[Show abstract][Hide abstract] ABSTRACT: Effective attraction between like-charged walls mediated by counterions is studied using local molecular field (LMF) theory. Monte Carlo simulations of the "mimic system" given by LMF theory, with short-ranged "Coulomb core" interactions in an effective single particle potential incorporating a mean-field average of the long-ranged Coulomb interactions, provide a direct test of the theory, and are in excellent agreement with more complex simulations of the full Coulomb system by Moreira and Netz [Eur. Phys. J. E 8, 33 (2002)]. A simple, generally applicable criterion to determine the consistency parameter sigma(min) needed for accurate use of the LMF theory is presented.
[Show abstract][Hide abstract] ABSTRACT: A simple model used to explore the interaction between like-charged macroions as mediated by intervening counterions is treated with local molecular field theory (LMF). LMF has recently been extended to general Coulombic systems by splitting the Coulomb potential 1/r into a short-ranged core that can be explicitly simulated and a long-ranged portion treated using a mean field approach; the potential separation is determined by a physically-relevant spacing parameter sigma. Here we show that LMF can treat the two-wall model system surprisingly well using an analytical Poisson-Boltzmann type technique. Also, combining self-consistent solution of LMF with simulation of the short-ranged core particles using the minimum image convention yields even more accurate results without using costly and complex Lekner or Ewald sums.
[Show abstract][Hide abstract] ABSTRACT: The importance of the presence of a small fraction of vacancies in a crystal structure is demonstrated from considerations of thermodynamic stability. We include in the density functional theory the effects due to the distortion of the lattice structure surrounding the vacancy and show that the free energy is less when vacancies are present. Near freezing point, our theoretical model obtains the equilibrium vacancy fraction in the hard sphere crystal to be approximately 10(-5) and it decreases with increase of the density.
[Show abstract][Hide abstract] ABSTRACT: The empirical relation (D(*))(alpha) = a exp[S] between the self-diffusion coefficient D(*) and the excess entropy S of a liquid is studied here in the context of theoretical model calculation. The coefficient alpha is dependent on the interaction potential and shows a crossover at an intermediate density, where cooperative dynamics become more important. Around this density a departure from the Stokes-Einstein relation is also observed. The above relation between entropy and diffusion is also tested for the scaled total diffusion coefficient in a binary mixture.
The Journal of Chemical Physics 08/2005; 123(3):34501. DOI:10.1063/1.1942488 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Structural and thermodynamic properties of ionic fluids are related to those of a simpler ``mimic'' system with short ranged intermolecular interactions in a spatially varying effective field by use of Local Molecular Field (LMF) theory, already successfully applied to nonuniform simple fluids. By consistently using the LMF approximation to describe only the slowly varying part of the Coulomb interaction, which we view as arising from a rigid Gaussian charge distribution with an appropriately chosen width $\sigma$, exceptionally accurate results can be found. In this paper we study a uniform system of charged hard spheres in a uniform neutralizing background, where these ideas can be presented in their simplest form. At low densities the LMF theory reduces to a generalized version of the Poisson-Boltzmann approximation, but the predicted structure factor satisfies the exact Stillinger-Lovett moment conditions, and with optimal choice of $\sigma$ the lowest order approximation remains accurate for much stronger couplings. At high density and strong couplings the pair correlation function in the uniform mimic system with short ranged interactions is very similar to that of the full ionic system. A simple analytic formula can then describe the difference in internal energy between the ionic system and the associated mimic system.
The Journal of Physical Chemistry B 12/2004; 108(51). DOI:10.1021/jp0469261 · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A Comment on the Letter by Alok Samanta, Sk. Musharaf Ali, and Swapan K. Ghosh [Phys. Rev. Lett.PRLTAO0031-9007 87, 245901 (2001)10.1103/PhysRevLett.87.245901]. The authors of the Letter offer a Reply.
[Show abstract][Hide abstract] ABSTRACT: We study tagged particle dynamics in a one-component simple liquid characterized by the Lennard-Jones (LJ) interaction potential. Extended mode coupling theory is used to obtain the correlation function which feeds back on the dynamics of the self-correlations. The cooperative dynamical effects are studied by evaluating various properties of tagged particle motion as influenced by the collective dynamics. Comparison between the results obtained for particles with purely repulsive interactions like the truncated LJ potential (or the hard-sphere interaction) and that of the full LJ potential are shown. The nature of the velocity autocorrelation function and the non-Gaussian variation of the van Hove self-correlation function is specifically highlighted here. The role of static structural input in the theory is considered especially in this regard.
[Show abstract][Hide abstract] ABSTRACT: The nature of the tagged particle motion in the strongly correlated state of a dense liquid is studied with the self-consistent mode-coupling model. The tagged particle time correlation function psi(s)(q,t) is computed by taking into account the nonlinear feedback effects on its dynamics from the coupling with density fluctuations. We consider the two cases where (a) the short-time dynamics is diffusive resembling colloidal system and (b) the short-time dynamics is Newtonian as in an atomic system. The non-Gaussian parameter alpha(2)(t) is evaluated using the fourth- and second-order spatial moments of the van Hove self-correlation function G(s)(r,t). We observe a two-peaked structure of alpha(2)(t) for both (a) and (b) types of dynamics. We also compare other characteristic aspects of tagged particle dynamics such as the mean square displacement, non-Gaussian nature of G(s)(r,t), and fraction of mobile particles. A qualitative comparison is drawn between the theoretical results with the experimental and computer simulation results on colloids.
[Show abstract][Hide abstract] ABSTRACT: We present here some features of supercooled liquids from both the structural and dynamical consideration. We discuss the existence of heterogenous structures characterized by weak mass localization. The analysis is done using the standard methods of density functional theory. The dynamics governing the supercooled states that lead to non-exponential relaxation of the time–correlation functions, are also studied. The dynamical heterogeneities are studied in terms of departure of the single particle dynamics from simple Gaussian behavior over intermediate timescales. Other relevant properties like the velocity auto-correlation function and the mean square displacement associated with the tagged particle dynamics are also computed.
Physica A: Statistical Mechanics and its Applications 02/2003; 318(1-2-318):121-128. DOI:10.1016/S0378-4371(02)01414-0 · 1.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The heterogeneous features of the supercooled state over different time regimes are explored in a self-consistent mode-coupling mode. The exponent a for the mean-square displacement <r(2)(t)> approximately t(a), of a tagged particle is computed. The non-Gaussian parameter alpha(2)(t) shows a peak in the short time regime in addition to a second peak over longer times. The position of the short-time peak in alpha(2)(t) hardly shifts, while that of the other grows with density.
[Show abstract][Hide abstract] ABSTRACT: One component Lennard–Jones fluid in the supercooled state has been recently studied by Angelani et al.  using the master equation approach to model the dynamics in the configuration space. Here we consider these results in the context of the extended mode coupling model (MCT). The crucial parameter related to the ergodicity restoring processes in the theoretical model is chosen such that transport coefficients calculated from the two approaches match. This parameter follows an activated behavior in order to produce the Arrhenius-type temperature dependence for the viscosity, as found in the master equation approach. Using the corresponding parameter values, we compute other dynamical properties of the supercooled LJ system.
Physics Letters A 07/2002; 300(2):291-297. DOI:10.1016/S0375-9601(02)00822-8 · 1.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The free energy of the supercooled liquid near freezing is studied in the density-functional approach using the modified weighted density approximation. A class of minima corresponding to heterogeneous structures characterized by weak mass localization are detected. The stability of these structures is found to be greater than the highly localized "hard-sphere glass" state in the intermediate density range above freezing.
[Show abstract][Hide abstract] ABSTRACT: The weighted density-functional theory is applied to investigate the free-energy landscape of dense supercooled liquids. Metastable states intermediate to the liquid and crystal phases are found, which can be identified with the supercooled states seen in computer simulations. These states are marked by a lower degree of mass localization as compared to the highly localized state termed as "hard-sphere glass" found in earlier studies. We evaluate the free energy using the modified weighted density approximation (MWDA), as formulated by Denton and Ashcroft (1989) Phys. Rev. A , 39 , 4701. The inhomogeneous density is parametrized in terms of Gaussian profiles centered around random lattice sites. The effects of heterogeneity coming from a fluctuation of the width of these Gaussian profiles show that the free energy of the system increases with increase in the fluctuations and, finally, the metastable minima disappear with growing fluctuations.
[Show abstract][Hide abstract] ABSTRACT: A metastable state, characterized by a low degree of mass localization is identified using Density Functional Theory. This free energy minimum, located through the proper evaluation of the competing terms in the free energy functional, is independent of the specific form of the DFT used. Computer simulation results on particle motion indicate that this heterogeneous state corresponds to the supercooled state. PACS number(s) : 64.70Pf, 64.60 Cn, 64.70 DvThe properties of supercooled liquids are of much current interest and are being studied from different approaches related to their thermodynamic as well as dynamic aspects -. A normal liquid is characterized by a homogenous density and when it is supercooled below the freezing transition it continues to stay in the amorphous phase. With the increase in density a solid like phase is formed with the particles being localized around their mean positions on a random structure. The underlying lattice on which such localized motion takes place is related to the time scales of relaxation in the supercooled liquid. While the supercooled liquid starts to attain solid like properties, structurally it does not have any long range order like the one present in a crystal. The Heterogeneity in glassy systems over
[Show abstract][Hide abstract] ABSTRACT: We incorporate the role of free volume in the density function of the amorphous structure and study its effects on the stability of such structures. The Density Functional Theory is used to explore this ``Free Volume Model'' of the supercooled structures. The Free energy minimization is carried out using the void concentration as a variational parameter. A critical value of this concentration exists corresponding to the Free energy minima of the amorphous structure. An increase in the stability is observed due to the inclusion of voids in the density structure. This study is conducted for both the weakly and highly localized amorphous structures. The free volume concentration shows a power law decrease with density for the weakly localized states and a linear decrease for the highly localized amorphous structures. Comment: 13 pages, 6 figures