Semifluid

Semifluid is a descriptive term used for substances whose flow behavior lies between that of a liquid and a solid. Semifluid materials can flow under applied stresses but often possess a finite yield stress and a structured microstructure that resists rapid deformation. In rheology, such materials are typically non-Newtonian and display viscoelasticity, time-dependent behavior, and viscosity that depends on shear rate or history.

Rheological models commonly used for semifluids include Bingham plastics and Herschel-Bulkley types, which combine a yield stress with either constant or shear-rate dependent viscosity. Many semifluids are shear-thinning (viscosity decreases with increasing shear rate) or thixotropic (viscosity decreases with time under constant shear and recovers after rest). Temperature, composition, particle interactions, and internal networks strongly influence their flow.

Common real-world examples include toothpaste, mayonnaise and ketchup, various gels and pastes, clay suspensions, and some cosmetic or food emulsions. In geology and materials science, the term is also used to describe partially molten rocks or polymer melts that behave as fluids on short timescales yet maintain structure over longer times. Understanding semifluid behavior is important for formulation, processing, pumping, and packaging of products, as well as for modeling natural flows.

Characterization relies on rheometry, slump tests, or other flow tests to measure yield stress, viscosity, and how these properties change with time, temperature, and shear history. The concept of semifluid sits within the broader category of non-Newtonian fluids and viscoelastic materials.