Studies of surface diffusion and dissipative particel dynamics

Abstract

In the first part of this thesis, we study diffusion of atoms on solid surfaces. To this end, we carry out Monte Carlo simulations for a lattice-gas model of O/W(110), first on clean surfaces and then on surfaces containing small concentrations of quenched (immobile) impurities. In both cases, we examine how thermodynamic non-equilibrium affects tracer diffusion and collective diffusion of atoms on a surface. We find that non-equilibrium effects can play an important role when there are strong interactions in the system. These effects persist even when there are only slight deviations from equilibrium. We also find that even minute impurity concentrations can lead to major changes in the diffusion coefficients. The combined effect is not, however, additive. Indeed, we find that the non-equilibrium effects are most pronounced on clean surfaces, while on surfaces covered by impurities the role of non-equilibrium conditions is weaker. This is essentially due to the similarity between disorder as induced either by the non-equilibrium condition or by the presence of impurities on the surface.

In the second part of the thesis, we consider methodological aspects of the dissipative particle dynamics (DPD) technique. First, we address the question: "How to integrate the equations of motion in DPD simulations?" We test and analyze several novel DPD integration schemes on an equal footing through DPD simulations of different model systems. By monitoring a number of physical observables including temperature, radial distribution function, radius of gyration for polymers, and tracer diffusion, we find that the methods by Lowe and Shardlow give the best overall performance and are superior also to the integrators tested in previous studies.

Second, we study the dynamics of polymer melts. In standard DPD, as well as in other coarse-grained soft potential models, there is a problem that polymers can penetrate through themselves. This is a clear artifact, and has direct consequences on polymer dynamics. To correct this problem, we tune the conservative forces within the polymer chain so strong that chains cannot cut through each other. Indeed, if a certain geometric criterion is met, it is impossible for polymer chains to cross. Through DPD simulations, we show that our approach is able to reproduce the Rouse-like dynamics for short chains and reptational dynamics for longer chains. The results are in good agreement with polymer theories and experiments.