Evolving information complexity of coarsening materials microstructures.

The temporal evolution of microstructural features in metals and ceramics has been the subject of intense investigation over many years because deviations from normal grain growth behavior are ubiquitous and strongly dictate observed mechanical and magnetic properties. To distinguish among different grain growth scenarios, we examine the time evolution of the information content of both synthetic and experimental coarsening microstructures as quantified by both a computable information density (CID) and a spectral entropy along with selected metrics and measures of shared information and interaction strength. In these approaches, microstructural evolution is described in terms of two time series representations, namely: (1) strings and their compressed counterparts that reflect the information contained in the configuration of a system over time, and (2) the spectra of graph Laplacians that embody the information contained in a coarsening grain network. These approaches permit one to characterize dynamically evolving microstructures and to identify correlation times associated with different coarsening scenarios. Moreover, as the information content of a system is a proxy for the entropy, a thermodynamic description of grain growth is also described.

Size And Locomotor Ecology Have Differing Effects on the External and Internal Morphologies of Squirrel (Rodentia: Sciuridae) Limb Bones.

Mammals exhibit a diverse range of limb morphologies that are associated with different locomotor ecologies and structural mechanics. Much remains to be investigated, however, about the combined effects of locomotor modes and scaling on the external shape and structural properties of limb bones. Here, we used squirrels (Sciuridae) as a model clade to examine the effects of locomotor mode and scaling on the external shape and structure of the two major limb bones, the humerus and femur. We quantified humeral and femoral morphologies using 3D geometric morphometrics and bone structure analyses on a sample of 76 squirrel species across their four major ecotypes. We then used phylogenetic generalized linear models to test how locomotor ecology, size, and their interaction influenced morphological traits. We found that size and locomotor mode exhibit different relationships with the external shape and structure of the limb bones, and that these relationships differ between the humerus and femur. External shapes of the humerus and, to a lesser extent, the femur are best explained by locomotor ecology rather than by size, whereas structures of both bones are best explained by interactions between locomotor ecology and scaling. Interestingly, the statistical relationships between limb morphologies and ecotype were lost when accounting for phylogenetic relationships among species under Brownian motion. That assuming Brownian motion confounded these relationships is not surprising considering squirrel ecotypes are phylogenetically clustered; our results suggest that humeral and femoral variation partitioned early between clades and their ecomorphologies were maintained to the present. Overall, our results show how mechanical constraints, locomotor ecology, and evolutionary history may enact different pressures on the shape and structure of limb bones in mammals.

EspañolLos mamíferos exhiben una amplia gama de morfologías de las extremidades, las cuales están asociadas con diferentes ecologías de locomoción y mecánicas estructurales. Sin embargo, aún queda mucho por investigar sobre cómo los tipos de locomoción y el tamaño corporal han afectado conjuntamente la forma externa y las propiedades estructurales de los huesos de las extremidades. En este estudio, usamos al clado de las ardillas (Sciuridae) como un modelo para examinar los efectos del tipo de locomoción y el tamaño en la forma externa y la estructura de los dos huesos principales de las extremidades, el húmero y el fémur. Utilizando morfometría geométrica en 3D y análisis de estructura ósea, cuantificamos la morfología humeral y femoral en una muestra de 76 especies de ardillas que abarcan sus cuatro ecotipos principales. Posteriormente, usamos modelos filogenéticos generalizados lineales para investigar cómo la ecología locomotora, el tamaño, y la interacción entre estos factores influencian los rasgos morfológicos. Encontramos que el tamaño y el tipo de locomoción exhiben diferentes relaciones con la forma externa y la estructura de los huesos de las extremidades, y que estas relaciones difieren entre el húmero y el fémur. La variación de la forma externa del húmero y, en menor medida, del fémur está más relacionada con la ecología locomotora que con el tamaño. Por otro lado, las diferencias en la estructura de ambos huesos se explican mejor por una combinación de efectos de la ecología locomotora y el tamaño. Curiosamente, las relaciones estadísticas entre la morfología de las extremidades y el ecotipo se pierden al incorporar las relaciones filogenéticas entre las especies bajo un modelo de movimiento browniano. El hecho de que asumir un modelo de movimiento browniano modifique estas relaciones no es sorprendente, considerando que los ecotipos de ardillas están agrupados filogenéticamente. Nuestros resultados además sugieren que la variación en morfología humeral y femoral se dividieron tempranamente entre clados y estas ecomorfologías se mantuvieron hasta el presente.En general, nuestros resultados demuestran cómo las restricciones mecánicas, la ecología locomotora y la historia evolutiva pueden ejercer diferentes presiones sobre la forma y la estructura de los huesos de las extremidades en los mamíferos.

Monte Carlo simulations of patch models with applications to soft matter.

Patch models have been employed to describe anisotropic interactions in disparate soft matter systems. In this work, we present a unified study of a patch model to explore particle self-assembly in both monodisperse and polydisperse systems, with applications to both proteins and colloids. In the first case, we obtained a temperature-density phase diagram for a model of the protein polyglutamine from Monte Carlo simulations. These simulations evinced clusters in the gas phase and, via a comparison with the corresponding coarse-grained PLUM model for this system, we verified that dense clustering in the gas phase falls into the supersaturation region of the saturation curve. In the second case, we have investigated the effect of size polydispersity on the phase behavior of binary colloidal mixtures. It was found that the width of gas-liquid phase coexistence increases with increasing polydispersity and that, in addition to the aforementioned particle clustering, small particles decorate patches on large particles, thereby creating large-particle bridges. Our aim is to compare and contrast self-assembly in these two prototypical systems.

Materials informatics for the screening of multi-principal elements and high-entropy alloys.

The field of multi-principal element or (single-phase) high-entropy (HE) alloys has recently seen exponential growth as these systems represent a paradigm shift in alloy development, in some cases exhibiting unexpected structures and superior mechanical properties. However, the identification of promising HE alloys presents a daunting challenge given the associated vastness of the chemistry/composition space. We describe here a supervised learning strategy for the efficient screening of HE alloys that combines two complementary tools, namely: (1) a multiple regression analysis and its generalization, a canonical-correlation analysis (CCA) and (2) a genetic algorithm (GA) with a CCA-inspired fitness function. These tools permit the identification of promising multi-principal element alloys. We implement this procedure using a database for which mechanical property information exists and highlight new alloys having high hardnesses. Our methodology is validated by comparing predicted hardnesses with alloys fabricated by arc-melting, identifying alloys having very high measured hardnesses.

Impact of windscreen scatter on laser eye dazzle.

This study investigates the extent to which a windscreen affects the severity of laser eye dazzle (disability glare produced by a laser) experienced by a human observer. Windscreen scatter measurements were taken for a range of windscreens in a variety of conditions, showing that windscreen scatter is similar in magnitude to scatter from the human eye. Human subject experiments verified that obscuration angles caused by laser eye dazzle could be increased by the presence of a windscreen when comparing a dirty automobile windscreen to an eye-only condition with a 532-nm laser exposure. However, a light aircraft windscreen with lower scatter did not exhibit increased obscuration angles at 532 nm, and neither windscreen exhibited an increase at 635 nm. A theoretical analysis of laser eye dazzle, using measured windscreen scatter functions, has provided insight into the delicate interplay between scatter, transmission and the angular extent of dazzle. A model based on this analysis has been shown to be a useful tool to predict the impact of windscreens on laser eye dazzle, with the goal of informing future updates to the authors' laser eye dazzle safety framework.

Early stage aggregation of a coarse-grained model of polyglutamine.

In this paper, we study the early stages of aggregation of a model of polyglutamine (polyQ) for different repeat lengths (number of glutamine amino acid groups in the chain). In particular, we use the Large-scale Atomic/Molecular Massively Parallel Simulator to study a generic coarse-grained model proposed by Bereau and Deserno. We focus on the primary nucleation mechanism involved and find that our results for the initial self-assembly process are consistent with the two-dimensional classical nucleation theory of Kashchiev and Auer. More specifically, we find that with decreasing supersaturation, the oligomer fibril (protofibril) transforms from a one-dimensional ß sheet to two-, three-, and higher layer ß sheets as the critical nucleus size increases. We also show that the results are consistent with several predictions of their theory, including the dependence of the critical nucleus size on the supersaturation. Our results for the time dependence of the mass aggregation are in reasonable agreement with an approximate analytical solution of the filament theory by Knowles and collaborators that corresponds to an additional secondary nucleation arising from filament fragmentation. Finally, we study the dependence of the critical nucleus size on the repeat length of polyQ. We find that for the larger length polyglutamine chain that we study, the critical nucleus is a monomer, in agreement with experiment and in contrast to the case for the smaller chain, for which the smallest critical nucleus size is four.

Wavelength and ambient luminance dependence of laser eye dazzle.

A series of experiments has been conducted to quantify the effects of laser wavelength and ambient luminance on the severity of laser eye dazzle experienced by human subjects. Eight laser wavelengths in the visible spectrum were used (458-647 nm) across a wide range of ambient luminance conditions (0.1-10,000 cd·m-2). Subjects were exposed to laser irradiance levels up to 600 µW·cm-2 and were asked to recognize the orientation of optotypes at varying eccentricities up to 31.6 deg of visual angle from the laser axis. More than 40,000 data points were collected from 14 subjects (ages 23-64), and these were consolidated into a series of obscuration angles for comparison to a theoretical model of laser eye dazzle. Scaling functions were derived to allow the model to predict the effects of laser dazzle on vision more accurately by including the effects of ambient luminance and laser wavelength. The updated model provides an improved match to observed laser eye dazzle effects across the full range of conditions assessed. The resulting model will find use in a variety of laser safety applications, including the estimation of maximum dazzle exposure and nominal ocular dazzle distance values.

Kinetics of first-order phase transitions with correlated nuclei.

We demonstrate that the time evolution of a first-order phase transition may be described quite generally in terms of the statistics of point processes, thereby providing an intuitive framework for visualizing transition kinetics. A number of attractive and repulsive nucleation scenarios is examined followed by isotropic domain growth at a constant rate This description holds for both uncorrelated and correlated nuclei, and may be employed to calculate the nonequilibrium, n-point spatiotemporal correlations that characterize the transition. Furthermore, it is shown that the interpretation of the one-point function in terms of a stretched-exponential, Kolmogorov-Johnson-Mehl-Avrami result is problematic in the case of correlated nuclei, but that the calculation of higher-order correlation functions permits one to distinguish among various nucleation scenarios.

Phase diagram of a model of the protein amelogenin.

There has been considerable recent interest in the self-assembly and phase behavior of models of colloidal and protein particles with anisotropic interactions. One example of particular interest is amelogenin, an important protein involved in the formation of dental enamel. Amelogenin is primarily hydrophobic with a 25-residue charged C-terminus tail. This protein undergoes a hierarchical assembly process that is crucial to mineral deposition, and experimental work has demonstrated that the deletion of the C-terminus tail prevents this self-assembly. A simplified model of amelogenin has been proposed in which the protein is treated as a hydrophobic sphere, interacting via the Asakura-Oosawa (AO) potential, with a tethered point charge on its surface. In this paper, we examine the effect of the Coulomb interaction between the point charges in altering the phase diagram of the AO model. For the parameter case specific to amelogenin, we find that the previous in vitro experimental and model conditions correspond to the system being near the low-density edge of the metastable region of the phase diagram. Our study illustrates more generally the importance of understanding the phase diagram for proteins, in that the kinetic pathway for self-assembly and the resulting aggregate morphology depends on the location of the initial state in the phase diagram.

Phase behavior of patchy spheroidal fluids.

We employ Gibbs-ensemble Monte Carlo computer simulation to assess the impact of shape anisotropy and particle interaction anisotropy on the phase behavior of a colloidal (or, by extension, protein) fluid comprising patchy ellipsoidal particles, with an emphasis on critical behavior. More specifically, we obtain the fluid-fluid equilibrium phase diagram of hard prolate ellipsoids having Kern-Frenkel surface patches under a variety of conditions and study the critical behavior of these fluids as a function of particle shape parameters. It is found that the dependence of the critical temperature on aspect ratio for particles having the same volume can be described approximately in terms of patch solid angles. In addition, ordering in the fluid that is associated with particle elongation is also found to be an important factor in dictating phase behavior.

Measuring the contribution of atmospheric scatter to laser eye dazzle.

An experiment has been conducted to determine the contribution of atmospheric scatter to the severity of the dazzle experienced by a human under illumination from a visible laser. A 15 W 532 nm laser was propagated over a 380 m outdoor range in San Antonio, Texas, over nine data collection sessions spanning June and July 2014. A narrow acceptance angle detector was used to measure scattered laser radiation within the laser beam at different angles from its axis. Atmospheric conditions were logged via a local weather station, and air quality data were taken from a nearby continuous air monitoring station. The measured laser irradiance data showed very little variation across the sessions and a single fitting equation was derived for the atmospheric scatter function. With very conservative estimates of the scatter from the human eye, atmospheric scatter was found to contribute no more than 5% to the overall veiling luminance across the scene for a human observer experiencing laser eye dazzle. It was concluded that atmospheric scatter does not make a significant contribution to laser eye dazzle for short-range laser engagements in atmospheres of good to moderate air quality, which account for 99.5% of conditions in San Antonio, Texas.

The impact of anisotropy and interaction range on the self-assembly of Janus ellipsoids.

We assess the roles of anisotropy and interaction range on the self-assembly of Janus colloidal particles. In particular, Monte Carlo simulation is employed to investigate the propensity for the formation of aggregates in a spheroidal model of a colloid having a relatively short-ranged interaction that is consistent with experimentally realizable systems. By monitoring the equilibrium distribution of aggregates as a function of temperature and density, we identify a "micelle" transition temperature and discuss its dependence on particle shape. We find that, unlike systems with longer ranged interactions, this system does not form micelles below a transition temperature at low density. Rather, larger clusters comprising 20-40 particles characterize the transition. We then examine the dependence of the second virial coefficient on particle shape and well width to determine how these important system parameters affect aggregation. Finally, we discuss possible strategies suggested by this work to promote self-assembly for the encapsulation of particles.

Hybrid atomistic simulation of fluid uptake in a deformable solid.

Fluid imbibition via diffusion in a deformable solid results in solid stresses that may, in turn, alter subsequent fluid uptake. To examine this interplay between diffusional and elastic fields, we employed a hybrid Monte Carlo-molecular dynamics scheme to model the coupling of a fluid reservoir to a deformable solid, and then simulated the resulting fluid permeation into the solid. By monitoring the instantaneous structure factor and solid dimensions, we were able to determine the compositional strain associated with imbibition, and the diffusion coefficient in the Fickian regime was obtained from the time dependence of the fluid uptake. Finally, for large, mobile fluid atoms, a non-Fickian regime was highlighted and possible mechanisms for this behavior were identified.

A numerical coarse-grained description of a binary alloy.

We employ Monte Carlo simulation in the semi-grand canonical ensemble to obtain the coarse-grained free energy corresponding to an embedded-atom method description of a binary alloy. In particular, the Ginzburg-Landau free energy for a Cu-Ni alloy was determined from a tabulated histogram of the joint probability density of composition, energy, and volume. Using histogram reweighting techniques, the free energy is extrapolated to a range of points in parameter space from a small number of simulations. The results are interpreted by comparing the free energy with that corresponding to a regular solution model of an alloy. In addition, we obtain expressions for thermodynamic quantities in terms of the joint cumulants of the probability density at a given temperature and chemical potential difference. These expressions may then be likewise extrapolated to obtain the dependence of the composition on the temperature and the chemical potential difference over a wide range of parameter space.

Elastic properties of a confined fluid.

Monte Carlo computer simulation is employed to determine the local, wave-number-dependent, high-frequency elastic properties of a Lennard-Jones fluid that is confined between two walls. In particular, the elastic constants are calculated from coarse-grained stress correlation functions and then related to the local fluid structure via planar radial distribution functions. We find that local fluid properties correlate with the inhomogeneous fluid density and the strength of the wall-fluid interaction. Finally, we discuss the utility of this analysis in the interpretation of experiments involving the characterization of confined fluids.

Elastic response of binary hard-sphere fluids.

We derive expressions for the high-frequency, wave-number-dependent elastic constants of a binary hard-sphere fluid and employ Monte Carlo computer simulation to evaluate these constants in order to highlight the impact of composition and relative sphere diameter on the elastic response of this system. It is found that the elastic constant c(11)(k) exhibits oscillatory behavior as a function of k whereas the high-frequency shear modulus, for example, does not. This behavior is shown to be dictated by the angular dependence (in k[over ⃗] space) of derivatives of the interatomic force at contact. The results are related to recent measurements of the compressibility of colloidal fluids in laser trapping experiments.

Kinetics and microstructure associated with nonisothermal nucleation and growth processes.

The impact of nonisothermal conditions on the kinetics and microstructure associated with a first-order phase transformation is investigated using simulation and a correlation function formalism. More specifically, equal-time correlation functions and microstructural descriptors of grain area are calculated using an N-fold Monte Carlo simulation and compared directly with their theoretical counterparts for a range of heating rates. Finally, connections between these results and those obtained experimentally via calorimetry and microscopy are discussed.

Role of segregating impurities in grain-boundary diffusion.

The impact of segregating impurities on grain-boundary diffusion is investigated using a kinetic Monte Carlo simulation of boundary transport on a lattice that is appropriate for both slab and pipe geometries. More specifically, the grain-boundary diffusivity is determined as a function of impurity concentration using an analog of the experimental sectioning method. The retardation of diffusion that results from segregation is interpreted in terms of site-blocking models that reflect the local free volume available to a diffusing tracer. Finally, the results of this investigation are interpreted in the context of experimental determinations of grain-boundary diffusivity in doped ceramic oxides.

Percutaneous endoscopic gastrostomy: psychological effects.

A mainly qualitative study, with some quantitative measurement tools, was undertaken to explore the psychological effects of percutaneous endoscopic gastrostomy (PEG) feeding on both patients and carers. A significant level of depression and stress was found in patients whose lifestyle had changed greatly; this was due partly to having to use a PEG and part to the underlying disease. Patients were, at the same time, grateful for the benefits to their nutritional state of having a PEG. A high level of stress was experienced by the relatives of patients whose functional and/or mental abilities had significantly changed for the worse and who had had a PEG tube placed. A need for more initial factual information and for ongoing practical and psychological support for both patients and carers was identified.