Fluctuation-stabilized marginal networks and anomalous entropic elasticity

We study the elastic properties of thermal networks of Hookean springs. In the purely mechanical limit, such systems are known to have vanishing rigidity when their connectivity falls below a critical, isostatic value. In this work we show that thermal networks exhibit a non-zero shear modulus $G$ well below the isostatic point, and that this modulus exhibits an anomalous, sublinear dependence on temperature $T$. At the isostatic point, $G$ increases as the square-root of $T$, while we find $G \propto T^{\alpha}$ below the isostatic point, where ${\alpha} \simeq 0.8$. We show that this anomalous $T$ dependence is entropic in origin.Comment: 9 pages, 7 figure

Frustration of the isotropic-columnar phase transition of colloidal hard platelets by a transient cubatic phase

Using simulations and theory, we show that the cubatic phase is metastable for three model hard platelets. The locally favored structures of perpendicular particle stacks in the fluid prevent the formation of the columnar phase through geometric frustration resulting in vitrification. Also, we find a direct link between structure and dynamic heterogeneities in the cooperative rotation of particle stacks, which is crucial for the devitrification process. Finally, we show that the life time of the glassy cubatic phase can be tuned by surprisingly small differences in particle shape.Comment: Submitted to Phys. Rev. Let

Surface Modification Influences on Electron Yield

Electron yield is sensitive to surface modifications such as charging effects, surface roughness, and contamination. Understanding these different surface modifications and how they influence electron yield is important to understand which measurements will accurately describe the yield of a material in its real-world environment

Why immigration has the potential to upend the Italian election

The failure of Marine Le Pen and Geert Wilders to cause an upset in 2017 has prompted some observers to argue that Europe's 'populist right' is now in retreat. But as James Dennison, Andrew Geddes and Matthew Goodwin highlight, the apparent fall in support for anti-immigration populism elsewhere in Europe has not been seen in Italy ahead of the country's general election in March. Immigration has risen from a non-issue to the second most important for Italian voters, and polls have shown growing support for the increasingly anti-immigration Forza Italia and Lega Nord

Electron Yield of Challenging Materials: Low Density Polyethylene and Carbon-composite Nanodielectrics

The electron yield—the ratio of the number of emitted electrons to incident electrons—is a key material property that characterizes how materials will charge due to exposure to electron fluxes. The USU Materials Physics Group has developed expertise in measuring this for a wide array of conductors, semiconductors and insulators, including many challenging materials. The basic definitions associated with electron yield and how they are measured will be discussed. We will highlight many critical applications investigated at USU, particularly those associated with spacecraft charging as materials interact with space plasma environments. Electron irradiation experiments conducted to investigate the electron transport, charging, discharging, and emission properties of two challenging and technologically useful materials are discussed. The first is the most structurally simple polymeric material, low density polyethylene (LDPE). The electron yield of this ubiquitous thermoplastic is influenced by the material’s very low conductivity and high negative electron affinity. Similar experiments were performed on an epoxy/carbon-fiber composite material used in extreme applications to understand how the results are influenced by the nanoscale structure of the conducting carbon fibers embedded in the dielectric epoxy matrix

Caesarean Sections at Juba Teaching Hospital 2008-2009

A summary and analysis of all recorded emergency and elective caesarean sections (CS) performed at Juba Teaching Hospital (JTH), Juba, Southern Sudan from October 2008 to September 2009 was made. During this period 430 CS were performed giving a mean of 1.2 each day, the main reason being cited as obstructed labour. Thirty of the babies delivered by CS died giving a neonatal morality rate of 7%. Due to various /non-comprehensive reporting methods it is difficult to measure the maternal mortality rate associated with CS. Overall 11.2% of all deliveries were CS, in accordance with WHO targets. The majority of caesarean sections were performed using a general anaesthetic or ketamine (79% for emergency and 62% for elective surgery). These rates are much higher than those in the published guidance of best practice in the UK (Royal College of Anaesthetists Guideline 2006)

Modeling the Effects of Surface Roughness on Electron Yield

Surface conditions—including surface morphology, composition, contamination, and oxidation—can significantly affect electron yields and consequently spacecraft charging. The effects of surface roughness on electron yield are modeled in this study by considering four aspects of electron yield calculations: (i) simple models of rough surface geometry, (ii) the angular distributions of electrons emitted from various points on these surfaces, (iii) the likelihood of these emitted electrons escaping the rough surface, and (iv) the relative fractions of smooth and rough surfaces. Three simple periodic one-dimensional surface profiles were considered—namely rectangular, triangular, and sawtooth features; each surface profile was characterized by an aspect ratio of the surface feature width to the height. Two different angular emission profiles were considered for lower energy secondary electrons (a Lambertian cosine distribution) and higher energy backscattered electrons (a much narrower, energy-dependent screened Rutherford model approximating a Mott scattering cross-section which also depends on the atomic number of the material). In this initial study, only normally-incident electron profiles were considered, and any emitted electrons were assumed to be recaptured if they intersect any surface. The relative fractions of smooth and rough surfaces (which could in general have different yields for materials in these regions) were taken into account using a simple 1D patch model. Combining the surface profiles with the emission distributions allowed the calculation of a roughness coefficient—which predicted the effect of the surface profile on a smooth surface electron yield—for both secondary and backscattered yields for each surface geometry. Generalized predictions are presented for the reduced secondary and backscattered yields (scaled as the ratio of yields for materials in the smooth and rough fractions) as functions of aspect ratio and the fraction of the surface profile occupied by surface features. Results for backscattered electrons of different incident energies are also presented

Defect-Driven Dynamic Model of Electrostatic Discharge and Endurance Time Measurements of Polymeric Spacecraft Materials

Charge buildup on insulating materials in the space environment can produce long exposure to electric fields, which can lead to Electrostatic Discharge (ESD). Charge buildup is the leading cause of spacecraft failure due to space environment interactions. ESD can be thought of as the point at which the buildup of charge in localized defects, found in polymeric insulating materials, leads to a catastrophic change in electrical conductivity, which can cause the materials to structurally breakdown. Defects produced by radiation, or prolonged exposure to electric fields, significantly alter the endurance time, the time it takes to produce enough defects to generate a current path to flow more readily. The literature discusses two competing theories for ESD in insulators, based on generation of either recoverable or irrecoverable defects. Such defects in the polymer chains can be produced by the electric field and result in localized trapped states for conduction electrons. Both mechanisms are characterized by the density of electron traps and the corresponding energy to create such defects. We propose a hybrid model for the aging process that predicts the endurance time as a function of electric field and temperature. The model incorporates both types of defects with an interdependence of the two mechanisms. Measurements of the endurance time dependence on electric fields in the insulating polymer Low Density Polyethylene (LDPE) are fit against this hybrid model. Understanding the electric field dependence of the time to ESD can assist designers in selecting appropriate materials for spacecraft construction and in mitigating destructive processes