Tesis doctoral--Valencia. / Contient le texte original en latin de F.M. Zarzoso : "In aequatorem planetarum..." et sa trad. en castillan. Le texte latin est reprod. en fac-sim. de l'éd. Paris : S. Colines, 1526. Bibliogr. p. 325-330.

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

Heidelberg, Universiẗat, Diss., 2007.

Extr. de: Diss.--Physique--Aachen--Technischen Hochschule, 2001. / Notes Bibliogr.

Ph. D.--Leiden--Rijksuniversiteit, 1997. / Bibliogr. p. 581-595. Index.

Texte remanié de: Diss.--Hamburg, 1999. / Bibliogr. p. 224-232. Notes bibliogr. Index.

University, Diss., 2002--Heidelberg.

University, Diss., 2001--Bonn.

The phenomenon of friction is on the one hand useful, for example for walking, which would not be so easy without friction, and on the other hand disturbing, for example in wheel bearings, where it slows down desired motion. Therefore, the origin and effect of friction is under intense research. One main point in this work is the analytic investigation of the coupling between friction force and (torsion) friction torque of a sliding and spinning disk. The local friction force at a contact area element was chosen to be an algebraic function of the local relative velocity with an exponent α > 0. It could be shown, that for α < 1 sliding and torsion friction dynamically reduce each other, while for α > 1 they amplify each other. In case of α = 1 sliding and torsion friction are decoupled. With respect to the velocity ratio of sliding and angular velocity, the final motion mode has been investigated, i.e. whether both motions stop together or whether one motion gets dominant. For α < 1 both motions stop together, while for α > 1 it depends on the initial velocity ratio. The mass distribution and contact area radius, which are encoded in the key parameter C of the corresponding differential equation, are the second important influence on the final motion mode. A phase diagram shows for given values C and α the possible final motion modes. The influence of an inhomogenous pressure distribution within the contact area on the coupling was investigated exemplarily for α = 0 with a cylinder as object. In contrast to the disk (homogenous pressure distribution) the cylinder is deflected from its initial sliding direction. In this context the motion of a curling rock on ice is discussed, as it is deflected towards the opposite direction compared to that of the cylinder. Another focal point is the investigation of the role of friction torques (rolling and torsion friction) in the compaction of nano-powders. For this three dimensional contact dynamics simulations with phenomenologically chosen contact laws were performed. With this it could be shown that torsion and rolling friction contribute significantly to the final porosity. Furthermore, these contributions of torsion and rolling friction are independent of each other and can be represented by a sum. In the chapter Conclusions and Outlook a brief introduction on recent research of atomic scale torsion friction is presented.

In this thesis, several stochastic models are investigated, which are subjected to irreversible dynamics. Motivation for the presented work stems, on the one hand, from particular physical systems under consideration, which are modeled by the studied stochastic processes. Besides that, the models discussed in this thesis are, on the other hand, generally interesting from the point of view of statistical physics, since they describe systems far from thermodynamic equilibrium. Interesting properties to be encountered are, e.g., dynamical scaling behavior or continuous phase transitions. The first issue to be addressed, is the investigation of irreversibly aggregating systems, where the main emphasis is laid on aggregation of monopolarly charged clusters suspended in a fluid. For this purpose, rate equations are analyzed and Brownian dynamics simulations are performed. It is shown that the system crosses over from power-law cluster growth to sub-logarithmic cluster growth. Asymptotically, the cluster size distribution evolves towards a universal scaling form, which implies a 'self-focussing' of the size distribution. Another emphasis of this thesis is the investigation of nonequilibrium critical phenomena, in particular, the study of phase transitions into absorbing states (states that may be reached irreversibly). To this end, the continuous nonequilibrium phase transition of directed percolation, which serves as a paradigm for absorbing-state phase transitions, is analyzed by a novel approach. Despite the lack of a partition function for directed percolation, this novel approach follows the ideas of Yang-Lee theory of equilibrium statistical mechanics, by investigating the complex roots of the survival probability. Stochastic models such as directed percolation mimic spreading processes, e.g., the spreading of an infectious disease. The effect of long-time memory, which is not included in directed percolation and which corresponds to immunization in epidemic spreading, is investigated through an appropriate model. This model includes dynamical percolation (perfect immunization) as a special case, as well as directed percolation (no immunization). The critical behavior of this model is studied by means of Monte Carlo simulations, in particular for weak immunization. A further generalization is investigated, which allows spontaneous mutations and different species of spreading agents (pathogens). Restricting the analysis to perfect immunization and two spatial dimensions, it is shown by Monte Carlo simulations, that immunization leads to a crossover from dynamical to directed percolation. Other properties of this model are discussed in detail.