Effective Soft-Core Potentials and Mesoscopic Simulations of Binary Polymer Mixtures

Mesoscopic molecular dynamics simulations are used to determine the large scale structure of several binary polymer mixtures of various chemical architecture, concentration, and thermodynamic conditions. By implementing an analytical formalism, which is based on the solution to the Ornstein-Zernike equation, each polymer chain is mapped onto the level of a single soft colloid. From the appropriate closure relation, the effective, soft-core potential between coarse-grained units is obtained and used as input to our mesoscale simulations. The potential derived in this manner is analytical and explicitly parameter dependent, making it general and transferable to numerous systems of interest. From computer simulations performed under various thermodynamic conditions the structure of the polymer mixture, through pair correlation functions, is determined over the entire miscible region of the phase diagram. In the athermal regime mesoscale simulations exhibit quantitative agreement with united atom simulations. Furthermore, they also provide information at larger scales than can be attained by united atom simulations and in the thermal regime approaching the phase transition.Comment: 19 pages, 11 figures, 3 table

Simulation study of the link between molecular association and reentrant miscibility for a mixture of molecules with directional interactions

The reentrant liquid-liquid miscibility of a symmetrical mixture with highly directional bonding interactions is studied by Gibbs ensemble Monte Carlo simulation. The resulting closed loop of immiscibility and the corresponding lower critical solution temperature are shown to be a direct consequence of the dramatic increase in association between unlike components as the temperature is lowered. Our exact calculations for an off-lattice system with a well-defined anisotropic potential confirm the findings of previous theoretical studies.Dirección General de Investigación Científica y Técnica PB94-144

Adaptive discontinuous evolution Galerkin method for dry atmospheric flow

We present a new adaptive genuinely multidimensional method within the framework of the discontinuous Galerkin method. The discontinuous evolution Galerkin (DEG) method couples a discontinuous Galerkin formulation with approximate evolution operators. The latter are constructed using the bicharacteristics of multidimensional hyperbolic systems, such that all of the infinitely many directions of wave propagation are considered explicitly. In order to take into account multiscale phenomena that typically appear in atmospheric flows, nonlinear fluxes are split into a linear part governing the acoustic and gravitational waves and a nonlinear part that models advection. Time integration is realized by the IMEX type approximation using the semi-implicit second-order backward differentiation formula (BDF2). Moreover in order to approximate efficiently small scale phenomena, adaptive mesh refinement using the space filling curves via the AMATOS function library is employed. Four standard meteorological test cases are used to validate the new discontinuous evolution Galerkin method for dry atmospheric convection. Comparisons with the Rusanov flux, a standard one-dimensional approximate Riemann solver used for the flux integration, demonstrate better stability and accuracy, as well as the reliability of the new multidimensional DEG method

Adaptive discontinuous evolution Galerkin method for dry atmospheric flow

We present a new adaptive genuinely multidimensional method within the frame-work of the discontinuous Galerkin method. The discontinuous evolution Galerkin (DEG) method couples a discontinuous Galerkin formulation with approximate evo-lution operators. The latter are constructed using the bicharacteristics of multidi-mensional hyperbolic systems, such that all of the infinitely many directions of wave propagation are considered explicitly. In order to take into account multiscale phe-nomena that typically appear in atmospheric flows nonlinear fluxes are split into a linear part governing the acoustic and gravitational waves and to the rest nonlinear part that models advection. Time integration is realized by the IMEX type ap-proximation using the semi-implicit second-order backward differentiation formulas (BDF2) scheme. Moreover in order to approximate efficiently small scale phenom-ena adaptive mesh refinement using the space filling curves via AMATOS function library is applied. Three standard meteorological test cases are used to validate the new discontinuous evolution Galerkin method for dry atmospheric convection. Comparisons with the standard one-dimensional approximate Riemann solver used for the flux integration demonstrate better stability, accuracy as well as reliability of the new multidimensional DEG method. Key words: dry atmospheric convection, steady states, systems of hyperbolic balance laws, Euler equations, large time step, semi-implicit approximation, evolution Galerkin scheme