ABSTRACT

In the following entry we shall restrict ourselves to discussing mesoscopic and continuum models for complex fluids in chemical physics. The wide span of time and length scales in these materials is illustrated in figure B3.6.1 for a blend of two polymers. On the atomistic scale each polymer consists of chemical repeat units joined together to form the chain molecule. The length scale is set by the distance between the atoms along the backbone of the polymer, typically in the range of 1-2 Å. The vibrations of the atoms occur on the timescale of picoseconds. In a dense melt, the flexible chain molecules adopt a random-walk-like conformation. The ‘step length’ of the random walk, or persistence length b, is typically of the order of a few nanometres. Since several thousands of repeat units form a polymer, the overall size of a single molecule, as specified by its radius of gyration, exceeds the persistence length by 1-3 orders of magnitude. On this range of length scales the structure of the polymer is self-similar. If the two components of the blend are not miscible, as it is generally the case, one species forms droplets that are dispersed in a matrix of the other species. The size of the droplets is in the micrometre range. On even larger length scales (say 1 mm) the material appears homogeneous. Clearly the properties on the mesoscopic length scale are important for application properties. A decrease of the droplet size or even the formation of a connected morphology (i.e. a microemulsion) improves the mechanical properties of the composite material. A similar span of time and length scales is encountered in many other systems (e.g., mixtures of oil, water and surfactant or glassy materials) and this behaviour is rather typical for complex fluids.