The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
Aalto

Studies of Surface Diffusion and Dissipative Particle Dynamics

Petri Nikunen

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Electrical and Communications Engineering for public examination and debate in Auditorium S4 at Helsinki University of Technology (Espoo, Finland) on the 18th of March, 2006, at 12 o'clock noon.

Overview in PDF format (ISBN 951-22-8059-0)   [1376 KB]
Dissertation is also available in print (ISBN 951-22-8058-2)

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.

This thesis consists of an overview and of the following 5 publications:

  1. P. Nikunen, I. Vattulainen, and T. Ala-Nissila, Non-equilibrium effects in profile evolution measurements of surface diffusion, Surface Science 447, L162 (2000). © 2000 Elsevier Science. By permission.
  2. P. Nikunen, I. Vattulainen, and T. Ala-Nissila, Density profile evolution and nonequilibrium effects in partial and full spreading measurements of surface diffusion, Journal of Chemical Physics 114, 6335 (2001). © 2001 American Institute of Physics. By permission.
  3. P. Nikunen, I. Vattulainen, and T. Ala-Nissila, Effects of quenched impurities on surface diffusion, spreading, and ordering of O/W(110), Journal of Chemical Physics 117, 6757 (2002). © 2002 American Institute of Physics. By permission.
  4. P. Nikunen, M. Karttunen, and I. Vattulainen, How would you integrate the equations of motion in dissipative particle dynamics simulations?, Computer Physics Communications 153, 407 (2003). © 2003 Elsevier Science. By permission.
  5. P. Nikunen, I. Vattulainen, and M. Karttunen, Reptational dynamics in dissipative particle dynamics simulations of polymer melts, Physical Review E, submitted for publication (December, 2005); Helsinki Institute of Physics preprint HIP–2005–54/Th. © 2005 by authors.

Keywords: computer simulation, integration scheme, non-equilibrium, soft matter

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© 2006 Helsinki University of Technology

Last update 2011-05-26