FließgrenzeFluids

FließgrenzeFluids are a class of non-Newtonian fluids that exhibit a finite yield stress, denoted tau_y. They behave as solid-like materials when the applied shear stress is below tau_y and begin to flow only when the stress exceeds this threshold. The yield stress represents the minimum force required to initiate macroscopic flow, after which the material's viscosity changes with shear rate.

The constitutive behavior of FließgrenzeFluids is commonly described by yield-stress models. The simplest is the Bingham plastic model, where tau equals tau_y plus a plastic viscosity term (mu_p times the shear rate) for tau above tau_y, and the shear rate is zero below tau_y. More general formulations, such as the Herschel-Bulkley model, express tau as tau_y plus K times the shear rate raised to the power n, enabling a broader range of flow responses including shear-thinning or shear-thickening after yield. Real fluids may also exhibit time-dependent effects like thixotropy or viscoelasticity, which can modify the flow onset and steady-state behavior.

Common examples include pastes, creams, mortars, clays, toothpaste, mayonnaise, and drilling muds. These materials flow under sufficient stress but resist deformation when at rest, which can complicate pumping, mixing, and molding operations. Measurement of yield stress is typically performed with rheometers, using vane tests, cone-and-plate geometry, or creep and oscillatory experiments to estimate tau_y and the flow parameters.

Applications span construction, food processing, cosmetics, and oil and gas industries. Understanding FließgrenzeFluids is essential for safe handling, efficient transport, and predictable processing of yield-stress materials.