426 research outputs found
Deformation-Driven Diffusion and Plastic Flow in Two-Dimensional Amorphous Granular Pillars
We report a combined experimental and simulation study of deformation-induced
diffusion in compacted two-dimensional amorphous granular pillars, in which
thermal fluctuations play negligible role. The pillars, consisting of
bidisperse cylindrical acetal plastic particles standing upright on a
substrate, are deformed uniaxially and quasistatically by a rigid bar moving at
a constant speed. The plastic flow and particle rearrangements in the pillars
are characterized by computing the best-fit affine transformation strain and
non-affine displacement associated with each particle between two stages of
deformation. The non-affine displacement exhibits exponential crossover from
ballistic to diffusive behavior with respect to the cumulative deviatoric
strain, indicating that in athermal granular packings, the cumulative
deviatoric strain plays the role of time in thermal systems and drives
effective particle diffusion. We further study the size-dependent deformation
of the granular pillars by simulation, and find that different-sized pillars
follow self-similar shape evolution during deformation. In addition, the yield
stress of the pillars increases linearly with pillar size. Formation of
transient shear lines in the pillars during deformation becomes more evident as
pillar size increases. The width of these elementary shear bands is about twice
the diameter of a particle, and does not vary with pillar size.Comment: 14 pages, 11 figure
Divergence of Voronoi Cell Anisotropy Vector: A Threshold-Free Characterization of Local Structure in Amorphous Materials
Characterizing structural inhomogeneity is an essential step in understanding the mechanical response of amorphous materials. We introduce a threshold-free measure based on the field of vectors pointing from the center of each particle to the centroid of the Voronoi cell in which the particle resides. These vectors tend to point in toward regions of high free volume and away from regions of low free volume, reminiscent of sinks and sources in a vector field. We compute the local divergence of these vectors, where positive values correspond to overpacked regions and negative values identify underpacked regions within the material. Distributions of this divergence are nearly Gaussian with zero mean, allowing for structural characterization using only the moments of the distribution. We explore how the standard deviation and skewness vary with the packing fraction for simulations of bidisperse systems and find a kink in these moments that coincides with the jamming transition
Identifying Structural Flow Defects in Disordered Solids Using Machine-Learning Methods
We use machine-learning methods on local structure to identify flow defectsâor particles susceptible to rearrangementâin jammed and glassy systems. We apply this method successfully to two very different systems: a two-dimensional experimental realization of a granular pillar under compression and a Lennard-Jones glass in both two and three dimensions above and below its glass transition temperature. We also identify characteristics of flow defects that differentiate them from the rest of the sample. Our results show it is possible to discern subtle structural features responsible for heterogeneous dynamics observed across a broad range of disordered materials
The development of path integration: combining estimations of distance and heading
Efficient daily navigation is underpinned by path integration, the mechanism by which we use self-movement information to update our position in space. This process is well-understood in adulthood, but there has been relatively little study of path integration in childhood, leading to an underrepresentation in accounts of navigational development. Previous research has shown that calculation of distance and heading both tend to be less accurate in children as they are in adults, although there have been no studies of the combined calculation of distance and heading that typifies naturalistic path integration. In the present study 5-year-olds and 7-year-olds took part in a triangle-completion task, where they were required to return to the startpoint of a multi-element path using only idiothetic information. Performance was compared to a sample of adult participants, who were found to be more accurate than children on measures of landing error, heading error, and distance error. 7-year-olds were significantly more accurate than 5-year-olds on measures of landing error and heading error, although the difference between groups was much smaller for distance error. All measures were reliably correlated with age, demonstrating a clear development of path integration abilities within the age range tested. Taken together, these data make a strong case for the inclusion of path integration within developmental models of spatial navigational processing
Pushing the mass limit for intact launch and photoionization of large neutral biopolymers
Since their first discovery by Louis Dunoyer and Otto Stern, molecular beams have conquered research and technology. However, it has remained an outstanding challenge to isolate and photoionize beams of massive neutral polypeptides. Here we show that femtosecond desorption from a matrix-free sample in high vacuum can produce biomolecular beams at least 25 times more efficiently than nanosecond techniques. While it has also been difficult to photoionize large biomolecules, we find that tailored structures with an abundant exposure of tryptophan residues at their surface can be ionized by vacuum ultraviolet light. The combination of these desorption and ionization techniques allows us to observe molecular beams of neutral polypeptides with a mass exceeding 20,000âamu. They are composed of 50 amino acids â 25 tryptophan and 25 lysine residues â and 26 fluorinated alkyl chains. The tools presented here offer a basis for the preparation, control and detection of polypeptide beams
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Demonstrations to Support Change to the >260 ppm Mercury Treatment Regulations
The U.S. Department of Energy (DOE) and the U. S. Environmental Protection Agency (EPA) are working together to justify a change in the Land Disposal Restriction for High Mercury (>260 ppm mercury) waste. The present regulation that requires roasting or retorting is based on recovering and recycling the mercury in the waste. However, most of DOEâs High Mercury waste is radioactively contaminated, eliminating the possibility of its recycle. The radioactive mercury recovered must be amalgamated and disposed. In addition, concern over fugitive emissions from retorting and roasting operations has raised the question of whether such processing is environmentally sound. A change to the regulation to allow stabilization and disposal would reduce the overall environmental threat, if the stabilization process can reduce the leachability of the mercury to regulatory levels. Demonstrations are underway to gather data showing that the High Mercury waste can be safely stabilized. At the same time, comparison tests are being conducted using an improved form of the baseline retorting technology to better quantify the fugitive emission problem and determine the full capability of thermal desorption systems. A first round of demonstrations stabilizing mercury in soil from Brookhaven National Laboratory (BNL) has been completed. Four groups demonstrated their process on the waste: 1) BNL demonstrated its Sulfur Polymer Stabilization/Solidification process; 2) Nuclear Fuel Services used their DeHg (de-merk) process, 3) Allied Technology Group used chemical stabilization, and 4) Sepradyne demonstrated their vacuum thermal desorption system. All groups were successful in their tests, reaching regulatory levels for mercury leachability. Data for each group will be presented. DOE, EPA, and the University of Cincinnati are presently working on another series of tests involving treatment of surrogate sludge and soil by commercial vendors. Protocols that better determine the waste formâs ability to withstand leaching are being used to analyze the stabilized surrogates. Results of these and the previous demonstrations will be used to determine whether the High Mercury treatment regulation can be safely changed
Organic farming provides reliable environmental benefits but increases variability in crop yields: a global meta-analysis
To promote food security and sustainability, ecologically intensive farming systems should reliably produce adequate yields of high-quality food, enhance the environment, be profitable, and promote social wellbeing. Yet, while many studies address the mean effects of ecologically intensive farming systems on sustainability metrics, few have considered variability. This represents a knowledge gap because producers depend on reliable provisioning of yields, profits, and environmental services to enhance the sustainability of their production systems over time. Further, stable crop yields are necessary to ensure reliable access to nutritious foods. Here we address this by conducting a global meta-analysis to assess the average magnitude and variability of seven sustainability metrics in organic compared to conventional systems. Specifically, we explored the effects of these systems on (i) biotic abundance, (ii) biotic richness, (iii) soil organic carbon, (iv) soil carbon stocks, (v) crop yield, (vi) total production costs, and (vii) profitability. Organic farms promoted biotic abundance, biotic richness, soil carbon, and profitability, but conventional farms produced higher yields. Compared to conventional farms, organic farms had lower variability in abundance and richness but greater yield variability. Organic farms thus provided a âwin-winâ (high means and low variability) for environmental sustainability, while conventional farms provided a âwin-winâ for production by promoting high crop yields with low variability. Despite lower yields, and greater yield variability, organic systems had similar costs to conventional systems and were more profitable due to organic premiums. Our results suggest certification guidelines for organic farms successfully promote reliable environmental benefits, but greater reliance on ecological processes may reduce predictability of crop production
The DisHuman child
In this paper we consider the relationship between the human and disability; with specific focus on the lives of disabled children and young people. We begin with an analysis of the close relationship between âthe disabledâ and âthe freakâ. We demonstrate that the historical markings of disability as object of curiosity and register of fear serve to render disabled children as non-human and monstrous. We then consider how the human has been constituted, particularly in the periods of modernity and the rise of capitalism, reliant upon the naming of disability as antithetical to all that counts as human. In order to find a place for disabled children in a social and cultural context that has historically denied their humanity and cast them as monstrous others, we develop the theoretical notion of the DisHuman: a bifurcated complex that allows us recognise their humanity whilst also celebrating the ways in which disabled children reframe what it means to be human. We suggest that the lives of disabled children and young people demand us to think in ways that affirm the inherent humanness in their lives but also allow us to consider their disruptive potential: this is our DisHuman child. We draw on our research projects to explore three sites where the DisHuman child emerges in moments where sameness and difference, monstrosity/disability and humanity are invoked simultaneously. We explore three locations â (i) DisDevelopment; (ii) DisFamily and (iii) DisSexuality â illuminating the ways in which the DisHuman child seeks nuanced, politicized and complicating forms of humanity
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