Hyperuniformity on spherical surfaces

In this work we present a study on the characterization of ordered and disordered hyperuniform point distributions on spherical surfaces. In spite of the extensive literature on disordered hyperuniform systems in Euclidean geometries, to date few works have dealt with the problem of hyperuniformity in curved spaces. As a matter of fact, some systems that display disordered hyperuniformity, like the space distribution of photoreceptors in avian retina, actually occur on curved surfaces. Here we will focus on the local particle number variance and its dependence on the size of the sampling window (which we take to be a spherical cap) for regular and uniform point distributions, as well as for equilibrium configurations of fluid particles interacting through Lennard-Jones, dipole-dipole and charge-charge potentials. We will show how the scaling of the local number variance enables the characterization of hyperuniform point patterns also on spherical surfaces

Multilayer approximation for a confined fluid in a slit pore

A simple Lennard-Jones fluid confined in a slit nanopore with hard walls is studied on the basis of a multilayer structured model. Each layer is homogeneous and parallel to the walls of the pore. The Helmholtz energy of this system is constructed following van der Waals-like approximations, with the advantage that the model geometry permits to obtain analytical expressions for the integrals involved. Being the multilayer system in thermodynamic equilibrium, a system of non-linear equations is obtained for the densities and widths of the layers. A numerical solution of the equations gives the density profile and the longitudinal pressures. The results are compared with Monte Carlo simulations and with experimental data for Nitrogen, showing very good agreement.Comment: 6 pages, 5 figures

Efectos del engorde a corral sobre la concentración de nitratos en el agua freática en el partido de Saladillo

En Saladillo, Pcia de Buenos Aires, se realizó un estudio de los efectos que produce un emprendimiento de engorde a corral (feedlot) sobre el agua freática, bajo suelos Hapludoles característicos de la zona. Se realizaron mapas equipotenciales de nivel freático por estación, entre invierno de 2015 y otoño de 2016 y por prospección geoeléctrica, se determinó un gradiente de conductividad eléctrica que sigue la dirección de flujo de la freática, aumentando en el tiempo, aguas abajo de los corrales. Se realizaron análisis de concentraciones de nitratos, fósforo, conductividad eléctrica, sólidos totales disueltos, pH, y oxígeno disuelto, y también bacteriológicos en muestras obtenidas del agua subterránea. Si bien la concentración de nitratos no supera los límites establecidos de 45 ppm, por el CAA para agua potable, los análisis bacteriológicos la caracterizan como no potable, con presencia de Pseudomonas y E. coli por lo que podría significar un riesgo de salud, tanto para humanos como para el ganado. El fósforo presenta concentraciones elevadas respecto al valor que establece la EPA como límite superior de 0.1 ppm, con el objetivo de disminuir la eutrofización de aguas superficiales. En los pozos de la zona de descarga del agua, los valores de concentraciones de fósforo superan ampliamente el límite máximo recomendado. Este comportamiento lo tuvo también el agua superficial del bajo lindante a los corrales, donde fluye el agua por escorrentía. Los mapas realizados por el método de kriging, señalan una estacionalidad de los aportes de iones, de manera desfasada con la cantidad de animales manejados en los corrales. Por lo que se infiere que tipo de suelo, con un Bt nátrico, actuaría como amortiguador de dichos aportes, siendo la ZNS un reservorio de iones, demostrado, por la prospección geoleléctrica.In Saladillo, Province of Buenos Aires, a study was made of the effects produced by a feedlot farm (feedlot) on groundwater under Hapludoles soils characteristic of the area. Equipotential maps of water table by season were made, between winter of 2015 and autumn of 2016 and by geoelectric prospecting, a gradient of electrical conductivity was determined that follows the direction of flow of the phreatic aquifer, increasing downstream of the pens. Analysis of concentrations of Nitrates, Phosphorus, Electric Conductivity, Dissolved Total Solids, pH, and Dissolved Oxygen were made, as well as bacteriological analysis in samples obtained from groundwater. Although the concentration of Nitrates does not exceed the established limits of 45 ppm, by the CAA for drinking water, the bacteriological analysis is characterize it as non-potable, with the presence of Pseudomonas and E. coli, which could mean a health risk, both for humans as for cattle. The Phosphorus presents high concentrations with respect to the value established by the EPA as an upper limit of 0.1 ppm, with the aim of reducing the eutrophication of surface waters. In the wells of the water discharge area, the Phosphorus concentration values greatly exceed the maximum recommended limit. This behavior also had the surface water of the lower zones adjoining the pens, where the water flows by runoff. The maps made by the kriging method indicate a seasonality of the ion contributions, in a way that is out of temporal phase with the number of animals handled in the pens. Therefore, it is inferred that this type of soil, with a natric Bt, would act as a buffer for said contributions, being the Non saturated zone (ZNS) an ion reservoir, demonstrated, by geolelectric prospecting.Facultad de Ciencias Naturales y Muse

Cluster pair correlation function of simple fluids: energetic connectivity criteria

We consider the clustering of Lennard-Jones particles by using an energetic connectivity criterion proposed long ago by T.L. Hill [J. Chem. Phys. 32, 617 (1955)] for the bond between pairs of particles. The criterion establishes that two particles are bonded (directly connected) if their relative kinetic energy is less than minus their relative potential energy. Thus, in general, it depends on the direction as well as on the magnitude of the velocities and positions of the particles. An integral equation for the pair connectedness function, proposed by two of the authors [Phys Rev. E 61, R6067 (2000)], is solved for this criterion and the results are compared with those obtained from molecular dynamics simulations and from a connectedness Percus-Yevick like integral equation for a velocity-averaged version of Hill’s energetic criterion.Facultad de Ciencias Exacta

Critical behaviour of the Ising ferromagnet confined in quasi-cylindrical pores: A Monte Carlo study

The critical behaviour of the Ising ferromagnet confined in pores of radius R and length L is studied by means of Monte Carlo computer simulations. Quasi-cylindrical pores are obtained by replicating n-times a triangular lattice disc of radius R, where L = na and a is the spacing between consecutive replications. So, spins placed at the surface of the pores have less nearest-neighbours (NN) as compared to 8 NN for spins in the bulk. These “missing neighbour” effects undergone by surface spins cause a strong suppression of surface ordering, leading to an ordinary surface transition. Also, the effect propagates into the bulk for small tubes (R ≤ 12) and the effective critical temperature of the pores is shifted towards lower values than in the bulk case. By applying the standard finite-size scaling theory, subsequently supported by numerical data, we concluded that data collapse of relevant observables, e.g., magnetization (m), susceptibility, specific heat, etc., can only be observed by comparing simulation results obtained by keeping the aspect ratio C ≡ R/L constant. Also, by extrapolating “effective” R-dependent critical temperatures to the thermodynamic limit (R → ∞, C fixed), we obtained TC(∞) = 6.208(4). As suggested by finite-size scaling arguments, the magnetization is measured at the critical point scales according to |m| TcR β ν ∝ R L 1 2 , where β and ν are the standard exponents for the order parameter and the correlation length, respectively. Furthermore, it is shown that close to criticality the axial correlation length decreases exponentially with the distance. That result is the signature of the formation of (randomly distributed) alternating domains of different magnetization, which can be directly observed by means of snapshot configurations, whose typical length (ξ ) is given by the characteristic length of the exponential decay of correlations. Moreover, we show that at criticality ξ = 0.43(2)R.Instituto de Física de Líquidos y Sistemas BiológicosConsejo Nacional de Investigaciones Científicas y TécnicasComisión de Investigaciones Científicas de la provincia de Buenos Aire

Critical behaviour of the Ising ferromagnet confined in quasi-cylindrical pores: A Monte Carlo study

The critical behaviour of the Ising ferromagnet confined in pores of radius R and length L is studied by means of Monte Carlo computer simulations. Quasi-cylindrical pores are obtained by replicating n-times a triangular lattice disc of radius R, where L = na and a is the spacing between consecutive replications. So, spins placed at the surface of the pores have less nearest-neighbours (NN) as compared to 8 NN for spins in the bulk. These “missing neighbour” effects undergone by surface spins cause a strong suppression of surface ordering, leading to an ordinary surface transition. Also, the effect propagates into the bulk for small tubes (R ≤ 12) and the effective critical temperature of the pores is shifted towards lower values than in the bulk case. By applying the standard finite-size scaling theory, subsequently supported by numerical data, we concluded that data collapse of relevant observables, e.g., magnetization (m), susceptibility, specific heat, etc., can only be observed by comparing simulation results obtained by keeping the aspect ratio C ≡ R/L constant. Also, by extrapolating “effective” R-dependent critical temperatures to the thermodynamic limit (R → ∞, C fixed), we obtained TC(∞) = 6.208(4). As suggested by finite-size scaling arguments, the magnetization is measured at the critical point scales according to |m| TcR β ν ∝ R L 1 2 , where β and ν are the standard exponents for the order parameter and the correlation length, respectively. Furthermore, it is shown that close to criticality the axial correlation length decreases exponentially with the distance. That result is the signature of the formation of (randomly distributed) alternating domains of different magnetization, which can be directly observed by means of snapshot configurations, whose typical length (ξ ) is given by the characteristic length of the exponential decay of correlations. Moreover, we show that at criticality ξ = 0.43(2)R.Instituto de Física de Líquidos y Sistemas BiológicosConsejo Nacional de Investigaciones Científicas y TécnicasComisión de Investigaciones Científicas de la provincia de Buenos Aire

Geometrical and physicochemical considerations of the pit membrane in relation to air seeding: The pit membrane as a capillary valve

A theoretical treatment of some of the factors influencing air seeding at the pit membranes of xylem vessels is given. Pit membrane structure, viewed as a three-dimensional mesh of intercrossing fibrils, and vulnerability to water-stress-induced air seeding are examined in the context of the Young-Laplace equation. Simple geometrical considerations of the porous membrane show that the vapor-liquid interface curvature radius is a function of fiber-fiber distance, fiber radius, wetting angle and position of the wetting line. Air seeding (maximum pressure) occurs at the minimum curvature radius, therefore air seeding is not simply determined by the fiber-fiber distance but is a function of the geometry of the pit membrane and of physicochemical quantities like surface tension and wetting angle. As a consequence of considering a wetting angle different from zero, the minimum curvature radius becomes larger than half the fiber-fiber distance. The present model considers that, for a given pressure difference at the pit membrane, all local interface curvatures are the same. In this sense, pit membranes work as variable capillary valves that allow or prevent air seeding by adjusting local curvatures and interface positions relative to the pore-forming fibers, following the pressure differences across the membranes. The theoretical prediction for the air seeding threshold is consistent with recent experimental data for angiosperm trees.Instituto de Física de Líquidos y Sistemas Biológico

Geometrical and physicochemical considerations of the pit membrane in relation to air seeding: The pit membrane as a capillary valve

A theoretical treatment of some of the factors influencing air seeding at the pit membranes of xylem vessels is given. Pit membrane structure, viewed as a three-dimensional mesh of intercrossing fibrils, and vulnerability to water-stress-induced air seeding are examined in the context of the Young-Laplace equation. Simple geometrical considerations of the porous membrane show that the vapor-liquid interface curvature radius is a function of fiber-fiber distance, fiber radius, wetting angle and position of the wetting line. Air seeding (maximum pressure) occurs at the minimum curvature radius, therefore air seeding is not simply determined by the fiber-fiber distance but is a function of the geometry of the pit membrane and of physicochemical quantities like surface tension and wetting angle. As a consequence of considering a wetting angle different from zero, the minimum curvature radius becomes larger than half the fiber-fiber distance. The present model considers that, for a given pressure difference at the pit membrane, all local interface curvatures are the same. In this sense, pit membranes work as variable capillary valves that allow or prevent air seeding by adjusting local curvatures and interface positions relative to the pore-forming fibers, following the pressure differences across the membranes. The theoretical prediction for the air seeding threshold is consistent with recent experimental data for angiosperm trees.Instituto de Física de Líquidos y Sistemas Biológico

Percolation of clusters with a residence time in the bond definition: Integral equation theory

We consider the clustering and percolation of continuum systems whose particles interact via the Lennard-Jones pair potential. A cluster definition is used according to which two particles are considered directly connected (bonded) at time t if they remain within a distance d, the connectivity distance, during at least a time of duration tau, the residence time. An integral equation for the corresponding pair connectedness function, recently proposed by two of the authors [Phys. Rev. E 61, R6067 (2000)], is solved using the orthogonal polynomial approach developed by another of the authors [Phys. Rev. E 55, 426 (1997)]. We compare our results with those obtained by molecular dynamics simulations.Instituto de Física de Líquidos y Sistemas BiológicosGrupo de Aplicaciones Matemáticas y Estadísticas de la Facultad de Ingenierí