Potential Energy Landscape of the Apparent First-Order Phase Transition between Low-Density and High-Density Amorphous Ice

The potential energy landscape (PEL) formalism is a valuable approach within statistical mechanics for describing supercooled liquids and glasses. Here we use the PEL formalism and computer simulations to study the pressure-induced transformations between low-density amorphous ice (LDA) and high-density amorphous ice (HDA) at different temperatures. We employ the ST2 water model for which the LDA-HDA transformations are remarkably sharp, similar to what is observed in experiments, and reminiscent of a first-order phase transition. Our results are consistent with the view that LDA and HDA configurations are associated with two distinct regions (megabasins) of the PEL that are separated by a potential energy barrier. At higher temperature, we find that low-density liquid (LDL) configurations are located in the same megabasin as LDA, and that high-density liquid (HDL) configurations are located in the same megabasin as HDA. We show that the pressure-induced LDL-HDL and LDA-HDA transformations occur along paths that interconnect these two megabasins, but that the path followed by the liquid is different than the path followed by the amorphous solid. At higher pressure, we also study the liquid-to-ice-VII first-order phase transition, and find that the behavior of the PEL properties across this transition are qualitatively similar to the changes found during the LDA-HDA transformation. This similarity supports the interpretation that the LDA-HDA transformation is a first-order-like phase transition between out-of-equilibrium states.Comment: 29 pages, 8 figure

Glass Polymorphism in TIP4P/2005 Water: A Description Based on the Potential Energy Landscape Formalism

The potential energy landscape (PEL) formalism is a statistical mechanical approach to describe supercooled liquids and glasses. Here we use the PEL formalism to study the pressure-induced transformations between low-density amorphous ice (LDA) and high-density amorphous ice (HDA) using computer simulations of the TIP4P/2005 molecular model of water. We find that the properties of the PEL sampled by the system during the LDA-HDA transformation exhibit anomalous behavior. In particular, at conditions where the change in density during the LDA-HDA transformation is approximately discontinuous, reminiscent of a first-order phase transition, we find that (i) the inherent structure (IS) energy, $e_\text{IS}(V)$, is a concave function of the volume, and (ii) the IS pressure, $P_\text{IS}(V)$, exhibits a van der Waals-like loop. In addition, the curvature of the PEL at the IS is anomalous, a non-monotonic function of $V$. In agreement with previous studies, our work suggests that conditions (i) and (ii) are necessary (but not sufficient) signatures of the PEL for the LDA-HDA transformation to be reminiscent of a first-order phase transition. We also find that one can identify two different regions of the PEL, one associated to LDA and another to HDA. Our computer simulations are performed using a wide range of compression/decompression and cooling rates. In particular, our slowest cooling rate (0.01 K/ns) is within the experimental rates employed in hyperquenching experiments to produce LDA. Interestingly, the LDA-HDA transformation pressure that we obtain at $T=80$ K and at different rates extrapolates remarkably well to the corresponding experimental pressure.Comment: Manuscript and Supplementary Materia

A Potential Energy Landscape Study of the Amorphous-Amorphous Transformation in H2_2O

We study the potential energy landscape explored during a compression-decompression cycle for the SPC/E (extended simple point charge) model of water. During the cycle, the system changes from low density amorphous ice (LDA) to high density amorphous ice (HDA). After the cycle, the system does not return to the same region of the landscape, supporting the interesting possibility that more than one significantly different configuration corresponds to LDA. We find that the regions of the landscape explored during this transition have properties remarkably different from those explored in thermal equilibrium in the liquid phase

Recepción de estructuras de hormigón: comparación de los criterios adoptados por los distintos códigos y reglamentos

Se hace un análisis comparativo de los criterios de evaluación que distintos reglamentos de seguridad estructural aplican para la recepción del hormigón. Dichos criterios fueron aplicados a ensayos de control de calidad disponibles de diversas obras en las cuales participaron los autores. Comprende hormigones de Clase H-170 a H-500 con desviaciones normales variables entre 25 kgf/cm2 y 40 kgf/cm2 Se informan los resultados obtenidos, incluyendo los volúmenes de hormigón rechazados según los diferentes criterios reglamentarios

Fluctuations of water near extended hydrophobic and hydrophilic surfaces

We use molecular dynamics simulations of the SPC-E model of liquid water to derive probability distributions for water density fluctuations in probe volumes of different shapes and sizes, both in the bulk as well as near hydrophobic and hydrophilic surfaces. To obtain our results, we introduce a biased sampling of coarse-grained densities, which in turn biases the actual solvent density. The technique is easily combined with molecular dynamics integration algorithms. Our principal result is that the probability for density fluctuations of water near a hydrophobic surface, with or without surface-water attractions, is akin to density fluctuations at the water-vapor interface. Specifically, the probability of density depletion near the surface is significantly larger than that in bulk. In contrast, we find that the statistics of water density fluctuations near a model hydrophilic surface are similar to that in the bulk

Brazilian Buffalo Genetic Variability by Cross-Specific Microsatellite Set

Buffaloes (Bubalus bubalis) are widely distributed and were introduced to Brazil in 1895. Most of the molecular genetic characterization of buffaloes has been done with cross-specific (cattle) markers, but few of them include Brazilian populations. Nineteen commonly used cattle microsatellites were tested to develop a multiplexed set of microsatellites and characterize Brazilian buffalo. Three PCR mixes were finally developed with the 11 markers that succeed in amplify and were polymorphic (58%). The average number of alleles was 5.42, with an average observed and expected heterocigozity of 0.441 and 0.695, respectively. As it was expected, Brazilian buffalo variability was lower than the previously reported from the domestication centres (China and India), but higher than the seriously selected European populations. The exclusion power calculated for the eleven markers in Brazilian buffalo was 0.9999999996, this allows its use in DNA based traceability.Instituto de Genética Veterinari

Polimorfismos del gen BoLA-DRB3.2* en ganado criollo colombiano

Objetivo Caracterizar el polimorfismo del gen BoLA-DRB3.2* en las razas bovinas criollas y colombianas. Materiales y métodos En 360 muestras de ADN de ocho razas bovinas criollas (Blanco Orejinegro, Casanareño, Costeño con Cuernos, Chino Santandereano, Caqueteño, Hartón del Valle, Romosinuano y San Martinero), dos razas sintéticas Colombianas (Lucerna y Velásquez) y dos razas foráneas (Brahman y Holstein) se evaluó el polimorfismo del gen BoLA-DRB3.2 mediante técnicas moleculares (PCR-RFLP); se calculó el número promedio de alelos (NPA), las frecuencias, la heterocigocidad esperada (He) y observada (Ho), el equilibrio de Hardy-Weinberg, la estructura genética y los valores de FST y FIS. Resultados El NPA fue 14.6 ± 3.8 siendo Caqueteño la raza con mayor NPA (25) y el menor el Chino Santandereano (10). Se encontraron 41 alelos BoLA-DRB3.2* los más frecuentes fueron *28, *37, *24, *23, *20, *27, *8, *16, *39 (0.17, 0.11, 0.10, 0.09, 0.09, 0.07, 0.07 y 0.06 respectivamente). Se encontró alta diversidad genética (He = 0.878) con mayor valor en Caqueteño (0.96) y menor en San Martinero (0.81). Todas las razas se encontraron en equilibrio de Hardy-Weinberg, se encontraron valores altamente significativos de diferenciación genética (FST= 0.044) y de coeficiente de endogamia (FIS = 0.249). Conclusiones El ganado criollo colombiano posee alto polimorfismo del gen BoLA-DRB3.2* representado en los altos valores de NPA y diversidad genética.Objective To characterize BoLA-DRB3.2*gen polymorphism in Colombian Creole breeds. Materials and methods. Using 360 DNA samples from eight Creole bovine breeds (Blanco Orejinegro, Casanareño, Costeño con Cuernos, Chino Santandereano, Caqueteño, Hartón del Valle, Romosinuano and San Martinero), two synthetic Colombian breeds (Lucerna and Velásquez) and two introduced breeds (Brahmán and Holstein), polymorphism of BoLA-DRB3.2* was evaluated using molecular techniques (PCR-RFLP). Allele average number (AAN), expected (He) and observed (Ho) allele frequencies, heterozygosity, Hardy-Weinberg equilibrium (HW), genetic structure and FST and FIS values were estimated. Results. AAN was 14.6 ± 3.8, Caqueteño breed displayed the highest AAN value (25) and Chino Santandereano the lowest (10). 41 alleles of BoLA-DRB3.2* were detected. The most frequent were *28, *37, *24, *23, *20, *27, *8, *16 and *39 (0.17, 0.11, 0.10, 0.09, 0.09, 0.07, 0.07 and 0.06 respectively). High genetic diversity was found (He=0.878) with the highest value for Caqueteño (0.96) and lowest for San Martinero (0.81). All breeds were in HW, and highly significant values of genetic differentiation (FST=0.044) and inbreeding coefficient (FIS=0.249) were found. Conclusions The Colombian Creole breeds have a high BoLA-DRB3.2*gen polymorphism represented by the high AAN and genetic diversity valuesInstituto de Genética Veterinari