The micromechanical behaviour of a biogenic carbonate sand

An experimental investigation of the micromechanical behaviour of a biogenic carbonate sand from the Philippines was conducted. Mechanical parameters of sands at the micro-scale are required in order to simulate the particle interactions in numerical analyses carried out using the Discrete Element Method (DEM). The tests were performed on particles obtained from the sedimentation of coral fragments and they were carried out by means of a custom-made inter-particle loading apparatus that enabled the investigation of both the normal and tangential loading behaviour at the particle contacts. The normal loading tests showed a reversible behaviour after the first loading, while the tangential loading behaviour of this sand appears to be dependent on the vertical confinement and mainly reversible for small displacement cycles. Also, the inter-particle friction coefficients at larger displacements are substantially higher than those calculated for other sands using the same experimental procedures

Spontaneous polarization and piezoelectricity in boron nitride nanotubes

Ab initio calculations of the spontaneous polarization and piezoelectric properties of boron nitride nanotubes show that they are excellent piezoelectric systems with response values larger than those of piezoelectric polymers. The intrinsic chiral symmetry of the nanotubes induces an exact cancellation of the total spontaneous polarization in ideal, isolated nanotubes of arbitrary indices. Breaking of this symmetry by inter-tube interaction or elastic deformations induces spontaneous polarization comparable to those of wurtzite semiconductors.Comment: 5 pages in PRB double column format, 3 figure

Nonlocal gravity and the diffusion equation

We propose a nonlocal scalar-tensor model of gravity with pseudodifferential operators inspired by the effective action of p-adic string and string field theory on flat spacetime. An infinite number of derivatives act both on the metric and scalar field sector. The system is localized via the diffusion equation approach and its cosmology is studied. We find several exact dynamical solutions, also in the presence of a barotropic fluid, which are stationary in the diffusion flow. In particular, and contrary to standard general relativity, there exist solutions with exponential and power-law scale factor also in an open universe, as well as solutions with sudden future singularities or a bounce. Also, from the point of view of quantum field theory, spontaneous symmetry breaking can be naturally realized in the class of actions we consider.Comment: 18 pages, 5 figures. v2: typos corrected, references added. Major changes are an expansion of the discussion of homogeneous perturbations and the inclusion of cosmological fluids in the dynamic

The inter-scale behaviour of two natural scaly clays

This paper describes the results of an experimental investigation of the inter-scale behaviour of two natural scaly clays. These have been tested by means of a custom-made inter-particle loading apparatus, which has enabled their mechanical response to be studied in both compression and shearing. The main features of the micromechanical behaviour of these clays have been compared, focusing on the influence of their composition, and the results are compared with those obtained testing the same materials using other devices (triaxial and ring shear apparatus). The results have shown that, contrary to expectations, the surfaces of the scales are not residual shear surfaces and the inter-scale angle of shearing resistance is actually closer to critical state or post-peak angles measured in conventional tests

New accessibility measures based on unconventional big data sources

In health econometric studies we are often interested in quantifying aspects related to the accessibility to medical infrastructures. The increasing availability of data automatically collected through unconventional sources (such as webscraping, crowdsourcing or internet of things) recently opened previously unconceivable opportunities to researchers interested in measuring accessibility and to use it as a tool for real-time monitoring, surveillance and health policies definition. This paper contributes to this strand of literature proposing new accessibility measures that can be continuously feeded by automatic data collection. We present new measures of accessibility and we illustrate their use to study the territorial impact of supply-side shocks of health facilities. We also illustrate the potential of our proposal with a case study based on a huge set of data (related to the Emergency Departments in Milan, Italy) that have been webscraped for the purpose of this paper every 5 minutes since November 2021 to March 2022, amounting to approximately 5 million observations

Complexity Analysis on Functional-Near Infrared Spectroscopy Time Series: A Preliminary Study on Mental Arithmetic

It is well known that physiological systems show complex and nonlinear behaviours. In spite of that, functional near-infrared spectroscopy (fNIRS) is usually analyzed in the time and frequency domains with the assumption that metabolic activity is generated from a linear system. To leverage the full information provided by fNIRS signals, in this study we investigate topological entropy in fNIRS series collected from 10 healthy subjects during mental mental arithmetic task. While sample entropy and fuzzy entropy were used to estimate time series irregularity, distribution entropy was used to estimate time series complexity. Our findings show that entropy estimates may provide complementary characterization of fNIRS dynamics with respect to reference time domain measurements. This finding paves the way to further investigate functional activation in fNIRS in different case studies using nonlinear and complexity system theory

Genomic and cytogenetic localization of the carotenoid genes in the aphid genome

Data published in the scientific literature suggests a possible link between chromosomal rearrangements involving autosomes 1 and 3 and the presence of red morphs in the peach-potato aphid Myzus persicae (Sulzer). In order to begin a study of this relationship, we analysed the genomic and chromosomal location of genes involved in carotenoid biosynthesis in M. persicae and the pea aphid, Acyrthosiphon pisum (Harris), since carotenoids are the basis of the colour in many aphid species. Genomic analysis identified a DNA sequence containing carotenoid genes in synteny between the 2 species. According to the results obtained using in situ PCR, carotenoid genes were located in a subterminal portion of autosome 1 in both species. The same localization has also been observed in the onion aphid Neotoxoptera formosana Takahashi that, as M. persicae and A. pisum, belongs to the tribe Macrosiphini, thereby suggesting a synteny of this chromosomal region in aphids. In situ PCR experiments performed on 2 M. persicae asexual lineages bearing heterozygous translocations involving autosomes 1 and 3 revealed that carotenoid genes were located within chromosomal portions involved in recurrent rearrangements. We also verified by bioinformatics analyses the presence of fragile sites that could explain these recurrent rearrangements in M. persicae

Ultra-Precise Optical and Microwave Synthesis With an Er/Yb:Glass Frequency Comb

This thesis focuses on the development of an optical frequency comb based on an Er/Yb:glass modelocked laser, and its application to precision optical and microwave metrology of atomic clocks. Frequency combs can be used for wideband generation of phase-coherent signals that span both the microwave and optical domains. This capability vastly expands the utility and application of optical atomic clocks, which are currently 100 times more accurate and stable than the current definition of the SI second, based on microwave 133Cs fountain clocks. Within national metrology labs like the National Institute of Standards and Technology (NIST), optical frequency combs are central to the characterization of atomic clocks and the realization of all-optical time scales, which rely on frequency combs to generate optical and microwave timing signals from ensembles of high-stability references.&nbsp; In this work, I detail the steps to design, build, and characterize a robust, low-noise and cost-effective frequency comb based on an Er/Yb:glass free-space modelocked laser. I developed such a comb and demonstrated its capability in a number of applications: 1) calibration of the NIST microwave time scale via optical frequency division of the 171Yb optical lattice clock, 2) generation of the highest spectral purity 10 GHz signals using a highly robust transfer oscillator technique, and 3) using the frequency comb to phase-coherently link NIST&rsquo;s 27Al+ single-ion clock and 177Yb optical-lattice clock in the first demonstration of differential spectroscopy, a technique that has enabled the highest-stability inter-species clock comparison to date. This technique allows for higher accuracy frequency ratio measurements of optical clocks and more accurate contributions of single-ion clocks in an optical time scale, demonstrating an order-of-magnitude improvement in stability.&nbsp; Finally, I have begun developing the infrastructure needed to support the creation of an all-optical time scale at NIST. This work has involved investigating the limiting noise sources in time/frequency transfer over optical fiber links, to support the highest resolution comparison of next-generation optical clocks. I have also worked on transmission of high stability 10 GHz microwave signals via radio-over-fiber, needed to upgrade the microwave timing capabilities of research laboratories within NIST Boulder. Both techniques will be valuable in the dissemination of high-stability and high-accuracy signals derived from optical clocks and will enable the development of next-generation technology with stringent synchronization requirements.</p

Electric Field Induced Phase Transitions in Polymers: A Novel Mechanism for High Speed Energy Storage

This article discusses electric field induced phase transitions in polymers

Optimal two-stage spatial sampling design for estimating critical parameters of SARS-CoV-2 epidemic: Efficiency versus feasibility

The COVID-19 pandemic presents an unprecedented clinical and healthcare challenge for the many medical researchers who are attempting to prevent its worldwide spread. It also presents a challenge for statisticians involved in designing appropriate sampling plans to estimate the crucial parameters of the pandemic. These plans are necessary for monitoring and surveillance of the phenomenon and evaluating health policies. In this respect, we can use spatial information and aggregate data regarding the number of verifed infections (either hospitalized or in compulsory quarantine) to improve the standard two-stage sampling design broadly adopted for studying human populations. We present an optimal spatial sampling design based on spatially balanced sampling techniques. We prove its relative performance analytically in comparison to other competing sampling plans, and we also study its properties through a series of Monte Carlo experiments. Considering the optimal theoretical properties of the proposed sampling plan and its feasibility, we discuss suboptimal designs that approximate well optimality and are more readily applicable