Multiscale Dissipative Mechanisms and Hierarchical Surfaces

Abstract

In the introduction chapter, the subjects and definitions of the surface science and tribology are discussed, as well as their relations to the concepts of hierarchy, mesoscale, energy dissipation and biomimetics. 1.1 Surfaces and Surface Free Energy Surface science is defined as the study of physical and chemical phenomena that occur at the interface of two phases (solid–liquid, solid–gas, solid–vacuum) or of different substances of the same phase (solid–solid, liquid–liquid) [6]. Various properties of matter (e.g., the density, ρ) can change rapidly at the interface. It is therefore convenient to assume that the interface is a geometrically two-dimensional surface in a sense that every point at the interface can be characterized by only two parameters. In reality, every interface has a nonzero thickness and the bulk properties change gradually at the interface; however, the thickness is so small compared to the two other dimensions that it can often be neglected. An important characteristic of every surface or interface is the surface free energy, γ . In the bulk of the body, chemical bonds exist between the molecules and certain energy has to be applied in order to break the bonds. The molecules that do not form the bonds have higher potential energy than those that form the bonds. Molecules at the surface do not form bonds at the side of the surface and thus they have higher energy. This additional energy is called surface or interface free energy and is measured in the energy per area units, that is, in the SI system, J/m2 or N/m. In order to create an interface (e.g., to form a vapor bubble inside boiling water), the energy should be applied which is equal to the area of the interface multiplied by the interface free energy. For the stable existence of the interface it is required that the free energy of formation of the interface be positive, so that accidental fluctuations do not result in the dispersion of one material into the other. The opposite example of an interface, which does not offer opposition to the dispersion, is that between two gases or between miscible liquids [6]. Any system tends to achieve a position that