semifluids

Semifluids are materials that exhibit both fluid-like flow and solid-like structure. They behave as liquids under some conditions but can retain shape or form when at rest due to a network of interactions that provides yield stress or elastic storage. In rheology, semifluids are typically non-Newtonian and viscoelastic, often possessing a yield stress that must be exceeded to initiate flow. Many semifluids show time-dependent behavior, being thixotropic or rheopectic, meaning their viscosity changes with the history of shear.

Classification and examples vary by discipline, but common members include pastes, gels, emulsions, foams, and suspensions with a percolating particle network. Everyday instances include toothpaste, mayonnaise, yoghurt, and cement paste, depending on composition, temperature, and confinement. Semifluids can exhibit shear-thinning (viscosity decreases with shear rate) or shear-thickening behavior, and their microstructure governs how they respond to applied stress.

Characterization typically relies on rheological measurements. Shear stress versus shear rate tests quantify flow behavior, while oscillatory tests yield storage modulus G' and loss modulus G''. The relationship of G' to G'' and the presence of a yield stress help distinguish semifluids from simple liquids and more rigid solids.

Applications and relevance span manufacturing and consumer products. Semifluid materials are used in 3D printing (bioinks and ceramic pastes), cosmetics and personal care, food processing, construction materials, and adhesives or coatings requiring controlled flow with shape retention. Biological secretions, such as mucus, also display semifluid properties that balance mobility with barrier function. The term remains descriptive and varies across disciplines, guiding the design and use of materials with tunable rheology.