Dipolar Fluids Research Articles

Structure and thermodynamics of a polar fluid at a discretely polarized wall

The structure and thermodynamics of a dipolar hard-sphere fluid at a discretely polarized hard wall are investigated using Monte Carlo computer simulation. In contrast to a continuously polarized wall, the discrete case exhibits significantly enhanced adsorption of the fluid at the surface relative to that observed for an unpolarized wall. Significant orientational ordering of the first liquid layer is observed plus correlation with the wall polarization that extends over several layers. The relative potential and free energy are calculated as a function of the polarization angle using thermodynamic integration.

Statistical mechanics of dipolar fluids: dielectric constant and sample shape

We give a new proof that the constitutive relation of macroscopic electrostatics holds in a dipolar fluid with a sample shape independent dielectric constant. Our approach is based on a BGY-like hierarchy equation which allows us to calculate the canonical one-body density function up to linear order in the electric field, in the thermodynamic limit. The dielectric constant comes out as an integral over a 3-point correlation function of the infinite unperturbed (unpolarized) system, from which one can recover the well-known formula for ε in terms of the 2-point direct correlation function.

Dielectric constant and sample's shape in dipolar fluids

We give a new proof that the constitutive relation of macroscopic electrostatics holds in a dipolar fluid with a sample's shape independent dielectric constant. Our approach is based on BGY-like hierarchy equations which allow us to calculate the canonical one-body density function up to linear order in the electric field, in the thermodynamic limit. The dielectric constant comes out as an integral over a 3-point correlation function of the infinite (unpolarized) system, from which one can recover the well known formula for e in terms of the direct correlation function.

Structure of polydisperse dipolar hard-sphere fluids

We report Monte Carlo simulations of dipolar hard-sphere fluids with a nonuniform distribution of the dipole strengths or different hard-sphere diameters. Dipolar polydispersity significantly reduces the spontaneous orientational order observed in the ferroelectric phase of monodisperse dipolar hard-sphere fluids at high densities and low temperatures. Equimolar binary mixtures of dipolar hard spheres of different diameters σa and σb also exhibit ferroelectric order. However, the order parameters are very dependent on the ratio σa/σb. Chains of dipolar hard spheres, typical of strongly interacting dipolar hard spheres at low densities and low temperatures, were observed in binary fluids of dipolar and neutral hard spheres of different sizes.

Freezing transition of a strongly dipolar simple fluid

The freezing transition of a strongly dipolar Lennard-Jones fluid is investigated using Monte Carlo simulation. It is found that the system undergoes a ferroelectric liquid-to-solid transition when cooled under a constant pressure. Near the triple point the stable solid phase is a body-centered orthorhombic ferroelectric crystal; a metastable ferroelectric solid phase with distorted hexagonal lattice structure is also discovered. To locate the freezing point, Gibbs free energies of the solid and liquid phase are determined using a thermodynamic integration method.

Phase behavior of magnetic nanoparticles dispersions in bulk and confined geometries

Dispersions of magnetic nanoparticles (ferrofluids) have been for a long time taken as examples of dipolar fluids. Theoretical papers conclude now that an important parameter is the ratio of the anisotropic attractions (dipolar ones) to the isotropic attractions (Van der Waals ones). It is confirmed by the recent experimental results concerning the behavior of small magnetic particles in bulk or confined 2D geometries.

Integral equation theory for dipolar hard sphere fluids with fluctuating orientational order

We present an integral equation approach to the structural and thermodynamic properties of a fluid of partially aligned dipolar hard spheres. To relate the two-particle correlation functions to the anisotropic singlet density, we mainly employ the Lovett–Mou–Buff–Wertheim equation. We show that, as in the isotropic case, the mean-spherical approximation and the reference hypernetted chain (RHNC) closures lead to quite different results. This is particularly true at high coupling strengths, where the RHNC theory shows a transition from an isotropic to a ferroelectric fluid phase. The predicted transition temperatures are very close to those one obtains from the RHNC theory for the isotropic fluid.

Strongly dipolar fluids at low densities

We propose a simple statistical mechanical theory for a strongly dipolar fluid at low densities, based on the analogy between a system of polydisperse linear chains and the equilibrium structure of these fluids as revealed by computer simulations. At low densities, both steric and dipolar interactions between long chains are weak and thus the dipolar fluid is well described as an ideal gas of polydisperse chains. We have further investigated how the residual dipolar interaction between monomers and/or additional isotropic attractions between the spheres cause the dissociation of the chains and/or the condensation of the dipolar fluid.

Isotropic fluid phases of dipolar hard spheres

Monte Carlo simulations are used to calculate the equation of state and free energy of dipolar hard sphere fluids at low temperatures and densities. Evidence for the existence of isotropic-fluid-isotropic-fluid phase transitions is presented and discussed. Condensation in the dipolar hard sphere fluid is unusual in that it is not accompanied by large energy or entropy changes. An explanation of this behavior is put forward.

A mapping equation of state: development for the dipolar hard sphere fluid

A new generalised equation of state is proposed and applied to a dipolar hard sphere fluid. This general equation of state has been successfully employed earlier for modelling a hard sphere chain fluid. It is shown that it is possible to map complicated equations which are derived by perturbation approaches on this simple equation. In addition different applications of the c-factor approach are discussed and implemented in the proposed equation of state. Furthermore, a simplified version of a molecular based dispersion term is proposed. The result is a simple fourth-order equation of state for molecular fluids.

Splitting Fingers under Strain: Pattern Formation of a Dipolar Fluid in a Polymer Medium

We study the formation and evolution of labyrinthine patterns for dipolar fluids inside a polymer medium under mechanical strain. A new general mechanism is proposed to describe the kinetics of the pattern formation that combines orientation ordering and subsequently breaking characteristics of dipolar fluids with branching characteristics of crack-propagation. This mechanism can lead to many of the patterns found in nature, including spots, stripes, labyrinthine, and starlike formations.

The second dielectric virial coefficient of the dipolar Gay–Berne fluid

We report the second dielectric virial coefficient Bε of a nonpolarizable dipolar Gay-Berne fluid. The second dielectric virial coefficient is evaluated for several sets of shape parameters and temperature. The second dielectric virial coefficient for rodlike molecules is negative while it is large and positive for disklike molecules. It decreases to zero on increasing the temperature.Key words: dipolar fluids, dielectric constant, intermolecular forces, shape parameters.

Orientational order in model dipolar fluids.

Fluids of hard spheres each carrying two parallel point dipoles have been investigated using constant-volume Monte Carlo computer simulations. The results show that both ferroelectric and antiferroelectric fluid phases can be stabilized at high density and low temperature by dipolar interactions alone, if the separation between the dipoles on each sphere is sufficiently large. A simple lattice calculation provides some insight into the balance between dipole energy and orientational entropy which governs the polarization state.

Strongly dipolar fluids: a theoretical and computational challenge

We propose a simple statistical mechanical theory for a strongly dipolar fluid at low densities, based on the analogy between a system of polydisperse polymer chains and the equilibrium structure of these fluids as shown by computer simulation. Both steric and dipolar interactions between long chains are weak at low densities and as a result the dipolar fluid, in this regime, is well described as an ideal gas of polydisperse chains. We further investigate how the residual dipolar interaction between monomers and attractive Van der Waals interactions between particles may cause dissociation of the chains and condensation of the dipolar fluid.

Exact solutions for the flow of a dipolar fluid on a suddenly accelerated flat plate

The Laplace transformation is used to determine the exact general solution for the unsteady and irrotational flow of an incompressible dipolar fluid set to motion by the acceleration of a flat plate from rest. The general solution is found for arbitrary values of the dipolar constantsd andl. In particular, attention is focused on the case of sudden plate motion for which the velocity field, the displacement thickness, the boundary layer thickness, and the dipolar stress component∑yyx have been determined. Special cases of the dipolar constants are also considered. It is shown that for a special boundary value of∑yyx, the velocity distribution becomes independent of time. In addition, some significant new results concerning steady flows are presented. Finally, results obtained are compared to the corresponding case for a viscous Newtonian fluid.

Thermodynamics and structural properties of the dipolar Yukawa fluid

We report computer simulations and a theoretical study of the thermodynamics and structure of a dipolar Yukawa system. A comparison between the analytical mean spherical approximation (MSA) solution, perturbation theory and Monte Carlo simulation data of pressure, internal energy and dielectric constant is given. In the perturbation theory, the MSA solution of hard core Yukawa fluid is used as a reference system. It was found that the MSA solution is reasonable only at lower dipole moments, while the perturbation theory gives good results at low and high values of dipole moment. Liquid–vapor coexistence data of dipolar Yukawa fluid are also obtained by Monte Carlo simulation and by both MSA and perturbation theory. It was found that at high dipole moments the liquid–vapor equilibrium disappears while chain-like structures appear in the low density fluid phase. The appearance of chain-like structures of dipolar Yukawa fluid is discussed in comparison with the Stockmayer fluid.

The mean spherical approximation for a dipolar Yukawa fluid

The dipolar hard sphere fluid (DHSF) is a useful model of a polar fluid. However, the DHSF lacks a vapor–liquid transition due to the formation of chain-like structures. Such chains are not characteristic of real polar fluids. A more realistic model of a polar fluid is obtained by adding a Lennard–Jones potential to the intermolecular potential. Very similar results are obtained by adding a Yukawa potential, instead of the Lennard–Jones potential. We call this fluid the dipolar Yukawa fluid (DYF). We show that an analytical solution of the mean spherical approximation (MSA) can be obtained for the DYF. Thus, the DYF has many of the attractive features of the DHSF. We find that, within the MSA, the Yukawa potential modifies only the spherically averaged distribution function. Thus, although the thermodynamic properties of the DYF differ from those of the DHSF, the MSA dielectric constant of the DYF is the same as that of the DHSF. This result, and some other predictions, are tested by simulations and are found to be good approximations.

Strongly dipolar fluids at low densities compared to living polymers

We carried out extensive canonical Monte Carlo (MC) simulations of the dipolar hard-sphere (DHS) fluid, with $N=1024$ particles at fixed reduced density ${\ensuremath{\rho}}^{*}=0.05,$ in order to investigate the chainlike structure that occurs at low densities, for sufficiently large reduced dipole moments ${\ensuremath{\mu}}^{*}.$ The dissociation and recombination of chains during equilibrium runs suggest an analogy between the DHS's and a system of living polymers. This was checked quantitatively by comparing the results of the simulations with those of a theory for living polymers taking into account the indistinguishability of the particles in self-assembled chains. Quantitative agreement between theoretical and simulated mean chain lengths and number of monomers, was found for particular choices of the parameters used in the working definition of the MC chains.

Wave structure in Stokes' second problem for a dipolar fluid with nonclassical heat conduction

The classical heat conduction law of Fourier associates an infinite speed of propagation to a thermal disturbance in a material body. Such behavior is a violation of the causality principle. In recent years, several modifications of Fourier's heat law have been proposed. In this work, a modified form of Fourier's heat law, based on the Maxwell-Cattaneo-Fox (MCF) model, is used to analyze the heat conduction effects in Stokes' second problem for a dipolar fluid. The structure of the waves and the influence of the dipolar constants on the velocity field is investigated. These results are then compared to the viscous fluid case. In addition, the displacement thickness and skin friction at the plate are determined.

Thermodynamic perturbation theory applied to the dipolar heteronuclear dumbbell fluid

The perturbation theory of Stell and coworkers as extended by Boublík and Vega is applied to the dipolar heteronuclear dumbbell fluid using the hard heteronuclear dumbbell fluid as a reference system. Monte Carlo simulations in the constant pressure and temperature ensemble have been carried out to determine the accuracy of the theory. The calculated thermodynamic properties are density, internal energy, and chemical potential. The theory shows good agreement with the simulation results over a large range of densities and dipole moments, similar to the findings of Patey and Valleau for dipolar hard spheres.