A characterization of micromanipulator controlled dry spinning of micro- and nanoscale polymer fibers.

No current microfabrication technique exists for producing room-temperature, high-precision, point-to-point polymer nanofibers in three dimensions. Producing rounded structures in a third dimension is particularly difficult to accomplish with conventional planar microfabrication techniques. Therefore, the purpose of this study is to characterize a novel method for fabricating such structures. In this investigation, PMMA micro- and nano-fibers have been fabricated using a technique which involves drawing a solvated polymer bridge between two liquid pools with a stylus positioned by an ultra-high-precision micromill. The solvent in the solution bridge rapidly evaporates, leaving a suspended PMMA fiber between the two pools. In order to characterize this process, fibers were drawn over a 1.8 mm silicon trench and fiber diameter was measured over a variety of different solution concentrations and polymer molecular weights. In addition, the fluid characteristics of the solutions were measured to allow for comparisons between fiber diameter and properties such as viscosity and surface tension. Fiber diameters ranging from 450 nm to 50 µm were drawn during the characterization experiments. In addition, fibers as small as 140 nm were drawn over distances less than 1.8 mm. It was observed that fiber diameter enlarged as both solution concentration and polymer molecular weight increased. In an attempt to decrease fiber diameter variance, different stylus materials were also examined, and it was found that a parylene-coated stylus resulted in a fair reduction, but not elimination, of the diameter variance. Although fiber diameter variances remain somewhat high, possibly due to unwanted solution buildup on the stylus tip, this fabrication technique presents a simple method for producing precisely positioned, low temperature, suspended polymer fiber structures on the micro- and nanoscale

Dispersal Patterns And Reproductive Strategies Of Trypanosoma Cruzi In An Urban Environment

Interactions with novel environments result in both population extinctions and explosions, depending on life history strategies, ecological circumstances and historical contingencies. Understanding how populations interact with novel environments is more important now than ever because of the rapid increase in anthropogenic environmental alterations. While many native populations perish during urbanization, other populations successfully invade and thrive in these novel environments including many pests and pathogens that are detrimental to human health and economy. Pathogens contain a diversity of life history strategies and are thus good models for understanding how life history strategies interact with novel environments to promote or hinder the dynamic processes of invasion. In order to determine how life history strategies affect the evolution of a population during invasion, we studied a population of Trypanosoma cruzi – a protozoan parasite and causative agent of Chagas disease in humans – in the city of Arequipa in southern Peru. To estimate population structure and evolutionary history, we sequenced and assembled 133 T. cruzi genomes collected throughout Arequipa (N=123) and South America, and performed phylogenetic and population genetic analyses. We found that (1) the extant T. cruzi population in Arequipa was founded by a single introduction; (2) T. cruzi readily disperses between houses in a city block, but rarely disperses between blocks; (3) We resolve an apparent contradiction between the perceived clonal population structure of T. cruzi and its capacity for sexual reproduction by showing that this population regularly underwent meiosis and fertilization as the it expanded throughout the city, but that exclusive inbreeding resulted in a clonal population structure. As urbanization increases worldwide, it is important to understand how life history strategies affect the invasion of urban environments

An exploration of defensive pessimism, explanatory style, and expectations in relation to the academic performance of college and university students.

Researchers have studied the concepts of optimism and pessimism as traits, expectations, strategies, and styles of explaining outcomes. Explanatory style and the strategy of defensive pessimism are two of these areas. In general, optimistic explanatory style has been associated with better outcomes including academic performance. Some studies have found that pessimistic explanatory style has been associated with better academic outcomes. One suggestion in the literature was that defensive pessimism might explain the cases where pessimistic explanatory style is associated with better academic outcomes. To evaluate this explanation, the Academic Attributional Style Questionnaire (AASQ, Revised Defensive Pessimism Questionnaire (DPQ), and Brief Symptom Inventory (BSI) were administered to 188 undergraduate and graduate students from five colleges and universities. Measures of academic achievement included official course and exam grade as reported by the instructor as well as self-reported GPA. There were no differences in academic performance between groups that made more pessimistic explanations and those who made more optimistic explanations. There were no differences within the group of those with more pessimistic explanatory styles regardless of level of use of defensive pessimism. Explanatory style was associated with expectations for course grade. There were no differences on outcome expectations or efficacy between defensive pessimists and low exam scorers. Defensive pessimism was associated with multiple psychological symptoms as measured by the Brief Symptom Inventory (BSI). Further investigation to determine if encouraging a strategy defensive pessimism in those with more pessimistic explanatory styles would lead to improved performance. More research needs to be devoted to the study of the relationships among explanatory style, defensive pessimism, and expectations

Revealing the Nature of Extreme Coronal-line Emitter SDSS J095209.56+214313.3

Extreme coronal-line emitter (ECLE) SDSSJ095209.56+214313.3, known by its strong, fading, high ionization lines, has been a long standing candidate for a tidal disruption event, however a supernova origin has not yet been ruled out. Here we add several new pieces of information to the puzzle of the nature of the transient that powered its variable coronal lines: 1) an optical light curve from the Lincoln Near Earth Asteroid Research (LINEAR) survey that serendipitously catches the optical flare, and 2) late-time observations of the host galaxy with the Swift Ultraviolet and Optical Telescope (UVOT) and X-ray telescope (XRT) and the ground-based Mercator telescope. The well-sampled, $\sim10$-year long, unfiltered LINEAR light curve constrains the onset of the flare to a precision of $\pm5$ days and enables us to place a lower limit on the peak optical magnitude. Difference imaging allows us to estimate the location of the flare in proximity of the host galaxy core. Comparison of the \textsl{GALEX} data (early 2006) with the recently acquired Swift UVOT (June 2015) and Mercator observations (April 2015) demonstrate a decrease in the UV flux over a $\sim 10$ year period, confirming that the flare was UV-bright. The long-lived UV-bright emission, detected 1.8 rest-frame years after the start of the flare, strongly disfavors a SN origin. These new data allow us to conclude that the flare was indeed powered by the tidal disruption of a star by a supermassive black hole and that TDEs are in fact capable of powering the enigmatic class of ECLEs.Comment: Submitted to Ap

Aeroheating Measurements of BOLT Aerodynamic Fairings and Transition Module

The Air Force Office of Scientific Research (AFOSR) has sponsored the Boundary Layer Transition (BOLT) Experiments to investigate hypersonic boundary layer transition on a low-curvature, concave surface with swept leading edges. This paper presents aeroheating measurements on a subscale model of the BOLT Flight Geometry, aerodynamic fairings, and Transition Module (TSM) in the NASA Langley 20-Inch Mach 6 Air Tunnel. The purpose of the test was to investigate and identify any areas of localized heating on the TSM for inclusion in the BOLT Critical Design Review (CDR). Surface heating distributions were measured using global phosphor thermography, and data were obtained for a range of model attitudes and free stream Reynolds numbers. Measurements showed low heating on the fairings and TSM. Additional analysis was completed after the CDR to compare heating on the TSM for the nominal BOLT vehicle reentry angle-of-attack with heating on the TSM for possible reentry angle-of-attack excursions. The results of this analysis were used in conjunction with thermal analyses from Johns Hopkins Applied Physics Lab (JHU/APL) and the Air Force Research Laboratory (AFRL) to assess the need for thermal protection on the flight vehicle TSM

Analysis of Dynamic Data from Supersonic Retropropulsion Experiments in NASA Langley's Unitary Plan Wind Tunnel

Recent experimental supersonic retropropulsion tests were conducted at the NASA Langley Research Center Unitary Plan Wind Tunnel Test Section 2 for a range of Mach numbers from 2.4 to 4.6. A 5-inch 70-degree sphere-cone forebody model with a 10-inch cylindrical aftbody experimental model was used which is capable of multiple retrorocket configurations. These configurations include a single central nozzle on the center point of the forebody, three nozzles at the forebody half-radius, and a combination of the first two configurations with no jets being plugged. A series of measurements were achieved through various instrumentation including forebody and aftbody pressure, internal pressures and temperatures, and high speed Schlieren visualization. Specifically, several high speed pressure transducers on the forebody and in the plenum were implemented to look at unsteady flow effects. The following work focuses on analyzing frequency traits due to the unsteady flow for a range of thrust coefficients for single, tri, and quad-nozzle test cases at freestream Mach 4.6 and angle of attack ranging from -8 degrees to +20 degrees. This analysis uses Matlab s fast Fourier transform, Welch's method (modified average of a periodogram), to create a power spectral density and analyze any high speed pressure transducer frequency traits due to the unsteady flow

Recommendations for Hypersonic Boundary Layer Transition Flight Testing

Much has been learned about the physics underlying the transition process at supersonic and hypersonic speeds through years of analysis, experiment and computation. Generally, the application of this knowledge has been restricted to simple shapes like plates, cones and spherical bodies. However, flight reentry vehicles are in reality never simple. They typically are highly complex geometries flown at angle of attack so three-dimensional effects are very important, as are roughness effects due to surface features and/or ablation. This paper will review our present understanding of the physics of the transition process and look back at some of the recent flight test programs for their successes and failures. The goal of this paper is to develop rationale for new hypersonic boundary layer transition flight experiments. Motivations will be derived from both an inward look at what we believe constitutes a good flight test program as well as an outward review of the goals and objectives of some recent US based unclassified proposals and programs. As part of our recommendations, this paper will address the need for careful experimental work as per the guidelines enunciated years ago by the U.S. Transition Study Group. Following these guidelines is essential to obtaining reliable, usable data for allowing refinement of transition estimation techniques

Damped Ly{\alpha} Absorption Systems in Semi-Analytic Models with Multiphase Gas

We investigate the properties of damped Ly{\alpha} absorption systems (DLAs) in semi-analytic models of galaxy formation, including partitioning of cold gas in galactic discs into atomic, molecular, and ionized phases with a molecular gas-based star formation recipe. We investigate two approaches for partitioning gas into these constituents: a pressure-based and a metallicity-based recipe. We identify DLAs by passing lines of sight through our simulations to compute HI column densities. We find that models with "standard" gas radial profiles - where the average specific angular momentum of the gas disc is equal to that of the host dark matter halo - fail to reproduce the observed column density distribution of DLAs. These models also fail to reproduce the distribution of velocity widths {\Delta}v, overproducing low {\Delta}v relative to high {\Delta}v systems. Models with "extended" radial gas profiles - corresponding to gas discs with higher specific angular momentum - are able to reproduce quite well the column density distribution of absorbers over the column density range 19 < log NHI < 22.5 in the redshift range 2 < z < 3.5. The model with pressure-based gas partitioning also reproduces the observed line density of DLAs, HI gas density, and {\Delta}v distribution at z < 3 remarkably well. However all of the models investigated here underproduce DLAs and the HI gas density at z > 3. If this is the case, the flatness in the number of DLAs and HI gas density over the redshift interval 0 < z < 5 may be due to a cosmic coincidence where the majority of DLAs at z > 3 arise from intergalactic gas in filaments while those at z < 3 arise predominantly in galactic discs. We further investigate the dependence of DLA metallicity on redshift and {\Delta}v, and find reasonably good agreement with the observations, particularly when including the effects of metallicity gradients (abbrv.).Comment: 27 pages, 15 figures, submitted to MNRA

Discrete Roughness Transition for Hypersonic Flight Vehicles

The importance of discrete roughness and the correlations developed to predict the onset of boundary layer transition on hypersonic flight vehicles are discussed. The paper is organized by hypersonic vehicle applications characterized in a general sense by the boundary layer: slender with hypersonic conditions at the edge of the boundary layer, moderately blunt with supersonic, and blunt with subsonic. This paper is intended to be a review of recent discrete roughness transition work completed at NASA Langley Research Center in support of agency flight test programs. First, a review is provided of discrete roughness wind tunnel data and the resulting correlations that were developed. Then, results obtained from flight vehicles, in particular the recently flown Hyper-X and Shuttle missions, are discussed and compared to the ground-based correlations