Godehard Sutmann

Semidilute Polymer Solutions at Equilibrium and under Shear Flow

The properties of semidilute polymer solutions are investigated at equilibrium and under shear flow by mesoscale simulations, which combine molecular dynamics simulations and the multiparticle collision dynamics approach. In semidilute solution, intermolecular hydrodynamic and excluded volume interactions become increasingly important due to the presence of polymer overlap. At equilibrium, the dependence of the radius of gyration, the structure factor, and the zero-shear viscosity on the polymer concentration is determined and found to be in good agreement with scaling predictions. In shear flow, the polymer alignment and deformation are calculated as a function of concentration. Shear thinning, which is related to flow alignment and finite polymer extensibility, is characterized by the shear viscosity and the normal stress coefficients.

Structure formation and dynamics of water in strong external electric fields

We have performed molecular dynamics computer simulations of water in homogeneous external electric fields which were varied in a wide range of field strengths. The dielectric response is found to be linear up to fields E0≈0.01 V/A from where dielectric saturation effects become important. At fields of E0≈3 V/A a phase transition into an ordered, ice-like structure is observed, which is stabilized through hydrogen-bonds. With an increasing external electric field, the frequency spectrum of the water dynamics shows a remarkable red shift of the intramolecular modes and a blue shift of the librational motions, where the frequency varies quadratically with the field strength. A simple analytical model is discussed which reproduces the observed behavior.

Frequency and wave-vector dependent dielectric function of water: Collective modes and relaxation spectra

The longitudinal frequency and wave-vector dependent complex dielectric response function χ(k,ω)=1−1/e(k,ω) is calculated in a broad range of k values by means of molecular dynamics computer simulation for a central force model of water. Its imaginary part, i.e., Im{e(k,ω)}/|e(k,ω)|2, shows two main contributions in the region of small k values: Debye-like orientational relaxation in the lower frequency part of the spectrum and a damped librational resonance at the high frequency wing. The Debye relaxation time does not follow a de Gennes-like pattern: τ(k) goes through a maximum at k≈k*≈1.7 A−1, while the static polar structure factor S(k) peaks at k≈3 A−1. The resonance frequency ω(k) and the decay decrement γ(k) show a dispersion law, indicative of a decaying optical-like mode, the libron. With an approximate normal mode approach, we analyze the origin of this mode on a molecular level which shows that it is due to a damped propagation of molecular orientational vibrations through the network of hydrog...

Cell-level canonical sampling by velocity scaling for multiparticle collision dynamics simulations

A local Maxwellian thermostat for the multiparticle collision dynamics algorithm is proposed. The algorithm is based on a scaling of the relative velocities of the fluid particles within a collision cell. The scaling factor is determined from the distribution of the kinetic energy within such a cell. Thereby the algorithm ensures that the distribution of the relative velocities is given by the Maxwell-Boltzmann distribution. The algorithm is particularly useful for non-equilibrium systems, where temperature has to be controlled locally. We perform various non-equilibrium simulations for fluids in shear and pressure-driven flow, which confirm the validity of the proposed simulation scheme. In addition, we determine the dynamic structure factors for fluids with and without thermostat, which exhibit significant differences due to suppression of the diffusive part of the energy transport of the isothermal system.

The shape of the nonlocal dielectric function of polar liquids and the implications for thermodynamic properties of electrolytes: A comparative study

Consequences of different forms of the nonlocal dielectric function e(k) of a polar solvent in thermodynamics of electrolytes are studied. This is done with the help of an analytical approximation which covers, as particular cases, different types of results obtained by several groups on the basis of molecular theories or computer simulations. It is shown that the forms of e(k) which include a range of negative values may not contradict experimental data for hydration energies (in contrast to which was argued before) when one goes beyond the simple Born model of an ion and accounts for higher frequency mode contributions. A study of the implications for the activity coefficents shows that oscillations of the interaction energy between ions, following from such forms of e(k), lead to a compensation effect which brings the results closer to the prediction of classical electrostatics and to experimental data.

Tumbling of polymers in semidilute solution under shear flow

The tumbling dynamics of individual polymers in semidilute solution is studied by large-scale non-equilibrium mesoscale hydrodynamic simulations. We find that the tumbling time is equal to the non-equilibrium relaxation time of the polymer end-to-end distance along the flow direction and strongly depends on concentration. In addition, the normalized tumbling frequency as well as the widths of the alignment distribution functions for a given concentration-dependent Weissenberg number exhibit a weak concentration dependence in the cross-over regime from a dilute to a semidilute solution. For semidilute solutions a universal behavior is obtained. This is a consequence of screening of hydrodynamic interactions at polymer concentrations exceeding the overlap concentration.

“Overscreening” in a polar liquid as a result of coupling between polarization and density fluctuations

Static nonlocal dielectric response of a polar liquid with linearly coupled polarization and density fluctuations is studied. We use a simple quadratic Landau-Ginzburg Hamiltonian to show that the symmetry of interaction between density and vectorial polarization might lead to a striking enhancement of the polarizability at a certain finite wavelength. A similar effect in water has been obtained in recent computer simulations in accordance with neutron diffraction data and predicted by molecular theories. Here, we suggest that the coupling between density and polarization fluctuations might be one of the sources of this behaviour and discuss the conditions for its realization. A closed-form expression is derived for the wave-vector (k-) dependent response function and compared with computer simulation and diffraction data. The expression may be used as a simple analytical framework for describing dielectric screening in polar liquids. Implications of the resonance shape of the response function in k-space are studied for the interionic mean force potential and for the reorganization energy of electron exchange reactions between two ions.

Nonlocal dielectric response of water and reorganization energies for outer sphere electron transfer reactions

New results on the nonlocal dielectric function of liquid water (Ph. A. Bopp, A. A. Kornyshev, G. Sutmann, Phys. Rev. Let. 76, 1280 (1996)) are discussed and used to calculate reorganization free energies for electron exchange reactions between solute ions. The energies are shown to be several times larger than those given by the classical Marcus formula for the born sphere model of an ion. However, they are reduced to Marcus or even lower values for modified charge density models, which take into account the delocalization of the ion excess charge distribution.

Polymer conformations in ionic microgels in the presence of salt: Theoretical and mesoscale simulation results

We investigate the conformational properties of polymers in ionic microgels in the presence of salt ions by molecular dynamics simulations and analytical theory. A microgel particle consists of coarse-grained linear polymers, which are tetra-functionally crosslinked. Counterions and salt ions are taken into account explicitly, and charge-charge interactions are described by the Coulomb potential. By varying the charge interaction strength and salt concentration, we characterize the swelling of the polyelectrolytes and the charge distribution. In particular, we determine the amount of trapped mobile charges inside the microgel and the Debye screening length. Moreover, we analyze the polymer extension theoretically in terms of the tension blob model taking into account counterions and salt ions implicitly by the Debye–Hückel model. Our studies reveal a strong dependence of the amount of ions absorbed in the interior of the microgel on the electrostatic interaction strength, which is related to the degree of the gel swelling. This implies a dependence of the inverse Debye screening length κ on the ion concentration; we find a power-law increase of κ with the Coulomb interaction strength with the exponent 3/5 for a salt-free microgel and an exponent 1/2 for moderate salt concentrations. Additionally, the radial dependence of polymer conformations and ion distributions is addressed.

GASPI – A Partitioned Global Address Space Programming Interface

At the threshold to exascale computing, limitations of the MPI programming model become more and more pronounced. HPC programmers have to design codes that can run and scale on systems with hundreds of thousands of cores. Setting up accordingly many communication buffers, point-to-point communication links, and using bulk-synchronous communication phases is contradicting scalability in these dimensions. Moreover, the reliability of upcoming systems will worsen.