Thixotropy is a time-dependent
shear thinning property. Certain gels or fluids that are thick (viscous) under static conditions will flow (become thin, less viscous) over time when shaken, agitated, or otherwise stressed (
time dependent viscosity). They then take a fixed time to return to a more viscous state. In more technical language: some
non-Newtonian pseudoplastic fluids show a time-dependent change in
viscosity; the longer the fluid undergoes
shear stress, the lower its viscosity. A thixotropic fluid is a fluid which takes a finite time to attain equilibrium viscosity when introduced to a step change in shear rate. Some thixotropic fluids return to a gel state almost instantly, such as ketchup, and are called
pseudoplastic fluids. Others such as yogurt take much longer and can become nearly solid. Many
gels and
colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated.