Yukawa Fluid Research Articles

Studies of force field effects on thermal conductivity of complex plasmas

The plasma thermal conductivity of three-dimensional (3D) strongly coupled Yukawa fluids (SCYFs) has been investigated under the influence of varying external force fields through the recently improved homogenous nonequilibrium molecular dynamics (MD) approach. The effects of external field strength (F*) have been calculated along with various combinations of plasma coupling (Γ) and screening parameter (κ) on the plasma thermal conductivity of SCYFs using homogenous nonequilibrium MD (HNEMD) simulations. New investigations show that the plasma thermal conductivity of complex plasmas decreases with an increase of external force field strength and plasma coupling (Γ). Our calculations show that the decreasing behavior is noted for plasma thermal conductivity with increasing screening parameter and system size. The HNEMD simulation results under different force field strengths are in satisfactory agreement with previous numerical results of nonequilibrium MD and equilibrium MD simulations and with reference data points and it showed that the deviations are within less than ±20% for the presented results. It has been shown that our numerical results extended the range of external force field strength up to 0.001 ≤ F* ≤ 0.1 exercised in the previous work in order to establish the size of the linear regimes and to clarify the nature of linearity of 3D SCYFs.

Surface tension of a Yukawa fluid according to mean-field theory.

Yukawa fluids consist of particles that interact through a repulsive or attractive Yukawa potential. A surface tension arises at the walls of the container that encloses the fluid or at the interface between two coexisting phases. We calculate that surface tension on the level of mean-field theory, thereby either ignoring the particle size (ideal Yukawa fluid) or accounting for a non-vanishing particle size through a nonideal contribution to the free energy, exemplified either on the level of a lattice gas (lattice Yukawa fluid) or based on the Carnahan-Starling equation of state (Carnahan-Starling Yukawa fluid). Our mean-field results, which do not rely on assuming small gradients of the particle concentrations, become exact in the limit of large temperature and large screening length. They are calculated numerically in the general case and analytically in the two limits of small particle concentration and close to the critical point for a phase-separating system. For a sufficiently small particle concentration, our predicted surface tension is accurate whereas for a phase boundary, we expect good agreement with exact calculations in the limit of a large screening length and if the mean-field model employs the Carnahan-Starling equation of state.

Oscillating Yukawa fluid at a hard wall: field theory description

ABSTRACTA field-theoretical approach is applied to study the structure of an oscillating Yukawa fluid in contact with a hard wall. The system is described beyond the mean field approximation with the Gaussian fluctuations taken into account. Analytical expressions for the pair correlation function, the density profiles, and the adsorption coefficient are derived. These results are analysed for different values of the temperature and the bulk density as well as the parameters of the pair potential. In order to estimate the validity of the applied approximations, the theoretical results are compared with the original computer simulations data. We show that the Gaussian approximation provides a much better description of the fluid density profile as compared to the mean field approximation.

Prediction of fluid-phase behavior of symmetrical binary Yukawa fluids using transition matrix Monte Carlo

Binary fluid mixtures display a wide array of fluid phase behavior ranging from simple vapor-liquid equilibrium diagrams to more complicated topology like azeotropy and heteroazeotropy. As a first step towards understanding the phenomena of equilibrium of binary molecular systems, the properties of binary monoatomic fluids have been studied in this work. An additional simplification made in this work is that of a symmetrical binary system where all similar molecules interact via the hard-core Yukawa (HCY) potential, u(r), while the dissimilar molecules interact via a potential, δu(r) where δ is a scalar parameter. The value of δ here is 0.75, which leads to the dissimilar molecules showing a tendency to dislike one another and the resulting phase diagram is such that the mixing-demixing line intersects the vapor-liquid equilibrium curve away from the vapor-liquid critical point. The effect of the variable potential range parameter of the HCY potential on the topology of the phase diagrams is investigated using grand canonical transition matrix Monte Carlo simulations and the conditions of temperature and pressure at which the system exhibits azeotropy and heteroazeotropy are ascertained. We find that the densities of the mixtures, as predicted by our simulation at equimolar concentrations, are in close agreement with the self-consistent Ornstein-Zernike approximation results of Schöll-Paschinger and co-workers.

Relations between the longitudinal and transverse sound velocities in strongly coupled Yukawa fluids

Two useful relations between the longitudinal and transverse sound velocities of the strongly coupled single component Yukawa fluids are derived. The first relates the sound velocities given by the quasilocalized charge approximation (QLCA) to the excess pressure of the system. This is shown to be a mathematical identity within QLCA, applicable to any soft isotropic interaction potential. The second relates the same quantities to the fluid sound velocity obtained via the thermodynamic route. Both three-dimensional and two-dimensional cases are considered. The accuracy of the relations is verified using the available results based on direct numerical simulations.

On the long-waves dispersion in Yukawa systems

A useful simplification of the quasilocalized charge approximations (QLCA) method to calculate the dispersion relations in strongly coupled Yukawa fluids is discussed. In this simplified version, a simplest possible model radial distribution function, properly related to the thermodynamic properties of the system, is used. The approach demonstrates good agreement with the dispersion relations obtained using the molecular dynamics simulations and the original QLCA in the long-wavelength regime.

Thermodynamics of Yukawa systems and sound velocity in dusty plasmas

The simple practical approaches to estimate thermodynamic properties of three-dimensional Yukawa systems across coupling regimes (in fluid and solid phases) are summarized. These approaches demonstrate very good accuracy when compared with the results of direct numerical simulations. To demonstrate possible applications, the sound velocity in a strongly coupled dusty plasma is evaluated by combining the conventional fluid description of multi-component plasma with the appropriate equation of state of Yukawa fluids. Limitations of the proposed approaches are briefly discussed.

On the estimation of sound speed in two-dimensional Yukawa fluids

The longitudinal sound speed in two-dimensional Yukawa fluids is estimated using the conventional hydrodynamic expression supplemented by appropriate thermodynamic functions proposed recently by Khrapak et al. [Phys. Plasmas 22, 083706 (2015)]. In contrast to the existing approaches, such as quasi-localized charge approximation (QLCA) and molecular dynamics simulations, our model provides a relatively simple estimate for the sound speed over a wide range of parameters of interest. At strong coupling, our results are shown to be in good agreement with the results obtained using the QLCA approach and those derived from the phonon spectrum for the triangular lattice. On the other hand, our model is also expected to remain accurate at moderate values of the coupling strength. In addition, the obtained results are used to discuss the influence of the strong coupling effects on the adiabatic index of two-dimensional Yukawa fluids.

Thermodynamics of Yukawa fluids near the one-component-plasma limit

Thermodynamics of weakly screened (near the one-component-plasma limit) Yukawa fluids in two and three dimensions is analyzed in detail. It is shown that the thermal component of the excess internal energy of these fluids, when expressed in terms of the properly normalized coupling strength, exhibits the scaling pertinent to the corresponding one-component-plasma limit (the scalings differ considerably between the two- and three-dimensional situations). This provides us with a simple and accurate practical tool to estimate thermodynamic properties of weakly screened Yukawa fluids. Particular attention is paid to the two-dimensional fluids, for which several important thermodynamic quantities are calculated to illustrate the application of the approach.

Effect of surface-screening parameter of the Yukawa potential model on vapour–liquid phase coexistence and critical-point properties of confined Yukawa fluid

We report the effect of surface-screening parameter of Yukawa potential model on vapour–liquid phase coexistence and critical-point properties of slit–pore-confined Yukawa fluid, using grand canonical transition-matrix Monte Carlo along with the histogram reweighting method. The effect of surface-screening parameter on the vapour–phase coexistence density is insignificant for the studied system. On the other hand, significant effect of surface-screening parameter is observed on liquid phase coexistence density. With increasing surface-screening parameter, liquid phase coexistence density decreases. Critical-point properties have shown monotonic decreasing trends with increase in surface-screening parameter. Moreover, the effect of change of surface-screening parameter is least on critical temperature changes as compared to critical density and critical pressure changes for the studied Yukawa system in this work.

A heuristic rule for classification of classical fluids: Master curves for Mie, Yukawa and square-well potentials

A shift of the vapor–liquid coexistence curves by the critical value of the reduced second virial coefficient yields striking data collapses to define master curves. This is observed for the Mie, Yukawa and square-well fluids of different attractive ranges. This modification of the extended corresponding-states law of Noro and Frenkel strongly improves the outcomes from the van der Waals principle. Moreover, this shifted extended principle makes the master curves from Mie and Yukawa potentials to be one on top of the other. The square-well potential forms two well defined master curves, each one corresponding to different effective critical exponents.

Fluid approach to evaluate sound velocity in Yukawa systems and complex plasmas.

The conventional fluid description of multicomponent plasma, supplemented by an appropriate equation of state for the macroparticle component, is used to evaluate the longitudinal sound velocity of Yukawa fluids. The obtained results are in very good agreement with those obtained earlier employing the quasilocalized charge approximation and molecular dynamics simulations in a rather broad parameter regime. Thus, a simple yet accurate tool to estimate the sound velocity across coupling regimes is proposed, which can be particularly helpful in estimating the dust-acoustic velocity in strongly coupled dusty (complex) plasmas. It is shown that, within the present approach, the sound velocity is completely determined by particle-particle correlations and the neutralizing medium (plasma), apart from providing screening of the Coulomb interaction, has no other effect on the sound propagation. The ratio of the actual sound velocity to its "ideal gas" (weak coupling) scale only weakly depends on the coupling strength in the fluid regime but exhibits a pronounced decrease with the increase of the screening strength. The limitations of the present approach in applications to real complex plasmas are briefly discussed.

Molecular dynamics simulations of soliton-like structures in a dusty plasma medium

The existence and evolution of soliton-like structures in a dusty plasma medium are investigated in a first principles approach using molecular dynamic (MD) simulations of particles interacting via a Yukawa potential. These localized structures are found to exist in both weakly and strongly coupled regimes with their structures becoming sharper as the correlation effects between the dust particles get stronger. A surprising result, compared to fluid simulations, is the existence of rarefactive soliton-like structures in our non-dissipative system, a feature that arises from the charge conjugation symmetry property of the Yukawa fluid. Our simulation findings closely resemble many diverse experimental results reported in the past.

Practical expressions for the internal energy and pressure of Yukawa fluids.

Simple practical expressions that allow estimation of thermodynamic properties of Yukawa fluids in a wide range of coupling, up to the fluid-solid phase transition, are presented. These expressions demonstrate excellent agreement with the available results from numerical simulations. The approach provides simple and accurate tools to estimate thermodynamic properties of Yukawa fluids and related systems in a broad range of parameters.

Virial expansions and augmented van der Waals approach: Application to Lennard-Jones-like Yukawa fluid

We argue that recently proposed [Melnyk et al., Fluid Phase Equilibr., 2009, Vol. 279, 1] a criterion to split the pair interaction energy into two parts, one of which is forced to be responsible the most accurate as possible for excluded volume energy in the system, results in expressions for the virial coefficients that improve the performance of the virial equation of state in general, and at subcritical temperatures, in particular. As an example, application to the Lennard-Jones-like hard-core attractive Yukawa fluid is discussed.

Adsorption of Yukawa fluids on a hard wall

We construct a closure theory for the bridge functional based on the third-order Ornstein–Zernike (OZ3) equation. The new closure is tested on the adsorption of the hard-core Yukawa fluid on a planar hard wall and its results are compared with Monte Carlo computer simulation. We show that in the particularly challenging cases of depletive adsorption, the density profiles near the wall obtained from the OZ3 theory improve considerably over conventional liquid-state theories, such as the hypernetted-chain and the Percus–Yevick equations. Some technical details of the implementation of the procedure of re-normalisation of the non-uniform indirect correlation function are also provided.

A generalization of random phase approximation as a new theory in the thermodynamic perturbation theory of simple fluids

A generalization of the optimized random phase approximation in the thermodynamic perturbation theory of liquids is proposed. By introducing a new method, we can derive an approximate analytical expression for the free energy of simple liquids. This is achieved by invoking a direct correlation function more general than the mean spherical approximation. By correcting the unphysical behavior of the corresponding radial distribution function in the hard core region, we attain a new perturbation expansion as a natural generalization of the optimized random phase approximation. In order to assess its accuracy and reliability, the theory is employed to find the structure and thermodynamics of hard-core Yukawa fluids. The new resulted optimized second order random phase approximation (OSORPA) is shown to be a little more accurate than the optimized random phase approximation in finding the thermodynamic properties, but it performs much better in predicting the static structure factor of hard-core Yukawa potentials.

Investigation of linear relationship between interfacial tension and excess adsorption of confined fluids in nanospherical pores

The modified fundamental measure theory is applied to investigate the interfacial tension and excess adsorption of confined fluids in nanospherical cavities. Results show that the curvature direction and wall–fluid interactions play important roles in the behavior of interfacial tension and excess adsorption, but only wall–fluid interactions determine their sign. The interfacial tension of hard sphere and two Yukawa fluids in contact with the inside (interfacial tension of confined fluids) and outside of the hard spherical cavities are both positive; but the former increases while the latter decreases with cavity size. For an attractive cavity, the signs may be negative or positive depending on both the wall–fluid interactions and the thermodynamic state of the fluid. The behavioral trend of the interfacial tension, with respect to cavity size, of a fluid in contact with an attractive wall is opposite to that of a fluid in contact with a hard wall. Therefore, the wall–fluid interaction and the curvature direction of the wall determine this trend. Investigation of the excess adsorption of fluids with cavity size led to a linear relationship established between excess adsorption and curvature. Also, linear relationships were found between the interfacial tensions of fluids inside and outside nanocavities, between interfacial tension and excess adsorption of the fluid inside nanocavities, and also between excess adsorption of a fluid in contact with the concave and convex sides of the cavity wall.

Prediction of volumetric properties of some fatty acid methyl esters, biodiesel fuels and their blends using perturbed Yukawa hard-core chain equation of state

In this paper, a perturbed Yukawa hard-core chain (PYHCC) equation of state (EOS) has been employed for predicting the volumetric properties of some fatty acid methyl esters (FAMEs) and methyl ester-based biodiesel fuels and their blends. Three pure-component parameters appear in the PYHCC EOS, which reflect the chain-length, dispersive energy parameter and hard-core diameter of Yukawa fluid. These parameters have been determined by fitting of 56 experimental density data at ambient pressure. The performance of the proposed model has been assessed by predicting 1165 density data over pressure range from 0.1 to 130MPa and temperature range from 278.4 to 396K, for which their measured values were available in the literature. The overall average absolute deviation (AAD) of the correlated and predicted densities of studied FAMEs and biodiesels from literature data were found to be 0.26% and 0.52%, respectively. The isothermal compressibility (κT) coefficients have also been estimated by the use of proposed perturbed hard-chain equation of state (PHC EOS) and the results have been compared with those obtained from Tait equation; from 596 data points examined, AAD was found to be 2.60%. Besides, we have extended the mixture version of the proposed PHC EOS to binary and ternary blends formed by methyl ester-based biodiesels for predicting their volumetric properties. Our prediction accuracy has been checked by taking 406 experimental data points for above-mentioned blends, AAD was found to be 0.43%.

The role of concavo-convex walls of a nanopore on the density profile, adsorption, solvation force, and capillary condensation of confined fluids: A DFT study

We investigate the effects of concavo-convex walls of a nanopore on the structure and certain thermodynamic properties of confined fluids. Adsorption, solvation force, and capillary condensation in a nanopore formed between two homocentric spheres will be determined using the MFMT. For hard sphere fluids, contact density is greater at the concave wall than it is at the convex wall. In Yukawa fluids, for the thermodynamic state in which the energy effect is the dominant factor, contact density at a concave wall is less than that at a convex wall; this will be reversed for the thermodynamic state in which the entropy effect is the dominant factor. It is possible to find thermodynamic states in which contact densities at concave and convex walls become identical. The adsorption and solvation force of hard sphere fluid show an oscillatory behavior versus H. Capillary condensation is in certain cases observed for Yukawa fluids.