Coupling of sedimentation and liquid structure: influence on hard sphere nucleation

The discrepancy in nucleation rate densities between simulated and experimental hard spheres remains staggering and unexplained. Suggestively, more strongly sedimenting colloidal suspensions of hard spheres nucleate much faster than weakly sedimenting systems. In this work we consider firstly the effect of sedimentation on the structure of colloidal hard spheres, by tuning the density mismatch between solvent and colloidal particles. In particular we investigate the effect on the degree of five fold symmetry present. Secondly we study the size of density fluctuations in these experimental systems in comparison to simulations. The density fluctuations are measured by assigning each particle a local density, which is related to the number of particles within a distance of 3.25 particle diameters. The standard deviation of these local densities gives an indication of the fluctuations present in the system. Five fold symmetry is suppressed by a factor of two when sedimentation is induced in our system. Density fluctuations are also increased by a factor of two in experiments compared to simulations. The change in five fold symmetry makes a difference to the expected nucleation rates, but we demonstrate that it is ultimately too small to resolve the discrepancy between experiment and simulation, while the fluctuations are shown to be an artefact of 3d particle tracking.Comment: 8 page

Determinants of Undergraduate GPA and Persistence at Connecticut College

This thesis uses a unique dataset of five class-years to determine the factors that affect the academic experience of students at Connecticut College, particularly their cumulative grade point averages (CGPA) and persistence into sophomore year. Chapter I examines how changes in high school inputs affect students’ performance at Connecticut College. This chapter finds that high school inputs (viz., pupil-teacher ratio, total expenditure per pupil, and expenditure on teacher salaries per pupil) have a significant effect on college GPA even after controlling for demographics and prior academic performance. Chapter II examines the factors affecting persistence at Connecticut College. It finds that various measures of fit between a student and his or her peers have a strong impact on persistence into sophomore year

Unmanned Aerial Vehicle Ground Control Point Deployment

According to Federal Aviation Regulation (FAR) Part 77, all airports have imaginary approach surfaces which must remain clear of obstructions in order to ensure safe air travel. Threats of penetration to these imaginary surfaces include new construction, telephone poles and lines, and trees. While most potential threats analyzed remain relatively constant in size, objects such as trees which grow require annual analysis for change detection. A variety of methods are available for surveying these surfaces for potential obstructions, one being an aerial mapping from photogrammetric data. Aerial mapping for surveying purposes is a process which ties overlapping photographs together using computer software which detects similar points between the images. These images requires ground control points, also known as GCPs, to create a scale which allows for accurate measurement data. When ground control points with known GPS locations are placed throughout the mapping area all the points within the model can then be tied to their respective GPS coordinates in the longitudinal, latitudinal, and altitude directions. The placement of these markers is one of the most time-consuming but necessary tasks when creating an aerial map. The main objective of this project was for the team to design and produce a method of streamlining the ground control point deployment process. This report introduces a device which, when implemented, can reduce the number of resources and labor needed for this process. A mechanical release system was designed and built to be carried by a drone. Remote control between two XBee RF modules was implemented to activate the rotating notch release system, following user commands. A 90 degree rotation by the servo of the flange would align the flange with the keyslot in the GCP, allowing it to fall. The final design was successfully operated by one pilot and three GCP’s were deployed at various locations. An important factor in designing this system was developing lightweight, high contrast GCPs that would not impact the flight of the drone. The weight and balance of the final product were suitable for the small 3DR Solo drone used in the project. This accomplishment proves this system could be applied to drones with greater payloads and longer flight times for preparing large surveying areas in minimal time

Structural signatures of ultrastability in a deposited glassformer

Glasses obtained from vapor deposition on a cold substrate have superior thermodynamic and kinetic stability with respect to ordinary glasses. Here we perform molecular dynamics simulations of vapor deposition of a model glass-former and investigate the origin of its high stability compared to that of ordinary glasses. We find that the vapor deposited glass is characterized by locally favoured structures (LFS) whose occurrence correlates with its stability, reaching a maximum at the optimal deposition temperature. The formation of LFS is enhanced near the free surface, hence supporting the idea that the stability of vapor deposited glasses is connected to the relaxation dynamics at the surface.Comment: Accepted for publication in Phys. Rev. Lett. (https://journals.aps.org/prl/accepted/7a07bY5cL7816472348d41f9d096a93c4bb5f035b) Transfer of copyright provided to the American Physical Society (APS

The long-standing dilemma of European summer temperatures at the mid-Holocene and other considerations on learning from the past for the future using a regional climate model

The past as an analogue for the future is one of the main motivations to use climate models for paleoclimate applications. Assessing possible model limitations in simulating past climate changes can lead to an improved understanding and representation of the response of the climate system to changes in the forcing, setting the basis for more reliable information for the future. In this study, the regional climate model (RCM) COSMO-CLM is used for the investigation of the mid-Holocene (MH, 6000 years ago) European climate, aiming to contribute to the solution of the long-standing debate on the reconstruction of MH summer temperatures for the region, and gaining more insights into the development of appropriate methods for the production of future climate projections. Two physically perturbed ensembles (PPEs) are first built by perturbing model physics and parameter values, consistently over two periods characterized by different forcing (i.e., the MH and pre-industrial, PI). The goal is to uncover possible processes associated with the considered changes that could deliver a response in MH summer temperatures closer to evidence from continental-scale pollen-based reconstructions. None of the investigated changes in model configuration produces remarkable differences with respect to the mean model behavior. This indicates a limited sensitivity of the model to changes in the climate forcing, in terms of its structural uncertainty. Additional sensitivity tests are further conducted for the MH, by perturbing the model initial soil moisture conditions at the beginning of spring. A strong spatial dependency of summer near-surface temperatures on the soil moisture available in spring is evinced from these experiments, with particularly remarkable differences evident over the Balkans and the areas north of the Black Sea. This emphasizes the role of soil–atmosphere interactions as one of the possible drivers of the differences in proxy-based summer temperatures evident between northern and southern Europe. A well-known deficiency of the considered land scheme of COSMO-CLM in properly retaining spring soil moisture, confirmed by the performed tests, suggests that more attention should be paid to the performance of the soil component of climate models applied to this case study. The consideration of more complex soil schemes may be required to help bridging the gap between models and proxy reconstructions. Finally, the distribution of the PPEs with changes in model configuration is analyzed for different variables. In almost all of the considered cases the results show that what is optimal for one period, in terms of a model configuration, is not the best for another characterized by different radiative forcing. These results raise concerns about the usefulness of automatic and objective calibration methods for RCMs, suggesting that a preferable approach is the production of small PPEs that target a set of model configurations, properly representing climate phenomena characteristic of the target region and that will be likely to contain the best model answer under different forcing

Seroprevalence of Ebola virus infection in Bombali District, Sierra Leone

A serosurvey of anti-Ebola Zaire virus nucleoprotein IgG prevalence was carried out among Ebola virus disease survivors and their Community Contacts in Bombali District, Sierra Leone. Our data suggest that the specie of Ebola virus (Zaire) responsible of the 2013-2016 epidemic in West Africa may cause mild or asymptomatic infection in a proportion of cases, possibly due to an efficient immune response