title compound, C21H15F3N2, the benzene ring bonded to the central C atom forms dihedral angles of 77.5 (7) and 89.0 (5)°, respectively, with the remaining two benzene rings. Weak inter­molecular C—H⋯F hydrogen bonds link the mol­ecules into chains propagated in [101]. The crystal packing exhibits weak π–π inter­actions as evidenced by relatively short distances between the centroids of the aromatic rings [3.820 (7) and 3.971 (5) Å]. A MOPAC PM3 optimization of the mol­ecular geometry in vacuo supports a suggestion that inter­molecular forces have a significnt influence on the mol­ecular conformation in the crystal

Multi-phonon Resonant Raman Scattering Predicted in LaMnO3 from the Franck-Condon Process via Self-Trapped Excitons

Resonant behavior of the Raman process is predicted when the laser frequency is close to the orbital excitation energy of LaMnO3 at 2 eV. The incident photon creates a vibrationally excited self-trapped ``orbiton'' state from the orbitally-ordered Jahn-Teller (JT) ground state. Trapping occurs by local oxygen rearrangement. Then the Franck-Condon mechanism activates multiphonon Raman scattering. The amplitude of the $n$-phonon process is first order in the electron-phonon coupling $g$. The resonance occurs {\it via} a dipole forbidden $d$ to $d$ transition. We previously suggested that this transition (also seen in optical reflectivity) becomes allowed because of asymmetric oxygen fluctuations. Here we calculate the magnitude of the corresponding matrix element using local spin-density functional theory. This calculation agrees to better than a factor of two with our previous value extracted from experiment. This allows us to calculate the absolute value of the Raman tensor for multiphonon scattering. Observation of this effect would be a direct confirmation of the importance of the JT electron-phonon term and the presence of self-trapped orbital excitons, or ``orbitons''.Comment: 8 pages and 3 embedded figures. The earlier short version is now replaced by a more complete paper with a slightly different title. This version includes a caculation by density-functional theory of the dipole matrix element for exciting the self-trapped orbital exciton which activates the multiphonon Raman signal

Force distributions near the jamming and glass transitions

We calculate the distribution of interparticle normal forces $P(F)$ near the glass and jamming transitions in model supercooled liquids and foams, respectively. $P(F)$ develops a peak that appears near the glass or jamming transitions, whose height increases with decreasing temperature, decreasing shear stress and increasing packing density. A similar shape of $P(F)$ was observed in experiments on static granular packings. We propose that the appearance of this peak signals the development of a yield stress. The sensitivity of the peak to temperature, shear stress and density lends credence to the recently proposed generalized jamming phase diagram.Comment: 4 pages, 3 postscript figures;Version 3 replaces figure 1 and removes figure 2 from version 1. Significant rewording of version 1 to emphasize the formation of peak in P(F) when these systems jam along five different routes of the recently proposed jamming phase diagram. Version 2 displayed the incorrect abstrac

Random Packings of Frictionless Particles

We study random packings of frictionless particles at T=0. The packing fraction where the pressure becomes nonzero is the same as the jamming threshold, where the static shear modulus becomes nonzero. The distribution of threshold packing fractions narrows and its peak approaches random close-packing as the system size increases. For packing fractions within the peak, there is no self-averaging, leading to exponential decay of the interparticle force distribution.Comment: 4 pages, 3 figure

Regional Differences in Prevalence of HIV-1 Discordance in Africa and Enrollment of HIV-1 Discordant Couples into an HIV-1 Prevention Trial

Background: Most HIV-1 transmission in Africa occurs among HIV-1-discordant couples (one partner HIV-1 infected and one uninfected) who are unaware of their discordant HIV-1 serostatus. Given the high HIV-1 incidence among HIV-1 discordant couples and to assess efficacy of interventions for reducing HIV-1 transmission, HIV-1 discordant couples represent a critical target population for HIV-1 prevention interventions and prevention trials. Substantial regional differences exist in HIV-1 prevalence in Africa, but regional differences in HIV-1 discordance among African couples, has not previously been reported. Methodology/Principal Findings: The Partners in Prevention HSV-2/HIV-1 Transmission Trial (“Partners HSV-2 Study”), the first large HIV-1 prevention trial in Africa involving HIV-1 discordant couples, completed enrollment in May 2007. Partners HSV-2 Study recruitment data from 12 sites from East and Southern Africa were used to assess HIV-1 discordance among couples accessing couples HIV-1 counseling and testing, and to correlate with enrollment of HIV-1 discordant couples. HIV-1 discordance at Partners HSV-2 Study sites ranged from 8–31% of couples tested from the community. Across all study sites and, among all couples with one HIV-1 infected partner, almost half (49%) of couples were HIV-1 discordant. Site-specific monthly enrollment of HIV-1 discordant couples into the clinical trial was not directly associated with prevalence of HIV-1 discordance, but was modestly correlated with national HIV-1 counseling and testing rates and access to palliative care/basic health care (r = 0.74, p = 0.09). Conclusions/Significance: HIV-1 discordant couples are a critical target for HIV-1 prevention in Africa. In addition to community prevalence of HIV-1 discordance, national infrastructure for HIV-1 testing and healthcare delivery and effective community outreach strategies impact recruitment of HIV-1 discordant couples into HIV-1 prevention trials

Interferometric imaging with the 32 element Murchison Wide-field Array

The Murchison Wide-field Array (MWA) is a low frequency radio telescope, currently under construction, intended to search for the spectral signature of the epoch of re-ionisation (EOR) and to probe the structure of the solar corona. Sited in Western Australia, the full MWA will comprise 8192 dipoles grouped into 512 tiles, and be capable of imaging the sky south of 40 degree declination, from 80 MHz to 300 MHz with an instantaneous field of view that is tens of degrees wide and a resolution of a few arcminutes. A 32-station prototype of the MWA has been recently commissioned and a set of observations taken that exercise the whole acquisition and processing pipeline. We present Stokes I, Q, and U images from two ~4 hour integrations of a field 20 degrees wide centered on Pictoris A. These images demonstrate the capacity and stability of a real-time calibration and imaging technique employing the weighted addition of warped snapshots to counter extreme wide field imaging distortions.Comment: Accepted for publication in PASP. This is the draft before journal typesetting corrections and proofs so does contain formatting and journal style errors, also has with lower quality figures for space requirement

Timing of immune escape linked to success or failure of vaccination

Successful vaccination against HIV should limit viral replication sufficiently to prevent the emergence of viral immune escape mutations. Broadly directed immunity is likely to be required to limit opportunities for immune escape variants to flourish. We studied the emergence of an SIV Gag cytotoxic T cell immune escape variant in pigtail macaques expressing the Mane-A*10 MHC I allele using a quantitative RT-PCR to measure viral loads of escape and wild type variants. Animals receiving whole Gag expressing vaccines completely controlled an SIVmac251 challenge, had broader CTL responses and exhibited minimal CTL escape. In contrast, animals vaccinated with only a single CTL epitope and challenged with the same SIVmac251 stock had high levels of viral replication and rapid CTL escape. Unvaccinated na&iuml;ve animals exhibited a slower emergence of immune escape variants. Thus narrowly directed vaccination against a single epitope resulted in rapid immune escape and viral levels equivalent to that of na&iuml;ve unvaccinated animals. These results emphasize the importance of inducing broadly directed HIV-specific immunity that effectively quashes early viral replication and limits the generation of immune escape variants. This has important implications for the selection of HIV vaccines for expanded human trials.<br /