Attraction Between Like-Charged Walls: Short-Ranged Simulations Using Local Molecular Field Theory

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

Heterogeneities in Supercooled liquids: A Density Functional Study

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.Comment: 10 pages, 6 figure

Stability of Amorphous Structures with Voids

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. The transformation of a continuum ergodic liquid into an amorphous glassy state with solidlike behaviour has been an area of much current research interest. The ambiguities concerning the glassy or the supercooled state have led to the development of a large number of theoretical approaches [1, 2, 3, 4] that are used to study the basic characteristics of

Dynamic heterogeneities in a simple liquid over different time scales

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 (r2(t))~ta, of a tagged particle is computed. The non-Gaussian parameter α 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 α 2(t) hardly shifts, while that of the other grows with density

Tagged particle motion in a dense liquid: feedback effects from the collective dynamics

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 ψ 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 α 2(t) is evaluated using the fourth- and second-order spatial moments of the van Hove self-correlation function Gs(r,t). We observe a two-peaked structure of α2(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 Gs(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

Metastable structures with modified weighted density-functional theory

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

Static and dynamical heterogeneities in supercooled liquids

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

Effect of attractive interactions on tagged particle dynamics

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

Nature of the entropy versus self-diffusivity plot for simple liquids

The empirical relation (D∗)α = 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 α 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

Optimum vacancy concentration in a crystal

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 ~10-5 and it decreases with increase of the density