Diffusion in colloidal and polymeric systems

Abstract

In this chapter we shall discuss diffusion properties of spherical and rod-like colloids, and of binary polymer melts. In addition, the principles of dynamic light scattering (DLS), fluorescence recovery after photobleaching (FRAP) and fluorescence correlation spectroscopy (FCS), which are methods commonly used to measure various types of diffusion coefficients, are introduced on a basic level. In Sect. 16.2 we discuss light scattering on a heuristic level, and we introduce various kinds of dynamic structure factors. These structure factors describe different types of diffusion processes, like self-, collective-, rotational- and interdiffusion. These diffusion processes, and their connection to the corresponding types of structure factors, are discussed in Sect. 16.3, first for very dilute systems (Sect. 16.3.1) and then for concentrated systems (Sect. 16.3.2). Interdiffusion is especially relevant for the dynamics in binary polymer melts. Since long-time self-diffusion is difficult to measure with light scattering, especially for non-spherical particles, two very direct methods to probe this type of diffusion process are introduced in Sect. 16.4: FRAP (Sect. 16.4.1) and FCS (Sect. 16.4.2). These sections also contain experimental data obtained with these techniques for rod-like macromolecules. Section 16.5 deals with diffusion of spherical colloids, where the various types of diffusion processes that were treated in Sect. 16.3 on a heuristic level, are quantified, and where theoretical short-time and long-time predictions, and computer simulation results, are explored and compared to experimental findings. The systems considered comprise three-dimensional dispersions of neutral and charged colloidal spheres, quasi two-dimensional suspensions of charged particle monolayers between narrow plates, and magnetically interacting particles confined to a liquid-gas interface. Section 16.5 addresses further the diffusion in binary polymer blends, where earlier discussed general relations for interdiffusion are explicitly quantified within the Flory-Huggins approach and the dynamic random phase approximation (RPA).