Ferroelectric liquid-crystal and solid phases formed by strongly interacting dipolar soft spheres

Simple molecular model for ferroelectric nematic liquid crystals exhibited by small rodlike mesogens

Nematic liquid crystals (NLCs) are the prime example of a liquid medium with an apolar orientational order. In the past couple of years, the ferroelectric nematic (FN) phase has been discovered in some compounds with small rodlike molecules with large longitudinal dipole moments and very restricted chemical structures, as the temperature is lowered from the NLC. We propose a simple model in which the molecules are idealized as cylindrical rods with longitudinal surface charge density waves. The usually strong electrostatic inter-rod interactions favoring antiparallel structures are shown to be subdued in magnitude, and those of parallel structures enhanced, by reducing the amplitudes of the half-waves at both ends of the rods. By introducing an additional increased amplitude of one interior wave, the energy per rod of a cluster of molecules with a pseudohexagonal order is shown to favor the ferroelectric order compared to the antiparallel order, below some value of the inter-rod separation. The model broadly accounts for the restriction in molecular structures for a compound to exhibit the FN phase. It is suggested that the weakly first-order nature of the NLC to FN transition arises from a coupling of the polar order and the density of the medium.

Dielectric continuum model examination of real-space electrostatic treatments

Electrostatic interaction is long ranged; thus, the accurate calculation is not an easy task in molecular dynamics or Monte Carlo simulations. Though the rigorous Ewald method based on the reciprocal space has been established, real-space treatments have recently become an attractive alternative because of the efficient calculation. However, the construction is not yet completed and is now a challenging subject. In an earlier theoretical study, Neumann and Steinhauser employed the Onsager dielectric continuum model to explain how simple real-space cutoff produces artificial dipolar orientation. In the present study, we employ this continuum model to explore the fundamental properties of the recently developed real-space treatments of three shifting schemes. The result of the distance-dependent Kirkwood function G_K(R) showed that the simple bare cutoff produces a well-known hole-shaped artifact, whereas the shift treatments do not. Two-dimensional mapping of electric field well explained how these shift treatments remove the hole-shaped artifact. Still, the shift treatments are not sufficient because they do not produce a flat G_K(R) profile unlike ideal no-cutoff treatment. To test the continuum model results, we also performed Monte Carlo simulations of dipolar particles. The results found that the continuum model could predict the qualitative tendency as to whether each electrostatic treatment produces the hole-shaped artifact of G_K(R) or not. We expect that the present study using the continuum model offers a stringent criterion to judge whether the primitive electrostatic behavior is correctly described or not, which will be useful for future construction of electrostatic treatments.

Observation of soft glassy behavior in a magnetic colloid exposed to an external magnetic field

We provide the first experimental evidence for soft glassy behavior in a sterically stabilized magnetic colloid (ferrofluid) of relatively low volume fraction (φ = 0.037) when a uniform magnetic field is applied at a sufficiently high rate (fast quench). Fast magnetic-field quenches favor structural arrest of field-induced aggregates, owing to insufficient time to settle into lower energy states, thereby pushing the system to a frustrated metastable configuration like a repulsive glass. Brownian dynamics simulations are used to show that the polydisperse ferrofluid (as in experiments) forms thick ropes aligned along the field direction, while a monodisperse ferrofluid does not. The simulations show that there is practically no ordering of the thin, monodisperse chains, while the thick, polydisperse ropes show positional ordering with a typical center-center separation between the particles in different ropes of about 0.39 μm. As a consequence of structural arrest, the ferrofluid exhibits aging with broken time-translational invariance, a hallmark of glassy dynamics. The superposition of strain and creep compliance curves obtained from rheological measurements at different waiting times in the effective time domain corroborates the soft glassy behavior when exposed to a magnetic field applied at a fast ramp rate.

Steady-state rheology and structure of soft hybrid mixtures of liquid crystals and magnetic nanoparticles

Using non-equilibrium molecular dynamics simulations, we study the rheology of a model hybrid mixture of liquid crystals (LCs) and dipolar soft spheres (DSS) representing magnetic nanoparticles. The bulk isotropic LC-DSS mixture is sheared with different shear rates using Lees-Edwards periodic boundary conditions. The steady-state rheological properties and the effect of the shear on the microstructure of the mixture are studied for different strengths of the dipolar coupling, λ, among the DSS. We find that at large shear rates, the mixture shows a shear-thinning behavior for all considered values of λ. At low and intermediate values of λ, a crossover from Newtonian to non-Newtonian behavior is observed as the rate of applied shear is increased. In contrast, for large values of λ, such a crossover is not observed within the range of shear rates considered. Also, the extent of the non-Newtonian regime increases as λ is increased. These features can be understood via the shear-induced changes of the microstructure. In particular, the LCs display a shear-induced isotropic-to-nematic transition at large shear rates with a shear-rate dependent degree of nematic ordering. The DSS show a shear-induced nematic ordering only for large values of λ, where the particles self-assemble into chains. Moreover, at large λ and low shear rates, our simulations indicate that the DSS form ferromagnetic domains.

Peptide Solubility Limits: Backbone and Side-Chain Interactions

We calculate the solubility limit of pentapeptides in water by simulating the phase separation in an oversaturated aqueous solution. The solubility limit order followed by our model peptides (GGRGG > GGDGG > GGGGG > GGVGG > GGQGG > GGNGG > GGFGG) is found to be the same as that reported for amino acid monomers from experiment (R > D > G > V > Q > N > F). Investigation of dynamical properties of peptides shows that the higher the solubility of a peptide is, the lower the time spent by the peptide in the aggregated cluster is. We also demonstrate that fluctuations in conformation and hydration number of peptide in monomeric form are correlated with the solubility of the peptide. We considered energetic mechanisms and dynamical properties of interbackbone CO-CO and CO···HN interactions. Our results confirm that CO-CO interactions more than the interbackbone H-bonds are important in peptide self-assembly and association. Further, we find that the stability of H-bonded peptide pairs arises mainly from coexisting CO-CO and CO···HN interactions.

Ferroelectric glass of spheroidal dipoles with impurities: polar nanoregions, response to applied electric field, and ergodicity breakdown

Using molecular dynamics simulation, we study dipolar glass in crystals composed of slightly spheroidal, polar particles and spherical, apolar impurities between metal walls. We present physical pictures of ferroelectric glass, which have been observed in relaxors, mixed crystals (such as KCN _x KBr_1-x ), and polymers. Our systems undergo a diffuse transition in a wide temperature range, where we visualize polar nanoregions (PNRs) surrounded by impurities. In our simulation, the impurities form clusters and their space distribution is heterogeneous. The polarization fluctuations are enhanced at relatively high T depending on the size of the dipole moment. They then form frozen PNRs as T is further lowered into the nonergodic regime. As a result, the dielectric permittivity exhibits the characteristic features of relaxor ferroelectrics. We also examine nonlinear response to cyclic applied electric field and nonergodic response to cyclic temperature changes (ZFC/FC), where the polarization and the strain change collectively and heterogeneously. We also study antiferroelectric glass arising from molecular shape asymmetry. We use an Ewald scheme of calculating the dipolar interaction in applied electric field.

Self-assembly of three-dimensional ensembles of magnetic particles with laterally shifted dipoles

We consider a model of colloidal spherical particles carrying a permanent dipole moment which is laterally shifted out of the particles' geometrical centres, i.e. the dipole vector is oriented perpendicular to the radius of the particles. Varying the shift δ from the centre, we analyse ground state structures for two, three and four hard spheres, using a simulated annealing procedure. We also compare earlier ground state results. We then consider a bulk system at finite temperatures and different densities. Using molecular dynamics simulations, we examine the equilibrium self-assembly properties for several shifts. Our results show that the shift of the dipole moment has a crucial impact on both the ground state configurations as well as the self-assembled structures at finite temperatures.

Ferroelectric domain formation in discotic liquid crystals: Monte Carlo study on the influence of boundary conditions

The realization of a spontaneous macroscopic ferroelectric order in fluids of anisotropic mesogens is a topic of both fundamental and technological interest. Recently we demonstrated that a system of dipolar achiral disklike ellipsoids can exhibit long-searched ferroelectric liquid crystalline phases of dipolar origin. In the present work, extensive off-lattice Monte Carlo simulations are used to investigate the phase behavior of the system under the influences of the electrostatic boundary conditions that restrict any global polarization. We find that the system develops strongly ferroelectric slablike domains periodically arranged in an antiferroelectric fashion. Exploring the phase behavior at different dipole strengths, we find existence of the ferroelectric nematic and ferroelectric columnar order inside the domains. For higher dipole strengths, a biaxial phase is also obtained with a similar periodic array of ferroelectric slabs of antiparallel polarizations. We have studied the depolarizing effects by using both the Ewald summation and the spherical cutoff techniques. We present and compare the results of the two different approaches of considering the depolarizing effects in this anisotropic system. It is explicitly shown that the domain size increases with the system size as a result of considering a longer range of dipolar interactions. The system exhibits pronounced system size effects for stronger dipolar interactions. The results provide strong evidence to the novel understanding that the dipolar interactions are indeed sufficient to produce long-range ferroelectric order in anisotropic fluids.

Ferroelectric order in liquid crystal phases of polar disk-shaped ellipsoids

The demonstration of a spontaneous macroscopic ferroelectric order in liquid phases in the absence of any long range positional order is considered an outstanding problem of both fundamental and technological interest. Recently, we reported that a system of polar achiral disklike ellipsoids can spontaneously exhibit a long searched ferroelectric nematic phase and a ferroelectric columnar phase with strong axial polarization. The major role is played by the dipolar interactions. The model system of interest consists of attractive-repulsive Gay-Berne oblate ellipsoids embedded with two parallel point dipoles positioned symmetrically on the equatorial plane of the ellipsoids. In the present work, we investigate in detail the profound effects of changing the separation between the two symmetrically placed dipoles and the strength of the dipoles upon the existence of different ferroelectric discotic liquid crystal phases via extensive off-lattice N-P-T Monte Carlo simulations. Ferroelectric biaxial phases are exhibited in addition to the uniaxial ferroelectric fluids where the phase biaxiality results from the dipolar interactions. The structures of all the ferroelectric configurations of interest are presented in detail. Simple phase diagrams are determined which include different polar and apolar discotic fluids generated by the system.

Size effect and stability of polarized fluid phases

The existence of a ferroelectric fluid phase for systems of 1000-2000 dipolar hard or soft spheres is well established by numerical simulations. Theoretical approaches proposed to determine the stability of such a phase are either in qualitative agreement with the simulation results or disagree with them. Experimental results for systems of molecules or particles with large electric or magnetic dipole moments are also inconclusive. As a contribution to the question of existence and stability of a fluid ferroelectric phase this simulation work considers system sizes of the order of 10 000 particles, thus an order of magnitude larger than those used in previous studies. It shows that although ferroelectricity is not affected by an increase of system size, different spatial arrangements of the dipolar hard spheres in such a phase are possible whose free energies seem to differ only marginally.

Phase behavior of an amphiphilic fluid

We invoke mean-field density functional theory (DFT) to investigate the phase behavior of an amphiphilic fluid composed of a hard-sphere core plus a superimposed anisometric Lennard-Jones perturbation. The orientation dependence of the interactions consists of a contribution analogous to the interaction potential between a pair of "spins" in the classical, three-dimensional Heisenberg fluid and another one reminiscent of the interaction between (electric or magnetic) point dipoles. At fixed orientation both contributions are short-range in nature decaying as r-6 (r being the separation between the centers of mass of a pair of amphiphiles). Based upon two mean-field-like approximations for the pair correlation function that differ in the degree of sophistication we derive expressions for the phase boundaries between various isotropic and polar phases that we solve numerically by the Newton-Raphson method. For sufficiently strong coupling between the Heisenberg "spins" both mean-field approximations generate three topologically different and generic types of phase diagrams that are observed in agreement with earlier work [see, for example, Tavares et al., Phys. Rev. E 52, 1915 (1995)]. Whereas the dipolar contribution alone is incapable of stabilizing polar phases on account of its short-range nature it is nevertheless important for details of the phase diagram such as location of the gas-isotropic liquid critical point, triple, and tricritical points. By tuning the dipolar coupling constant suitably one may, in fact, switch between topologically different phase diagrams. Employing also Monte Carlo simulations in the isothermal-isobaric ensemble the general topology of the DFT phase diagrams is confirmed.

Thermodynamics of ferrofluids in applied magnetic fields

The thermodynamic properties of ferrofluids in applied magnetic fields are examined using theory and computer simulation. The dipolar hard sphere model is used. The second and third virial coefficients (B(2) and B(3)) are evaluated as functions of the dipolar coupling constant λ, and the Langevin parameter α. The formula for B(3) for a system in an applied field is different from that in the zero-field case, and a derivation is presented. The formulas are compared to results from Mayer-sampling calculations, and the trends with increasing λ and α are examined. Very good agreement between theory and computation is demonstrated for the realistic values λ≤2. The analytical formulas for the virial coefficients are incorporated in to various forms of virial expansion, designed to minimize the effects of truncation. The theoretical results for the equation of state are compared against results from Monte Carlo simulations. In all cases, the so-called logarithmic free energy theory is seen to be superior. In this theory, the virial expansion of the Helmholtz free energy is re-summed in to a logarithmic function. Its success is due to the approximate representation of high-order terms in the virial expansion, while retaining the exact low-concentration behavior. The theory also yields the magnetization, and a comparison with simulation results and a competing modified mean-field theory shows excellent agreement. Finally, the putative field-dependent critical parameters for the condensation transition are obtained and compared against existing simulation results for the Stockmayer fluid. Dipolar hard spheres do not undergo the transition, but the presence of isotropic attractions, as in the Stockmayer fluid, gives rise to condensation even in zero field. A comparison of the relative changes in critical parameters with increasing field strength shows excellent agreement between theory and simulation, showing that the theoretical treatment of the dipolar interactions is robust.

Monte Carlo simulations of spontaneous ferroelectric order in discotic liquid crystals

The demonstration of a spontaneous macroscopic ferroelectric order in liquid phases in the absence of any long-range positional order is considered as an outstanding problem of great fundamental and technological interest. We report here off-lattice Monte Carlo simulations of a system of polar achiral disklike ellipsoids which spontaneously exhibit a novel ferroelectric nematic phase which is a liquid in three dimensions, considering attractive-repulsive pair interaction suitable for the anisotropic particles. At lower temperature, the ferroelectric nematic phase condenses to a ferroelectric hexagonal columnar fluid with an axial macroscopic polarization. A spontaneous ferroelectric order of dipolar origin is established here for the first time in columnar liquid crystals. Our study demonstrates that simple dipolar interactions are indeed sufficient to produce a class of novel ferroelectric fluids of essential interest. The present work reveals the structure-property relationship of achieving long searched ferroelectric liquid crystal phases and transitions between them, and we hope these findings will help in future development of technologically important fluid ferroelectric materials.

Isotropic-polar phase transitions in an amphiphilic fluid: density functional theory versus computer simulations

We investigate the critical line separating isotropic from polar phases in an amphiphilic bulk fluid by means of density functional theory (DFT) and Monte Carlo (MC) simulations in the isothermal-isobaric ensemble. The intermolecular interactions are described by a Lennard-Jones potential in which the attractive contribution is modified by an orientation-dependent function. The latter consists of two terms: The first one has the orientation dependence of a classical three-dimensional Heisenberg interaction, whereas, the second one has the orientation dependence of a classical dipole-dipole interaction. However, both contributions are short range. Employing DFT together with a modified mean-field (MMF) approximation for the orientation-dependent pair correlation function, we derive an analytical expression for the critical line separating isotropic from polar liquidlike phases. In parallel MC simulations, we locate the line of critical points through an analysis of Binder's second-order cumulant of the polar-order parameter. Comparison with DFT shows that the dipolelike contribution is irrelevant for the isotropic-polar phase transition. As far as the Heisenberg contribution is concerned, the MC data are in semiquantitative agreement with the DFT predictions for sufficiently strong coupling between molecular orientations. For weaker coupling, the variation in the ratio of critical density and temperature ρ(c)/T(c) with the Heisenberg coupling constant ε(H) is underestimated by the MMF treatment. The MC results suggest that this is because ρ(c) increases with decreasing ε(H) such that the assumption on which the MMF approach rests becomes less applicable in the weaker-coupling limit.

Role of dipole elongation in orientationally ordered liquids

A recent study shows that the dipole elongation in the extended dipole model plays a significant role in the phase transitions of liquid crystal phases. In this paper, molecular dynamics (MD) simulations were performed for the dipole model with different distances between the two charges keeping the total dipole moment the same. The potential energy consists of the Lennard-Jones potential and the site-site electrostatic contribution of partial charges. Detailed analyses were made with respect to the average order parameters (P(1)) and (P(2)) as functions of density along with other thermodynamic properties. When the reduced dipole elongations are 0.16 and 0.32, respectively, it is shown that the chainlike structures in the low density regime, liquid phases with columnar and smectic orders, and solid phases are formed; the phase with nematic order is not present anymore. At 0.64, the phases with antiferroelectric order were favored. The transition is found at the reduced elongation 0.55. It shows that the phase transitions are quite sensitive to the molecular charge distribution; this simple system could exhibit rather rich phase behaviors, which represents a significant advance in identifying molecular and state parameters of the future ferroelectric liquids.

Steric effects in a mean-field model for polar nematic liquid crystals

The existence of uniaxial liquid crystals comprising polar molecules, with all the dipoles aligned in a parallel pattern, is classically ruled out. Generally, there are two different avenues to a mean-field theory for liquid crystals: one is based on short-range, repulsive, steric forces, and the other is based on long-range, globally attractive, dispersion forces. Purely polar steric interactions have been shown to have the potential of inducing unexpected orientationally ordered states. In real molecules, anisotropies both in shape and in polarizability coexist; it has been shown that dispersion forces interaction can be combined with hard-core repulsion in a formal theory, based on a steric tensor. Starting from this, we build an interaction Hamiltonian featuring the average electric dipolar energy exchanged between molecules with the same excluded region. Under the assumption that the molecular shape is spheroidal, we propose a mean-field model for polar nematic liquid crystals which can exhibit both uniaxial and biaxial polar phases. By means of a numerical bifurcation analysis, we discuss the stability of the equilibrium against the choice of two model parameters, one describing the degree of molecular shape biaxiality and the other describing the relative orientation of the electric dipole within each molecule. We find only uniaxial stable phases, which are effectively characterized by a single scalar order parameter.

Dipolar sticky hard spheres within the Percus-Yevick approximation plus orientational linearization

We consider a strongly idealized model for polar fluids, which consists of spherical particles, having, in addition to a hard-core repulsion, a "surface dipolar" interaction, acting only when particles are exactly at contact. A fully analytic solution of the molecular Orstein-Zernike equation is found for this potential, within the Percus-Yevick approximation complemented by a linearization of the angular dependence on molecular orientations (Percus-Yevick closure with orientational linearization). Numerical results are also presented in a detailed analysis about the local orientational structure. From the pair correlation function g(1,2), we first derive the best orientations of a test particle which explores the space around an arbitrary reference molecule. Then some local and global order parameters, related to the polarization induced by the reference particle, are also calculated. The local structure of this model with only short-ranged anisotropic interactions turns out to be, at least within the chosen approximation, qualitatively different from that of hard spheres with fully long-ranged dipolar potentials.

Molecular dynamics simulations of dipolar fluids in orientationally ordered phases

It has been established that the strongly interacting dipoles form orientationally ordered liquid phases. However, most of the computer simulations adapt the point dipole model. In this paper, we report molecular dynamics simulations of orientationally order phases formed by extended dipoles, where the potential energy consists of the site-site Lennard-Jones potential and electrostatic contribution of partial charges. The calculations were performed for a range of densities along an isotherm and for different temperatures at the same reduced densities. It is found that orientationally ordered phases are present in the wide density regime, the extended dipole tends to form chains at low density, and the isotropic liquid phase is not seen in the density regime studied for a specific temperature.

Orientational order in high density dipolar hard sphere fluids

Taking advantage of recent estimates, by one of us, of the critical temperature of the isotropic-ferroelectric transition of high density dipolar hard spheres, we performed new Monte Carlo simulations in the close vicinity of these estimates and applied histogram reweighting methods to obtain refined values of the critical temperatures from the crossing of the fourth-order cumulant for different system sizes. The ferroelectric line is determined in the density range rho*=0.80-0.95, and the onset of columnar ordering is located.

Pair formation and global ordering of strongly interacting ferrocolloid mixtures: an integral equation study

Using the reference hypernetted chain (RHNC) integral equation theory and an accompanying stability analysis we investigate the structural and phase behaviors of model bidisperse ferrocolloids based on correlations of the homogeneous isotropic high-temperature phase. Our model consists of two species of dipolar hard spheres (DHSs) which dipole moments are proportional to the particle volume. At small packing fractions our results indicate the onset of chain formation, where the (more strongly coupled) A species behaves essentially as a one-component DHS fluid in a background of B particles. At high packing fractions, on the other hand, the RHNC theory indicates the appearance of isotropic-to-ferromagnetic transitions (volume ratios close to one) and demixing transitions (smaller volume ratios). However, contrary with the related case of monodisperse DHS mixtures previously studied by us [Phys. Rev. E 70, 031201 (2004)], none of the present bidisperse systems exhibit demixing within the isotropic phase, rather we observe coupled ferromagnetic/demixing phase transitions.

The ferroelectric transition of dipolar hard spheres

We investigate by Monte Carlo simulation the size dependence of the variation of the polarization and the dielectric constant with temperature for dipolar hard spheres at the two densities rho sigma3=0.80 and 0.88. From the crossing of the fourth-order cumulant for different system sizes first more precise estimates of the ferroelectric transition temperatures are obtained. Theoretical approaches, when predicting an ordering transition, are shown to generally overestimate the critical temperature.

Computer simulations of the structure of colloidal ferrofluids

The structure of a ferrofluid under the influence of an external magnetic field is expected to become anisotropic due to the alignment of the dipoles into the direction of the external field, and subsequently to the formation of particle chains due to the attractive head to tail orientations of the ferrofluid particles. Knowledge about the structure of a colloidal ferrofluid can be inferred from scattering data via the measurement of structure factors. We have used molecular-dynamics simulations to investigate the structure of both monodispersed and polydispersed ferrofluids. The results for the isotropic structure factor for monodispersed samples are similar to previous data by Camp and Patey that were obtained using an alternative Monte Carlo simulation technique, but in a different parameter region. Here we look in addition at bidispersed samples and compute the anisotropic structure factor by projecting the q vector onto the XY and XZ planes separately, when the magnetic field was applied along the z axis. We observe that the XY-plane structure factor as well as the pair distribution functions are quite different from those obtained for the XZ plane. Further, the two-dimensional structure factor patterns are investigated for both monodispersed and bidispersed samples under different conditions. In addition, we look at the scaling exponents of structure factors. Our results should be of value to interpret scattering data on ferrofluids obtained under the influence of an external field.

Demixing in simple dipolar mixtures: Integral equation versus density functional results

Using reference hypernetted chain (RHNC) integral equations and density functional theory in the modified mean-field (MMF) approximation we investigate the phase behavior of binary mixtures of dipolar hard spheres. The two species (A and B) differ only in their dipole moments m(A) and m(B), and the central question investigated is under which conditions these asymmetric mixtures can exhibit demixing phase transitions in the fluid phase regime. Results from our two theoretical approaches turn out to strongly differ. Within the RHNC (which we apply to the isotropic high-temperature phase) demixing does indeed occur for dense systems with small interaction parameters Gamma= m(2)(B)/m(2)(A). This result generalizes previously reported observations on demixing in mixtures of dipolar and neutral hard spheres (Gamma=0) to the case of true dipolar hard sphere mixtures. The RHNC approach also indicates that these demixed fluid phases are isotropic at temperatures accessible by the theory, whereas isotropic-to-ferroelectric transitions occur only at larger Gamma. The MMF theory, on the other hand, yields a different picture in which demixing occurs in combination with spontaneous ferroelectricity at all Gamma considered. This discrepancy underlines the relevance of correlational effects for the existence of demixing transitions in dipolar systems without dispersive interactions. Indeed, supplementing the dipolar interactions by small, asymmetric amounts of van der Waals-like interactions (and thereby supporting the systems tendency to demix) one finally reaches good agreement between MMF and RHNC results.

Liquid-liquid phase transitions for soft-core attractive potentials

Using event-driven molecular dynamics simulations, we study a three-dimensional one-component system of spherical particles interacting via a discontinuous potential combining a repulsive square soft core and an attractive square well. In the case of a narrow attractive well, it has been shown that this potential has two metastable gas-liquid critical points. Here we systematically investigate how the changes of the parameters of this potential affect the phase diagram of the system. We find a broad range of potential parameters for which the system has both a gas-liquid critical point C1 and a liquid-liquid critical point C2. For the liquid-gas critical point we find that the derivatives of the critical temperature and pressure, with respect to the parameters of the potential, have the same signs: they are positive for increasing width of the attractive well and negative for increasing width and repulsive energy of the soft core. This result resembles the behavior of the liquid-gas critical point for standard liquids. In contrast, for the liquid-liquid critical point the critical pressure decreases as the critical temperature increases. As a consequence, the liquid-liquid critical point exists at positive pressures only in a finite range of parameters. We present a modified van der Waals equation which qualitatively reproduces the behavior of both critical points within some range of parameters, and gives us insight on the mechanisms ruling the dependence of the two critical points on the potential's parameters. The soft-core potential studied here resembles model potentials used for colloids, proteins, and potentials that have been related to liquid metals, raising an interesting possibility that a liquid-liquid phase transition may be present in some systems where it has not yet been observed.

Density functional study of the phase behavior of asymmetric binary dipolar mixtures

Using density functional theory in the modified mean-field (MMF) approximation we study the phase behavior of asymmetric binary mixtures of equisized dipolar hard spheres with different dipole moments in the fluid phase regime. We focus on "dipole-dominated" systems where isotropic attractive interactions are absent. Despite these restrictions our results reveal complex fluid-fluid phase behavior involving demixing and first- and second-order isotropic-to-ferroelectric phase transitions the relative importance of which depends on two "tuning" parameters, that is, the parameter Gamma measuring the ratio of the dipolar coupling strengths, and the chemical potential difference Deltamu controlling the composition. The interplay of these effects then yields three different types of phase behavior differing in the degree to which demixing dominates the system. A generic feature of the resulting diagrams is that the isotropic-to-ferroelectric transition is shifted towards significantly higher densities compared to the one-component case, and is therefore destabilized. Furthermore, demixing in the MMF approach turns out to be always accompanied by spontaneous ferroelectricity, which is in contrast to recent integral equation and simulation results for the limiting case of a mixture of dipolar and pure hard spheres (Gamma=0).

Structure and magnetic properties of polydisperse ferrofluids: a molecular dynamics study

We study by Langevin molecular dynamics simulations systematically the influence of polydispersity in the particle size, and subsequently in the dipole moment, on the physical properties of ferrofluids. The polydispersity is in a first approximation modeled by a bidisperse system that consists of small and large particles at different ratios of their volume fractions. In the first part of our investigations the total volume fraction of the system is fixed, and the volume fraction phiL of the large particles is varied. The initial susceptibility chi and magnetization curve of the systems show a strong dependence on the value of phiL. With the increase of phiL, the magnetization M of the system has a much faster increment at weak fields, and thus leads to a larger chi. We performed a cluster analysis that indicates that this is due to the aggregation of the large particles in the systems. The average size of these clusters increases with increasing phiL. In the second part of our investigations, we fixed the volume fraction of the large particles, and increased the volume fraction phiS of the small particles in order to study their influence on the chain formation of the large ones. We found that the average aggregate size formed by large particles decreases when phiS is increased, demonstrating a significant effect of the small particles on the structural properties of the system. A topological analysis of the structure reveals that the majority of the small particles remain nonaggregated. Only a small number of them are attracted to the ends of the chains formed by large particles.

Computer simulations of hard pear-shaped particles

We report results obtained from Monte Carlo simulations investigating mesophase formation in two model systems of hard pear-shaped particles. The first model considered is a hard variant of the truncated Stone-expansion model previously shown to form nematic and smectic mesophases when embedded within a 12-6 Gay-Berne-like potential [R. Berardi, M. Ricci, and C. Zannoni, ChemPhysChem 7, 443 (2001)]. When stripped of its attractive interactions, however, this system is found to lose its liquid crystalline phases. For particles of length to breadth ratio k=3, glassy behavior is seen at high pressures, whereas for k=5 several bi- layerlike domains are seen, with high intradomain order but little interdomain orientational correlation. For the second model, which uses a parametric shape parameter based on the generalized Gay-Berne formalism, results are presented for particles with elongation k=3, 4, and 5. Here, the systems with k=3 and 4 fail to display orientationally ordered phases, but the system with k=5 shows isotropic, nematic and, unusual for a hard-particle model, interdigitated smectic A2 phases.

Spontaneous ferromagnetic ordering in magnetic fluids

This paper is devoted to the theoretical justification of spontaneous orientational order in magnetic fluids. We study the self-consistent solutions of the Bogoliubov-Born-Green-Kirkwood-Yvon equation connecting the one-particle distribution function with the pair correlation function. This self-consistent approach is used in the specific density functional method and proves to be equivalent to the mean field theory. On the basis of the second-order perturbation method over the intensity of dipole-dipole interparticle interaction the following effect is discovered: the self-consistent density functional approach leads to the spontaneous "ferrimagnetic" state of the magnetic fluid induced by the dipole-dipole interaction. This strange result seems to be physically meaningless and prejudices the validity of the density functional methods and mean field theories applied to orientational microstructure in ferrofluids.