Monatomic Liquids Research Articles

Self-diffusion in a monatomic glass-forming liquid embedded in the hyperbolic plane

We study, by molecular dynamics simulation, the slowing down of particle motion in a two-dimensional monatomic model: a Lennard-Jones liquid on the hyperbolic plane. The negative curvature of the embedding space frustrates the long-range extension of the local hexagonal order. As a result, the liquid avoids crystallization and forms a glass. We show that, as temperature decreases, the single-particle motion displays the canonical features seen in real glass-forming liquids: the emergence of a ‘plateau’ at intermediate times in the mean square displacement, and a decoupling between the local relaxation time and the (hyperbolic) diffusion constant.

Vibration-transit theory of self-dynamic response in a monatomic liquid

A theoretical model for self-dynamic response is developed using vibration-transit theory, and is applied to liquid sodium at all wave vectors q from the hydrodynamic regime to the free particle limit. In this theory the zeroth-order Hamiltonian describes the vibrational motion in a single random valley harmonically extended to infinity. This Hamiltonian is tractable, is evaluated a priori for monatomic liquids, and the same Hamiltonian (the same set of eigenvalues and eigenvectors) is used for equilibrium and nonequilibrium theory. Here, for the self-intermediate scattering function F;{s}(q,t) , we find the vibrational contribution is in near perfect agreement with molecular dynamics (MD) through short and intermediate times, at all q . This is direct confirmation that normal mode vibrational correlations are present in the motion of the liquid state. The primary transit effect is the diffusive motion of the vibrational equilibrium positions, as the liquid transits rapidly among random valleys. This motion is modeled as a standard random walk, and the resulting theoretical F;{s}(q,t) is in excellent agreement with MD results at all q and t . In the limit q-->infinity , the theory automatically exhibits the correct approach to the free-particle limit. Also, in the limit q-->0 , the hydrodynamic limit emerges as well. In contrast to the benchmark theories of generalized hydrodynamics and mode coupling, the present theory is near a priori, while achieving modestly better accuracy. Therefore, in our view, it constitutes an improvement over the traditional theories.

Tuning the Fragility of a Glass-Forming Liquid by Curving Space

We investigate the influence of space curvature, and of the associated frustration, on the dynamics of a model glass former: a monatomic liquid on the hyperbolic plane. We find that the system's fragility, i.e., the sensitivity of the relaxation time to temperature changes, increases as one decreases the frustration. As a result, curving space provides a way to tune fragility and make it as large as wanted. We also show that the nature of the emerging "dynamic heterogeneities", another distinctive feature of slowly relaxing systems, is directly connected to the presence of frustration-induced topological defects.

Interatomic interaction in metallic and dielectric monatomic liquids

The simplified model of pair potential of interatomic interaction is used for considering the nature of qualitative difference in the pattern of correlation between the critical temperature and critical volume for monatomic metallic and dielectric liquids. The groups of alkali metals and inert gases are considered.

Glasses of simple liquids with double-well interaction potential

Abstract Glasses of simple monatomic liquids were studied in cubic models containing 2744 atoms by using the molecular dynamics (MD) method under periodic boundary conditions. We use the pair double-well interaction potentials, which were recently developed by Engel et al. We found a glass transition in the system via cooling it from the melt toward T=0. Microstructure of a model is analyzed via radial distribution function (RDF), coordination number distributions, bond-angle distributions and Honeycutt–Andersen analysis. We found the existence of an icosahedral local order in the system which is enhanced with decreasing temperature. Evolution of structure of the system upon cooling from the melt and annealing effects on the structure of model are discussed.

Simulation of an atomistic dynamic field theory for monatomic liquids: Freezing and glass formation

We examine a phase field crystal model for simple liquid-solid systems consisting of a free energy functional related to the Ramakrishnan-Yussouff free energy of classical density functional theory and an equation of motion capable of describing long-time-scale behavior in the deeply supercooled regime. The thermodynamics and dynamics of freezing and glass formation in this model system are studied through large-scale three-dimensional Langevin simulations. At low cooling rates bcc crystals are formed by nucleation and growth from the melt. At large cooling rates no clear glass transition is observed, but a kinetically driven first-order transition from supercooled liquid to a disordered glasslike solid does occur. Despite the peculiarities of the transition, the structure and properties of the resulting disordered solid are shown to strongly resemble those of a typical glass. Consequences of pseudocritical behavior and heterogeneity near the liquid spinodal are also discussed.

Collective dynamics of simple liquids: A mode-coupling description

We use the mode-coupling theory (MCT), which has been highly successful in accounting for the anomalous relaxation behaviour near the liquid-to-glass transition, for describing the dynamics of monatomic (i.e. simple) liquids away from the glass formation regime. We find that the dynamical structure factor predicted by MCT compares well to experimental findings and results of computer simulations. The memory function exhibits a two-step decay as found frequently in experimental and simulation data. The long-time relaxation regime, in which the relaxation rate strongly depends on the density and is identified as the α relaxation. At high density this process leads the glass instability. The short-time relaxation rate is fairly independent of density.

低密度アルカリ金属流体の構造研究

Liquid alkali metals in the vicinity of the triple point behave as a simple monatomic metallic liquid. However, as the volume expands under high-pressure and temperature condition, they undergo a metal-nonmetal transition around the critical density. In order to investigate the correlation between the electronic and structural properties, we have performed x-ray diffraction and small angle x-ray scattering measurements for fluid rubidium and cesium up to supercritical conditions using synchrotron radiation at SPring-8. Experimental observation indicates that with volume expansion the nearest neighbor distance starts to decrease and the density fluctuation increases even in the density range where the fluids are metallic. The electronic density where such structural inhomogeneity appears corresponds to the theoretically calculated threshold density where the compressibility of the homogeneous interacting electron gas turns to negative. This finding suggests that the observed structural changes reflect instability of the electron gas.

Sedimentation of isotope atoms in monatomic liquid Se

A strong gravitational field resulted in the sedimentation of isotope atoms in monatomic liquid. The concentration ratio Se82∕Se76 increased by greater than 3.5% in specimen ultracentrifuged at (0.7–0.9)×106G and at 300°C. The recovered sample had a feather-shaped crystalline morphology. The concentration gradient was nearly twice that of the steady state analytical result (ideal gas system), indicating a nonideal system diffusion. The present result is evidence of sedimentation of substitutional atoms in condensed matter via self-diffusion and suggestes its possible application to isotope separation, crystalline control, and matter dynamics in massive star.

Single-random-valley approximation in vibration-transit theory of liquid dynamics

The first goal of vibration-transit theory is to be able to calculate from a tractable partition function and without adjustable parameters the thermodynamic properties of the elemental monatomic liquids. The key hypothesis is that the random class of potential energy valleys dominates the statistical mechanics of the liquid at temperatures above melting T approximately greater than Tm and that these valleys are macroscopically uniform in the thermodynamic limit. This allows us to use a single random valley to calculate the vibrational contribution to liquid properties, exactly in the thermodynamic limit, and as an approximation at finite number of particles N . This approximation is tested here for liquid Na with a physically realistic potential based on electronic structure theory. Steepest descent quenches were made from the molecular dynamics equilibrium liquid (N=500) at temperatures from 0.90Tm to 3.31Tm, and six potential parameters were calculated for each structure, namely, the potential energy and five principal moments of the vibrational frequency distribution. The results show temperature-independent means and small standard deviations for all potential parameters, consistent with random valley uniformity at N-->infinity, and with finite- N broadening at N=500. The expected error in the single random valley approximation for Na at N=500 and T approximately greater than Tm is 0.1% for the entropy and 0.5% for the internal energy, negligible in the current development of liquid dynamics theory. In related quench studies of recent years, the common finding of nearly temperature-independent means of structural potential energy properties at T approximately greater than Tm suggests that the single random valley approximation might also apply to systems more complicated than the elemental liquids.

Absence of a structural glass phase in a monatomic model liquid predicted to undergo anideal glass transition

We study numerically a two-dimensional monodisperse model of interacting classicalparticles predicted to exhibit a static liquid–glass transition. Using a dynamical MonteCarlo method we show that the model does not freeze into a glassy phase at lowtemperatures. Instead, depending on the choice of the hard-core radius for the particles, thesystem either collapses trivially or a polycrystalline hexagonal structure emerges.

Vitrification of a monatomic metallic liquid

Although the majority of glasses in use in technology are complex mixtures of oxides or chalcogenides, there are numerous examples of pure substances-'glassformers'-that also fail to crystallize during cooling. Most glassformers are organic molecular systems, but there are important inorganic examples too, such as silicon dioxide and elemental selenium (the latter being polymeric). Bulk metallic glasses can now be made; but, with the exception of Zr50Cu50 (ref. 4), they require multiple components to avoid crystallization during normal liquid cooling. Two-component 'metglasses' can often be achieved by hyperquenching, but this has not hitherto been achieved with a single-component system. Glasses form when crystal nucleation rates are slow, although the factors that create the slow nucleation conditions are not well understood. Here we apply the insights gained in a recent molecular dynamics simulation study to create conditions for successful vitrification of metallic liquid germanium. Our results also provide micrographic evidence for a rare polyamorphic transition preceding crystallization of the diamond cubic phase.

Experimental study of the heat transport processes in dusty plasma fluid

The results are given of an experimental investigation of heat transport processes in fluid dusty structures in rf-discharge plasmas under different conditions: for discharge in argon, and for discharge in air under an action of electron beam. The analysis of steady-state and unsteady-state heat transfer is used to obtain the coefficients of thermal conductivity and thermal diffusivity under the assumption that the observed heat transport is associated with a thermal conduction in the dusty component of plasmas. The temperature dependence of these coefficients is obtained, which agrees qualitatively with the results of numerical simulation for simple monatomic liquids.

Classical vs Quantum Vibrational Energy Relaxation Pathways in Solvated Polyatomic Molecules

Vibrational energy relaxation (VER) of solvated polyatomic molecules can occur via different pathways. In this paper, we address the question of whether treating VER classically or quantum-mechanically can lead to different predictions with regard to the preferred pathway. To this end, we consider the relaxation of the singly excited asymmetric stretch of a rigid, symmetrical, and linear triatomic molecule (A-B-A) in a monatomic liquid. In this case, VER can occur either directly to the ground state or indirectly via intramolecular vibrational relaxation (IVR) to the symmetric stretch. We have calculated the rates of these two different VER pathways via classical mechanics and the linearized semiclassical (LSC) method. When the mass of the terminal A atoms is significantly larger than that of the central B atom, we find that LSC points to intermolecular VER as the preferred pathway, whereas the classical treatment points to IVR. The origin of this trend reversal appears to be purely quantum-mechanical and can be traced back to the significantly weaker quantum enhancement of solvent-assisted IVR in comparison to that of intermolecular VER.

Shear rate dependence of viscosity and configurational entropy of the Zr 41.2Ti 13.8Cu 12.5Ni 10.0Be 22.5 metallic glass forming liquid

In this study the viscosity of Zr 41.2Ti 13.8Cu 12.5Ni 10.0Be 22.5 (Vit 1) has been measured as a function of temperature and shear rate above the liquidus temperature, T liq = 1026 K, using a high temperature Couette Rheometer. It has been discovered that there exists a pronounced decrease in viscosity with increasing shear rate. This is contrary to the general belief that metallic systems above the liquidus temperature should show Newtonian behavior. It has also been discovered that this shear rate dependence of the viscosity decreases with increasing temperature and approaches the Newtonian behavior and viscosities of simple monatomic liquids or binary alloys at approximately 1300 K. These results are discussed in terms of configurational entropy, S c, by using the Adam–Gibbs entropy model for viscous flow. It is suggested that a large amount of short-range order still exists above the liquidus temperature. At high shear rates this order is destroyed causing the configurational entropy to increase by a magnitude that is on the order of the entropy of fusion.

Vibrational Energy Relaxation of Polyatomic Molecules in Liquid Solution via the Linearized Semiclassical Method

Vibrational energy relaxation (VER) of polyatomic, as opposed to diatomic, molecules can occur via different, often solvent assisted, intramolecular and/or intermolecular pathways. In this paper, we apply the linearized semiclassical (LSC) method for calculating VER rates in the prototypical case of a rigid, symmetrical and linear triatomic molecule (A-B-A) in a monatomic liquid. Starting at the first excited state of either the symmetric or asymmetric stretches, VER can occur either directly to the ground state or indirectly via intramolecular vibrational relaxation (IVR). The VER rate constants for the various pathways are calculated within the framework of the Landau-Teller formalism, where they are expressed in terms of two-time quantum-mechanical correlation functions. The latter are calculated by the LHA-LSC method, which puts them in a "Wignerized" form, and employs a local harmonic approximation (LHA) in order to compute the necessary multidimensional Wigner integrals. Results are reported for the LHL/Ar model of Deng and Stratt [J. Chem. Phys. 2002, 117, 1735], as well as for CO(2) in liquid argon and in liquid neon. The LHA-LSC method is shown to give rise to significantly faster VER and IVR rates in comparison to the classical treatment, particularly at lower temperatures. We also find that the type and extent of the quantum rate enhancement is strongly dependent on the particular VER pathway. Finally, we find that the classical and semiclassical treatments can give rise to opposite trends when it comes to the dependence of the VER rates on the solvent.

Atomic dynamics in liquid alkali metals at the melting point

The atomic dynamics in liquid alkali metals at the melting point has been studied with the help of an equation of motion in terms of the velocity autocorrelation function. An atom is considered to be moving in a mean-time-dependent field, in a series of steps much smaller than the average interatomic spacing (small-step diffusion approximation). The theoretical formulation presented here is a special case of our generalized formulation for simple monatomic liquids [Phys. Rev. E. 70, 51201 (2004)]. It gives a good account of the single-particle motion in liquid Li, Na, K, Rb, and Cs. The calculated self-diffusion coefficients are found to be in excellent agreement with the experiments. The formulation is also used to investigate the collective excitations arising due to density fluctuations. The dispersion curves for the collective excitations for Li, Na, and K show a decent overall agreement with the inelastic neutron scattering and inelastic x-ray scattering results. The effect of quantum-mechanical correction has been observed for Li, Na, and K. It suggests a significant improvement over the classical results. Since the applicability of the small-step diffusion approximation is limited to weak electron-ion coupling at high electron densities, the present formulation gives satisfactory dispersion curves for Rb and Cs in a small intermediate $q$ range. It has been observed that an abrupt change in the dynamical properties occurs at the position of potassium in accordance with the change in atomic number density.

Diffusivity, excess entropy, and the potential-energy landscape of monatomic liquids

The connection between thermodynamic, transport, and potential-energy landscape features is studied for liquids with Lennard-Jones-type pair interactions using both microcanonical molecular-dynamics and isothermal-isobaric ensemble Monte Carlo simulations. Instantaneous normal-mode and saddle-point analyses of two variants of the monatomic Lennard-Jones liquid have been performed. The diffusivity is shown to depend linearly on several key properties of instantaneous and saddle configurations-the energy, the fraction of negative curvature directions, and the mean, maximum, and minimum eigenvalues of the Hessian. Since the Dzugutov scaling relationship also holds for such systems [Nature (London) 381, 137 (1996)], the exponential of the excess entropy, within the two-particle approximation, displays the same linear dependence on energy landscape properties as the diffusivity.

A model for transits in dynamic response theory

The first goal of vibration-transit (V-T) theory was to construct a tractable approximate Hamiltonian from which the equilibrium thermodynamic properties of monatomic liquids can be calculated. The Hamiltonian for vibrations in an infinitely extended harmonic random valley, together with the universal multiplicity of such valleys, gives an accurate first-principles account of the measured thermodynamic properties of the elemental liquids at melt. In the present paper, V-T theory is extended to nonequilibrium properties, through an application to the dynamic structure factor S(q,omega). It was previously shown that the vibrational contribution alone accurately accounts for the Brillouin peak dispersion curve for liquid sodium, as compared both with molecular-dynamics (MD) calculations and inelastic x-ray scattering data. Here it is argued that the major effects of transits will be to disrupt correlations within the normal-mode vibrational motion and to provide an additional source of inelastic scattering. We construct a parametrized model for these effects and show that it is capable of fitting MD results for S(q,omega) in liquid sodium. A small discrepancy between model and MD at large q is attributed to multimode vibrational scattering. In comparison, mode coupling theory formulates S(q,omega) in terms of processes through which density fluctuations decay. While mode coupling theory is also capable of modeling S(q,omega) very well, V-T theory is the more universal since it expresses all statistical averages, thermodynamic functions, and time correlation functions alike, in terms of the same motional constituents, vibrations and transits.

Molecular Dynamics Study of the Influence of Surfactant Structure on Surfactant-Facilitated Spreading of Droplets on Solid Surfaces

The spreading of a partially wetting aqueous drop in air on a hydrophobic surface can be facilitated by the adsorption of surfactants from the drop phase onto the air/aqueous and aqueous/hydrophobic solid interfaces of the drop. At the contact line at which these interfaces meet, conventional surfactants with a linear alkyl hydrophobic chain attached to a polar group adsorb onto the surfaces, forming monolayers which remain distinct as they merge at the contact juncture. The adsorption causes a decrease in the interfacial tensions and reduction in the contact angle but the angle remains above zero so the drop is still nonwetting. Trisiloxane surfactants with a T-shaped geometry in which the hydrophobic group is composed of a trisiloxane oligomer with a polar group attached at the center of the chain can give rise to a zero contact angle at the contact line and complete wetting (superspreading). Experimental evidence suggests the adsorption of the T-shaped molecule, in addition to significantly decreasing the tensions of the interfaces (relative to the conventional surfactants), promotes the formation of a precursor film consisting of a surfactant bilayer at the contact line which facilitates the spreading. The aim of this study is to use molecular dynamics to examine if the T-shaped structure can promote spreading by the formation of a bilayer and to contrast this case with that of the linear chain surfactant where complex assembly does not occur. The simulation models the solvent as a monatomic liquid, the substrate as a particle lattice, and the surfactants as united atom structures, with all interactions given by Lennard-Jones potentials. We start with a base case in which the solvent partially wets a substrate comprised of a lattice of particles. We demonstrate that adsorbed T-shaped surfactant monolayers can, when the interaction between the solvent and the hydrophile particles is strong enough, assemble into a bilayer, allowing the drop to extend to a thin planar film. In the case of the flexible linear chain surfactant, there is no interaction between the monolayers on the two interfaces in the case of a strong hydrophile-solvent interaction and less coordination for a weaker interaction. In either case, the monolayers remain distinct, as the surfactant only marginally improves wetting.