FestFlüssig

Festflüssig is a term used in German-language discussions of materials science to describe substances that combine solid-like and liquid-like behavior. It refers to systems that form a network or structure giving them solid-like properties at rest or under small stresses, yet can flow like a liquid when subjected to sufficient stress or over extended timescales. The concept is often discussed in the context of rheology, where the material’s response depends on time, temperature, and history of applied forces.

Characteristic features include a yield stress, meaning the material behaves as a solid until a certain stress threshold is exceeded; viscoelasticity, with both stored elastic energy and dissipative flow components; and sometimes thixotropy, where structure gradually rebuilds after the stress is removed. The relative balance between solid-like and liquid-like behavior is commonly described by rheological measurements such as the storage modulus (G′) and the loss modulus (G″), as well as observations of flow under shear.

Examples of festflüssig systems encompass gels, pastes, and certain emulsions and suspensions. Common illustrative materials are toothpaste, mayonnaise, ketchup, concrete or mortar pastes, and various food emulsions or cosmetic gels. In geology and materials engineering, clays, cementitious pastes, and other yield-stress substances also exhibit festflüssig-like behavior.

Applications and implications include improved handling and processing in food, cosmetic, and construction industries, as well as advances in additive manufacturing and coatings where controlled flow and recovery are desirable. See also rheology, yield-stress fluids, gels, and non-Newtonian fluids.