arXiv subjects

Marcia C. Barbosa

Publications and source records attributed to Marcia C. Barbosa.

At least 37 records · Page 2Linked to original sources

arXiv2019

Quantum density anomaly in optically trapped ultracold gases

We show that the Bose-Hubbard Model exhibits an increase in density with temperature at fixed pressure in the regular fluid regime and in the superfluid phase. The anomaly at the Bose-Einstein condensate is the first density anomaly observed in a quantum state. We propose that the mechanism underlying both the normal phase and the superfluid phase anomalies is related to zero point entropies and ground state phase transitions. A connection with the typical experimental scales and setups is also addressed. This key finding opens a new pathway for theoretical and experimental studies of water-like anomalies in the area of ultracold quantum gases.

cond-mat.quant-gas↗

arXiv2018

Diffusion behavior of water confined in deformed carbon nanotubes

We use molecular dynamics simulations to study the diffusion of water inside deformed carbon nanotubes, with different degrees of eccentricity at 300K. We found a water structural transition between tubular-like to single-file for the (7,7) nanotubes associated with a change from a high to low mobility regimes. The water which in the undeformed (9,9) nanotubes is frozen, becomes liquid for the distortion above a certain threshold. These water diffusion enhancement (suppresion) is related to a reduction (increase) in the number of hydrogen bonds. This suggests that the shape of the nanotube is a particularly important ingredient when considering the dynamical and structural properties of confined water.

cond-mat.soft↗

arXiv2018

Sodium Chloride, NaCl/ε : New Force Field

A new computational model for Sodium Chloride, the NaCl/ε, is proposed. The Force Fields employed here for the description of the NaCl is based on a set of radial particle-particle pair potentials involving Lennard-Jones (LJ) and Coulombic forces. The parameterization is obtained fitting the density of the crystal and the density and the dielectric constant of the mixture of salt with water at diluted solution. Our model shows good agreement with the experimental values for the density and surface tension for the pure system and for the density, the viscosity, the diffusion, and the dielectric constant for the mixture with water at various molal concentrations. The NaCl/ε together with the water TIP4P/ε model provide a good approximation for studying electrolyte solutions.

cond-mat.soft↗

arXiv2018

Flexible Bond and Angle, FBA/epsilon model of water

We propose a new flexible force field for water. The model in addition to the Lennard-Jones and electrostatic parameters, includes the flexibility of the OH bonds and angles. The parameters are selected to give the experimental values of the density and dielectric constant of water at at 1 bar at 240K and the dipole moment of minimum density. The FBA/epsilon reproduces the experimental values of structural, thermodynamic and the phase behavior of water in a wide range of temperatures with better accuracy than atomistic and other flexible models. We expect that this new approach would be suitable for studying water solutions.

physics.comp-ph↗

arXiv2018

Relation between boundary slip mechanisms and water-like fluid behavior

The slip of a fluid layer in contact with a solid confining surface is investigated for different temperatures and densities using molecular dynamic simulations. We show that for an anomalous water-like fluid the slip goes as follows: for low levels of shear, the defect slip appears and is related with the particle exchange between the fluid layers; at high levels of shear, the global slip occurs and is related to the homogeneous distribution of the fluid in the confining surfaces. The oscillations in the transition velocity from the defect to the global slip is shown to be associated with changes in the layering distribution in the anomalous fluid.

cond-mat.soft↗

arXiv2018

2D nanoporous membrane for cation removal from water: effects of ionic valence, membrane hydrophobicity and pore size

Using molecular dynamic simulations we show that single-layers of molybdenum disulfide (MoS$_2$) and graphene can effectively reject ions and allow high water permeability. Solutions of water and three cations with different valence (Na$^+$, Zn$^{2+}$ and Fe$^{3+}$) were investigated in the presence of the two types of membranes and the results indicate a high dependence of the ion rejection on the cation charge. The associative characteristic of ferric chloride leads to a high rate of ion rejection by both nanopores, while the monovalent sodium chloride induces lower rejection rates. Particularly, MoS$_2$ shows 100\% of Fe$^{3+}$ rejection for all pore sizes and applied pressures. On the other hand, the water permeation did not varies with the cation valence, having dependence only with the nanopore geometric and chemical characteristic. This study helps to understand the fluid transport through nanoporous membrane, essential for the development of new technologies for pollutants removal from water.

cond-mat.soft↗

arXiv2017

Waterlike anomalies in a 2D core-softened potential

We investigate the structural, thermodynamic and dynamic behavior of of a two dimensional core-corona system using Langevin Dynamics simulations. The particles are modeled using a core-softened potential that exhibits waterlike anomalies for the 3D case. For quasi-2D systems we have observed previously a new region of structural anomaly. Now, our results show that a new region of structural, density and diffusion anomalies arises for the 2D system. Our findings indicates that while the anomalous region at lower densities observed is due the competition between the two length scales in the potential, the traditional mechanism, the higher densities anomalous region is related to changes in the particles conformation and a melting region.

cond-mat.soft↗

arXiv2016

Potassium bromide, KBr/ε : New Force Field

The correct description of the ionic interaction and stable equilibrium of the simulations of biomolecular structure, dynamics, folding, catalysis, and function, an accurate model of the monovalent ions is very important. The force field needs to reproduce coincident many properties of ions, like their structure, solvation, and moreover both the interactions of these ions with each other in the crystal and in solution and the interactions of ions with other molecules. Using a similar strategy employed in the parameterization of the NaCl/epsilon , in this paper, we first propose a force field for the Potassium Bromide, the KBr/epsilon. This new model is compared with the experimental values of cristal density and structure for the salt and the density, the viscosity, the dielectric constant and the solubility in the water solution for a range of concentrations. Next, the transferability, of this new model KBr/epsilon and the NaCl/epsilon, is verified by creating the KCl/epsilon and the NaBr/epsilon models. The strategy is to employ the same parameters obtained for the NaCl/epsilon and for the KBr/epsilon force fields. The two new models derived are also compared with the experimental values for the density, the viscosity, the dielectric constant and the solubility in the water solution for a range of concentrations.

cond-mat.soft↗

arXiv2016

Flow and structure of fluids in functionalized nanopores

We investigate through non-equilibrium Molecular Dynamics simulations the structure and flow of fluids in functionalized nanopores. The nanopores are modeled as cylindrical structures with solvophilic and solvophobic sites. Two fluids are modeled. The first is a standard Lennard Jones fluid. The second one is modeled with a isotropic two-length scale potential, which exhibits in bulk water-like anomalies. Our results indicates distinct dependence of the overall mass flux for each species of fluid with the number of solvophilic sites for different nanotubes radii. Also, the density and fluid structure are dependent from the nanotube radius and the solvophilic properties of the nanotube. This indicates that the presence of a second length scale in the fluid-fluid interaction will lead to distinct behavior. Also, our results shows that chemically functionalized nanotubes with different radius will have distinct nano-fluidic features. Our results are explained in the basis of the characteristic scale fluid properties and the effects of nanoconfinement.

cond-mat.soft↗

arXiv2015

Universality, Correlations, and Rankings in the Brazilian Universities National Admission Examinations

The scores obtained by students that have performed the ENEM exam, the Brazilian High School National Examination used to admit students at the Brazilian universities, is analyzed. The average high school's scores are compared between different disciplines through the Pearson correlation coefficient. The results show a very large correlation between the performance in the different subjects. Even thought the students' scores in the ENEM due to the standardization form a Gaussian, we show that the high schools' scores form a bimodal distribution that can not be used to evaluate and compare performance over time. We also show that this high schools distribution reflects the correlation between school performance and economic level of the students. The ENEM's scores are compared with a Brazilian non standardized exam, the entrance exam at the Universidade Federal do Rio Grande do Sul. The comparison of the performance of the same individuals in both tests is compared showing that the two tests not only select different abilities but chooses a different set of individuals. Our results indicates that standardized exams might be an interesting tool to compare performance over the years but only of individuals and not of institutions.

physics.soc-ph↗

arXiv2015

Thermodynamic and dynamic anomalous behavior in the TIP4P/ε water model

The model Tip4p/ε for water is tested for the presence of thermodynamic and dy- namic anomalies. Molecular dynamic simulations for this model were performed and we show that for this system the density versus temperature at constant pressure ex- hibits a maximum. In addition we also show that the diffusion coefficient versus density at constant temperature has a maximum and a minimum. The anomalous behavior of the density and of the diffusion coefficient obey the water hierachy. The results for the Tip4p- are consistent with experiments and when compared with the Tip4p-2005 model show similar results a variety of physical properties and better performance for the dielectric constant.

cond-mat.soft↗

arXiv2015

Order-Disorder structural transition in a confined fluid

In this paper the amorphous/solid to disorder liquid structural phase transitions of an anomalous confined fluid is analyzed using their local fractal dimension. The model is a system of particles interacting through a two length scales potentials confined by two infinite plates. In the bulk, this fluid exhibit water-like anomalies and under confinement forms layers of particles. The particle distributions of them, present different arrangements related to amorphous/solid phases. Here only the contact layer is analyzed through fractal singularity spectrum. At high densities the structural transition its quantify by the order degree to determine the phases affected by the confinement. This mapping shows that the system as the temperature increased, the fractal dimension decreases, which is consistent with the behavior studying in such systems. This result suggests that under thermodynamic perturbations, an anomalous confined liquid, presents different phase transitions achieving be characterized by its fractality.

cond-mat.soft↗

arXiv2015

Self-Assembly and Waterlike Anomalies in Janus Nanoparticles

We explore the pressure versus temperature phase diagram of dimeric Janus nanoparticles using Molecular Dynamics simulations. The nanoparticle was modeled as a dumbbells particle, and have one monomer that interacts by a standard Lennard Jones potential and another monomer that is modeled using a two-length scale shoulder potential. Monomeric and dimeric systems modeled by this shoulder potential show waterlike anomalies, and we investigate if a Janus nanoparticle composed by one anomalous monomer will exhibit anomalous behavior and self-assembly structures. The influence of the non-anomalous monomer in the dimeric system properties was explored. We show that the diffusion anomaly is maintained, while the density anomaly can disappear depending on the non-anomalous monomer characteristics. As well, the self-assembled structures are affected. Our results are discussed in the basis of the distinct monomer-monomer interactions and on the two-length scale fluid characteristics.

cond-mat.soft↗

arXiv2015

Effects of nanopore and fluid structure on anomalies and phase transitions of confined core-softened fluids

We use Molecular Dynamics simulations to study how the nanopore and the fluid structures affects the dynamic, thermodynamic and structural properties of a confined anomalous fluid. The fluid is modeled using an effective pair potential derived from the ST4 atomistic model for water. This system exhibits density, structural and dynamical anomalies and the vapor-liquid and liquid-liquid critical points similar to the quantities observed in bulk water. The confinement is modeled both by smooth and structured walls. The temperatures of extremum density and diffusion for the confined fluid show a shift to lower values while the pressures move to higher amounts for both smooth and structured confinement. In the case of smooth walls, the critical points and the limit between fluid and amorphous phases show a non-monotonic change in the temperatures and pressures when the the nanopore size is increase. In the case of structured walls the pressures and temperatures of the critical points varies monotonicaly with the porous size. Our results are explained on basis of the competition between the different length scales of the fluid and the wall-fluid interaction.

cond-mat.soft↗

arXiv2014

Surface Phase Transition in Anomalous Fluid in Nanoconfinement

We explore by molecular dynamic simulations the thermodynamical behavior of an anomalous fluid confined inside rigid and flexible nanopores. The fluid is modeled by a two length scale potential. In the bulk this system exhibits the density and diffusion anomalous behavior observed in liquid water. We show that the anomalous fluid confined inside rigid and flexible nanopores forms layers. As the volume of the nanopore is decreased the rigid surface exhibits three consecutive first order phase transitions associated with the change in the number of layers. These phase transitions are not present for flexible confinement. Our results indicate that the nature of confinement is relevant for the properties of the confined liquid what suggests that confinement in carbon nanotubes should be quite different from confinement in biological channels.

cond-mat.soft↗

arXiv2014

Phase Diagram and Thermodynamic and Dynamic Anomalies in a Pure Repulsive Model

Using Monte Carlo simulations a lattice gas model with only repulsive interactions was checked for the presence of anomalies. We show that this system exhibits the density (temperature of maximum density - TMD) and diffusion anomalies as present in liquid water. These anomalous behavior exist in the region of the chemical potential vs temperature phase diagram where two structured phases are present. A fragile-to-strong dynamic transition is also observed in the vicinity of the TMD line.

cond-mat.stat-mech↗

arXiv2013

A water-like model under confinement for hydrophobic and hydrophilic particle-plate interaction potentials

Molecular dynamic simulations were employed to study a water-like model confined between hydrophobic and hydrophilic plates. The phase behavior of this system is obtained for different distances between the plates and particle-plate potentials. For both hydrophobic and hydrophilic walls there are the formation of layers. Crystallization occurs at lower temperature at the contact layer than at the middle layer. In addition, the melting temperature decreases as the plates become more hydrophobic. Similarly, the temperatures of maximum density and extremum diffusivity decrease with hydrophobicity.

cond-mat.soft↗

arXiv2013

Phase diagram and water-like anomalies in core-softened shoulder-dumbbells system

Using molecular dynamics we studied the role of the anisotropy on the phase boundary and on the anomalous behavior of 250 dimeric particles interacting by a core-softened potential. This study led us to an unexpected result: the introduction of a rather small anisotropy, quantified by the distance between the particles inside each dimer, leads to the increase of the size of the regions in the pressure-temperature phase diagram of anomalies when compared to the isotropic monomeric case. However, as the anisotropy increases beyond a threshold the anomalous regions shrinks. We found that this behavior can be understood by decoupling the translational and non-translational kinetic energy components into different translational and non-translational temperatures.

cond-mat.stat-mech↗