Beyond Finger-Pointing and Test Scores

This report examines high-stakes interventions in low-performing schools in six cities in Cross City Campaign's network

The Devil is in the details:Pentagonal bipyramids and dynamic arrest

Colloidal suspensions have long been studied as a model for atomic and molecular systems, due to the ability to fluorescently label and individually track each particle, yielding particle-resolved structural information. This allows various local order parameters to be probed that are otherwise inaccessible for a comparable molecular system. For phase transitions such as crystallisation, appropriate order parameters which emphasise 6-fold symmetry are a natural choice, but for vitrification the choice of order parameter is less clear cut. Previous work has highlighted the importance of icosahedral local structure as the glass transition is approached. However, counting icosahedra or related motifs is not a continuous order parameter in the same way as, for example, the bond-orientational order parameters $Q_{6}$ and $W_6$. In this work we investigate the suitability of using pentagonal bipyramid membership, a structure which can be assembled into larger, five-fold symmetric structures, as a finer order parameter to investigate the glass transition. We explore various structural and dynamic properties and show that this new approach produces many of the same findings as simple icosahedral membership, but we also find that large instantaneous displacements are often correlated with significant changes in pentagonal bipyramid membership, and unlike the population of defective icosahedra, the pentagonal bypyramid membership and spindle number do not saturate for any measured volume fraction, but continues to increase.Comment: accepted by JStat Mech: Theory and Experiment 201

Interactive effects of low molecular weight carbon compounds on N2O emissions

Low molecular weight carbon (C) compounds in hotspots such as the rhizosphere can greatly affect nitrate reduction processes. Towards a better prediction of N2O emission from denitrification, we are still lacking understanding of responses to the supply of complex C compound mixtures such as rhizodeposits versus the often examined response to individually amended C compounds. In a laboratory study, we applied three C compounds, glucose, citric acid and glutamic acid, individually or as a three-compound mixture to 14NH415NO3 amended soil at 80% water-filled pore space. For the individual C compound treatments, the substrateswere enriched in 13C-C. The mixture was enriched in 13C-C either in all constituent compounds or in one of the compounds only, resulting in four different treatments. This set-up enabled quantification of the utilization of each compound for heterotrophic respiration when applied individually and in combination, and for this to be related to the dynamics of 15N-NO3- reduction to 15N-N2O. The total 15N-N2O emission from the compound mixture over 10 days was similar to the total emission predicted from the average of the individual compound treatments This could suggest potential predictability of denitrification responses to complex mixtures of C based on knowledge of its constituents. However, immediate and simultaneous peaks of 15N-N2O and 13C-CO2 fluxes from the compound mixture contrasted with observed delays in 15N-N2O and 13C-CO2 fluxes when the compounds had been applied individually. Moreover, relative contributions of the C compounds to 13C-CO2 respiration from the compound mixture were different from the predicted contributions based on their individual application. While contributions of glutamic acid-C and citric acid-C to respiration in mixture during peak 15N-N2O emission were increased, glucose utilization in the mixture treatment was significantly lower. These findings give a glimpse of the challenges we are facing when trying to predict nitrate reduction occurring in the rhizosphere where interactions between C compounds and the soil matrix, as well as within the wider heterotrophic community, determine process rates. Given that most of our understanding of the role of C in regulating nitrate reduction, is informed from single compound studies, we require more evidence on the effects and innate interactions of compound mixtures to be able to predict responses to C sources

Preparation of films of a highly aligned lipid cubic phase

We demonstrate a method by which we can produce an oriented film of an inverse bicontinuous cubic phase (QII D) formed by the lipid monoolein (MO). By starting with the lipid as a disordered precursor (the L3 phase) in the presence of butanediol, we can obtain a film of the QII D phase showing a high degree of in-plane orientation by controlled dilution of the sample under shear within a linear flow cell. We demonstrate that the direction of orientation of the film is different from that found in the oriented bulk material that we have reported previously; therefore, we can now reproducibly form QII D samples oriented with either the [110] or the [100] axis aligned in the flow direction depending on the method of preparation. The deposition of MO as a film, via a moving fluid− air interface that leaves a coating of MO in the L3 phase on the capillary wall, leads to a sample in the [110] orientation. This contrasts with the bulk material that we have previously demonstrated to be oriented in the [100] direction, arising from flow producing an oriented bulk slug of material within the capillary tube. The bulk sample contains significant amounts of residual butanediol, which can be estimated from the lattice parameter of the QII D phase obtained. The sample orientation and lattice parameters are determined from synchrotron small-angle X-ray scattering patterns and confirmed by simulations. This has potential applications in the production of template materials and the growth of protein crystals for crystallography as well as deepening our understanding of the mechanisms underlying the behavior of lyotropic liquid-crystal phases

Local structure in deeply supercooled liquids exhibits growing lengthscales and dynamical correlations

Glasses are among the most widely used of everyday materials, yet the process by which a liquid’s viscosity increases by 14 decades to become a glass remains unclear, as often contradictory theories provide equally good descriptions of the available data. Knowledge of emergent lengthscales and higher-order structure could help resolve this, but this requires time-resolved measurements of dense particle coordinates—previously only obtained over a limited time interval. Here we present an experimental study of a model colloidal system over a dynamic window significantly larger than previous measurements, revealing structural ordering more strongly linked to dynamics than previously found. Furthermore we find that immobile regions and domains of local structure grow concurrently with density, and that these regions have low configurational entropy. We thus show that local structure plays an important role at deep supercooling, consistent with a thermodynamic interpretation of the glass transition rather than a principally dynamic description

Experimental Determination of Configurational Entropy in a Two-Dimensional Liquid under Random Pinning

A quasi two-dimensional colloidal suspension is studied under the influence of immobilisation (pinning) of a random fraction of its particles. We introduce a novel experimental method to perform random pinning and, with the support of numerical simulation, we find that increasing the pinning concentration smoothly arrests the system, with a cross-over from a regime of high mobility and high entropy to a regime of low mobility and low entropy. At the local level, we study fluctuations in area fraction and concentration of pins and map them to entropic structural signatures and local mobility, obtaining a measure for the local entropic fluctuations of the experimental system