Sten Sarman

Bio: Sten Sarman is an academic researcher from Stockholm University. The author has contributed to research in topics: Liquid crystal & Shear flow. The author has an hindex of 24, co-authored 69 publications receiving 1566 citations. Previous affiliations of Sten Sarman include Oak Ridge National Laboratory & Australian National University.

Microstructural and Dynamical Heterogeneities in Ionic Liquids.

Abstract: Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of ILs lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

Recent developments in non-Newtonian molecular dynamics

Abstract: In just 25 years, nonequilibrium molecular dynamics (NEMD) has gone from a largely empirical molecular simulation methodology based on reproducing planar Couette flow in brute force fashion to a fully developed subfield of molecular simulation, underpinned rigorously by linear and nonlinear response theory, with prescriptions now available to simulate synthetically, in thermodynamically homogeneous systems, all of the transport properties measured experimentally (viscosity, self- and mutual diffusion coefficients, thermal conductivity, and Soret and Dufor coefficients). Many of these developments were reviewed in the 1990 monograph by Evans and Morriss (Statistical Mechanics of Nonequilibrium Liquids, Academic Press, New York, 1990). However, progress in this field has been very rapid since 1990, and this review describes some of the major developments over the intervening period. These include extensions of the NEMD simple-fluid algorithms for viscosity and thermal conductivity to rigid nonspherical molecules, coupling of thermal conductivity and mass diffusion in mixtures, calculation of transport properties (diffusion coefficient and thermal conductivity) in systems subjected to nonlinear shear, application of NEMD to model liquid crystal systems, and the use of NEMD simulation to understand the nonlinear dynamics of nonequilibrium steady states.

Statistical mechanics of viscous flow in nematic fluids

Abstract: We derive Green–Kubo (GK) relations for the viscosity coefficients of nematic liquid crystals. These GK relations are similar to, but considerably more complicated than, those of an isotropic fluid. In addition to shear viscosities there are also twist viscosities and cross couplings between the symmetric strain rate and the antisymmetric pressure tensor and vice versa. We show that the twist viscosity is inversely proportional to the mean square displacement of the director. Using the so‐called SLLOD equations of motion we construct nonequilibrium molecular dynamics (NEMD) algorithms that can be used to efficiently calculate the viscosity coefficients of nematic liquid crystals from atomistic computer simulations. We also devise an additional NEMD algorithm for controlling the angular velocity of the director in a nematic fluid. We derive a fluctuation relation for the alignment angle between the director and the streamlines in planar Couette flow and also for the shear induced molecular angular velocity. In an isotropic fluid, close to equilibrium, this angular velocity is equal to half the vorticity. In a nematic liquid crystal it is nearly zero because of cross couplings with the symmetric part of the strain rate tensor. We test the Green–Kubo relations and the NEMD algorithms in a nematic liquid crystal modeled using a modified version of the Gay–Berne potential. In general, the Green–Kubo and NEMD results agree very well.

Heat flow and mass diffusion in binary Lennard-Jones mixtures

Abstract: We have used the Evans-Cummings (EC) nonequilibrium-molecular-dynamics (NEMD) heat-flow algorithm for liquid mixtures to calculate the thermal conductivity and the Dufour coefficient of a binary Lennard-Jones mixture. A consistency check has been carried out by applying the NEMD color-conductivity algorithm to evaluate the mutual diffusion and the Soret coefficients, which, according to the Onsager reciprocal relations, are equal. Further checking was carried out by comparing these NEMD results with equilibrium Green-Kubo calculations. This work extends that recently carried out by us [S. Sarman and D. J. Evans, Phys. Rev. A 45, 2370 (1992), hereafter referred to as I] to a much lower density and higher temperature than that studied in I. At this state point the Soret and Dufour coefficients are much larger than their corresponding triple-point values in I. This enables considerably more accurate checks of algorithm validity. The NEMD and the Green-Kubo estimates for the thermal conductivity and the mutual diffusion coefficients agree within statistical uncertainties of about 2%. The difference between the cross-coupling coefficients obtained by the various methods is more than twice as accurate as in I, namely, 6% compared to 15%. This provides a convincing check of the correctness of the EC algorithm. We have also developed a way of applying the EC algorithm that increases the accuracy of estimates of the Dufour coefficient. When this method was applied to the triple-point fluid, the results agreed with those in I; however, the errors were reduced by a factor of 3.

The chemical potential of simple fluids in a common class of integral equation closures

Abstract: An explicit formula for the chemical potential (μ) of simple fluids is derived for a whole class of integral equation theories, including the Percus–Yevick approximation and some other, more recently proposed closures. This formula only requires the pair correlation functions for one single state of the system, and applies to both homogeneous and inhomogeneous fluids. The coupling parameter integration method to calculate the chemical potential—where one particle is gradually coupled to the rest—is also investigated. It is shown that the μ value obtained in the approximate theories is not unique, but depends on the integration path. This behavior is due to inconsistencies of the approximation, which are discussed in some detail. For a certain choice of integration path the μ values obtained using the latter method agree with those from the explicit formula. Numerical results for the different closures are presented for a hard sphere fluid. The accuracy of μ depends strongly on the quality of the bridge function inside the hard core diameter.

Phase separation in confined systems

Abstract: We review the current state of knowledge of phase separation and phase equilibria in porous materials. Our emphasis is on fundamental studies of simple fluids (composed of small, neutral molecules) and well-characterized materials. While theoretical and molecular simulation studies are stressed, we also survey experimental investigations that are fundamental in nature. Following a brief survey of the most useful theoretical and simulation methods, we describe the nature of gas‐liquid (capillary condensation), layering, liquid‐liquid and freezing/melting transitions. In each case studies for simple pore geometries, and also more complex ones where available, are discussed. While a reasonably good understanding is available for phase equilibria of pure adsorbates in simple pore geometries, there is a need to extend the models to more complex pore geometries that include effects of chemical and geometrical heterogeneity and connectivity. In addition, with the exception of liquid‐liquid equilibria, little work has been done so far on phase separation for mixtures in porous media.

From 1970 until present: the Keller-Segel model in chemotaxis and its consequences

Abstract: This article summarizes various aspects and results for some general formulations of the classical chemotaxis models also known as Keller-Segel models. It is intended as a survey of results for the most common formulation of this classical model for positive chemotactical movement and offers possible generalizations of these results to more universal models. Furthermore it collects open questions and outlines mathematical progress in the study of the Keller-Segel model since the first presentation of the equations in 1970.

Dynamical ensembles in stationary states

Abstract: We propose, as a generalization of an idea of Ruelle's to describe turbulent fluid flow, a chaotic hypothesis for reversible dissipative many-particle systems in nonequilibrium stationary states in general. This implies an extension of the zeroth law of thermodynamics to nonequilibrium states and it leads to the identification of a unique distribution μ describing the asymptotic properties of the time evolution of the system for initial data randomly chosen with respect to a uniform distribution on phase space. For conservative systems in thermal equilibrium the chaotic hypothesis implies the ergodic hypothesis. We outline a procedure to obtain the distribution μ: it leads to a new unifying point of view for the phase space behavior of dissipative and conservative systems. The chaotic hypothesis is confirmed in a nontrivial, parameter-free, way by a recent computer experiment on the entropy production fluctuations in a shearing fluid far from equilibrium. Similar applications to other models are proposed, in particular to a model for the Kolmogorov-Obuchov theory for turbulent flow.

Motions and relaxations of confined liquids

Abstract: When a liquid is confined in a narrow gap (as near a cell membrane, in a lubricated contact between solids, or in a porous medium), new dynamic behavior emerges. The effective shear viscosity is enhanced compared to the bulk, relaxation times are prolonged, and nonlinear responses set in at lower shear rates. These effects are more prominent, the thinner the liquid film. They appear to be the manifestation of collective motions. The flow of liquids under extreme confinement cannot be understood simply by intuitive extrapolation of bulk properties. Practical consequences are possible in areas from tribology and materials processing to membrane physics.

Fluctuation-dissipation: Response theory in statistical physics

Abstract: General aspects of the Fluctuation–Dissipation Relation (FDR), and Response Theory are considered. After analyzing the conceptual and historical relevance of fluctuations in statistical mechanics, we illustrate the relation between the relaxation of spontaneous fluctuations, and the response to an external perturbation. These studies date back to Einstein’s work on Brownian Motion, were continued by Nyquist and Onsager and culminated in Kubo’s linear response theory. The FDR has been originally developed in the framework of statistical mechanics of Hamiltonian systems, nevertheless a generalized FDR holds under rather general hypotheses, regardless of the Hamiltonian, or equilibrium nature of the system. In the last decade, this subject was revived by the works on Fluctuation Relations (FR) concerning far from equilibrium systems. The connection of these works with large deviation theory is analyzed. Some examples, beyond the standard applications of statistical mechanics, where fluctuations play a major role are discussed: fluids, granular media, nanosystems and biological systems.