K. Mussawisade

Dynamics of polymers in a particle-based mesoscopic solvent

We study the dynamics of flexible polymer chains in solution by combining multiparticle-collision dynamics (MPCD), a mesoscale simulation method, and molecular-dynamics simulations. Polymers with and without excluded-volume interactions are considered. With an appropriate choice of the collision time step for the MPCD solvent, hydrodynamic interactions build up properly. For the center-of-mass diffusion coefficient, scaling with respect to polymer length is found to hold already for rather short chains. The center-of-mass velocity autocorrelation function displays a long-time tail which decays algebraically as (Dt)(-3/2) as a function of time t, where D is the diffusion coefficient. The analysis of the intramolecular dynamics in terms of Rouse modes yields excellent agreement between simulation data and results of the Zimm model for the mode-number dependence of the mode-amplitude correlation functions.

Rod-like colloids and polymers in shear flow: a multi-particle-collision dynamics study

The effect of the hydrodynamic interaction on the dynamics of flexible and rod-like polymers in solution is investigated. The solvent is simulated by the multi-particle-collision dynamics (MPCD) algorithm, a mesoscale simulation technique. The dynamics of the solvent is studied and the self-diffusion coefficient is calculated as a function of the mean free path of a particle. At small mean free paths, the hydrodynamic interaction strongly influences the dynamics of the fluid particles. This solvent model is then coupled to a molecular dynamics simulation algorithm. We obtain excellent agreement between our simulation results for a flexible polymer and the predictions of Zimm theory. The study of the translational diffusion coefficient of rod-like polymers confirms the predicted chain-length dependence. In addition, we study the influence of shear on the structural properties of rod-like polymers. For shear rates exceeding the rotational relaxation time, the rod-like molecule aligns with the shear flow, leading to an orientational symmetry breaking transverse to the flow direction. The comparison of the obtained shear rate dependencies with theoretical predictions exhibits significant deviations. The properties of the orientational tensor and the rotational velocity are discussed in detail as a function of shear rate.

Simulation of complex fluids by multi-particle-collision dynamics

The particle-based mesoscale simulation technique called Multi-Particle-Collision Dynamics (MPCD) (also denoted as Stochastic Rotation Dynamics (SDR)) is introduced. The algorithm is outlined and applications to complex fluids are described. Results for the dynamics of a Gaussian polymer and a rodlike colloid are discussed. Moreover, the density dependence of the diffusion coefficient of a colloidal fluid is presented. Our investigations show that the MPCD algorithm accounts for hydrodynamic interactions.

Combination of random-barrier and random-trap models

The temperature dependence of the diffusion coefficient of particles is studied on lattices with disorder. A model is investigated with both trap and barrier disorder that was introduced earlier by Y Limoge and J L Bocquet (1990 Phys. Rev. Lett. 65 60) to explain an Arrhenian temperature dependence of the diffusion coefficient in amorphous substances. We have used a generalized effective-medium approximation (EMA) by introducing weighted transition rates as inferred from an exact expression for the diffusion coefficient in one-dimensional disordered chains. Monte Carlo simulations were made to check the validity of the approximations. Approximate Arrhenian behaviour can be achieved in finite temperature intervals in three- and higher-dimensional lattices by adjusting the relative strengths of the barrier and trap disorder. Exact Arrhenian behaviour of the diffusion coefficient can only be obtained in infinite dimensions.

Mesoscale hydrodynamics simulations of attractive rod-like colloids in shear flow

Suspensions of rod-like colloids show in equilibrium an isotropic–nematic coexistence region, which depends on the strength of an attractive interaction between the rods. We study the behavior of this system in shear flow for various interaction strengths. A hybrid simulation approach is employed, which consists of a mesoscale particle-based hydrodynamics technique (multi-particle collision dynamics) for the solvent and molecular dynamics simulations for the colloidal rods. The shear flow induces alignment in the initially isotropic phase, which generated an additional free volume around each rod and causes the densification of the isotropic phase at the expense of an erosion of the initially nematic phase. Furthermore, the nematic phase exhibits a collective rotational motion. The associated rotational time decreases linearly in with increasing shear rate , and increases with increasing attraction strength between the rods. The density difference between these two regions at different shear rates allows us to determine the binodal line of the phase diagram. For large applied shear rates, the difference between the phases disappears in favor of a homogeneous flow-aligned state.

ANNIHILATING RANDOM WALKS IN ONE-DIMENSIONAL DISORDERED MEDIA

We study diffusion-limited pair annihilation

Single-particle diffusion coefficient on surfaces with Ehrlich–Schwoebel barriers

A+A\to 0

Low-Reynolds-number hydrodynamics of complex fluids by multi-particle-collision dynamics

on one-dimensional lattices with inhomogeneous nearest neighbour hopping in the limit of infinite reaction rate. We obtain a simple exact expression for the particle concentration

Dynamic regimes of fluids simulated by multiparticle-collision dynamics.

\rho_k(t)

Rectification by hopping motion through nonsymmetric potentials with strong bias

of the many-particle system in terms of the conditional probabilities