thixotrop

Thixotropy is a time-dependent rheological behavior where a fluid's viscosity decreases over time when subjected to shear stress, and then gradually recovers to its original viscosity when the stress is removed. This phenomenon is also known as shear thinning with a time delay.

Materials exhibiting thixotropy are often described as having a "gel-like" structure in their quiescent state. When disturbed or stirred, this structure breaks down, allowing the material to flow more easily. Upon standing undisturbed, the structure reforms, and the viscosity increases again. This reversible process is a key characteristic distinguishing thixotropy from simple shear thinning, where viscosity decreases instantaneously with shear and returns to its original state immediately upon removal of stress.

Examples of thixotropic materials include certain paints, inks, yogurts, and some drilling fluids. In the case of paint, thixotropy allows it to be easily spread with a brush or roller (low viscosity under shear) but prevents it from dripping excessively once applied to a vertical surface (high viscosity when at rest). The degree of thixotropy can vary significantly between different substances. The underlying mechanism often involves the reversible breaking and reforming of weak physical bonds or aggregates within the fluid.