Soft Matter

Soft condensed matter encompasses systems with structures in the size range of nanometres to a few micrometres. Hence, a sample contains very many structures or particles, typically on the order of the Avogadro number. This leads to new (collective) phenomena such as the occurrence of different phases (gas, liquid, solid), a concept which is not applicable to individual particles. Furthermore, the systems and their behaviours are governed by interactions in the range of (only) the thermal energy. Entropy thus plays an important role.

Concepts and phenomena observed in soft condensed matter are also relevant for other areas, in particular hard condensed matter (metals, semi-conductors etc.) and biological systems (proteins, DNA, cell membranes etc.), where collective phenomena are also crucial. In contrast to these systems, soft matter, due to the prevailing length scales, can easily be observed on an individual-particle level, their behaviour followed in time and their particle-particle interactions tuned in a broad range.

Soft matter includes different kinds of systems:

• Colloids, i.e. small particles which do not change their size and shape and whose internal structure is not of prime interest (although, of course, it affects their behaviour). Everyday examples are paints, tooth paste or mud.

• Polymers, i.e. threadlike molecules which are possibly branched and consist of individual 'monomers' and thus possess (interesting) internal structure and dynamics. Synthetic (e.g. plastic bags and bottles, nappies, car tyres) and natural (e.g. DNA, starch) polymers are ubiquitous in everyday life.

• Surfactants which self-assemble into structures which not only exhibit (interesting) internal structure and dynamics, but continuously disintegrate and recombine. Surfactants are used as soap, dishwashing or laundry detergent and other cleaning applications.

Soft matter systems are found in many natural and industrial products, in particular food stuffs, drug delivery systems and cleaning products.