Rheopexy

Rheopexy is a rheological phenomenon in which a material’s viscosity increases with time under a constant shear rate, i.e., a rheopectic fluid. It is the opposite of thixotropy, where viscosity decreases under shear and recovers when the stress is removed.

Mechanism and characteristics: The effect arises from time‑dependent structural buildup within the material during sustained shear. Particles may form flocs or networks, or polymer chains may become entangled or aligned, increasing resistance to flow. When shear is stopped, the induced structure gradually relaxes and viscosity returns toward its resting value. The onset and magnitude of rheopexy depend on composition, particle interactions, and the level of applied shear. It is typically observed in certain concentrated suspensions and complex colloidal or polymer systems.

Measurement and interpretation: Rheopexy is studied with rheometers using controlled-shear-rate or controlled-stress protocols. At a fixed shear rate, viscosity or related properties may rise with time, producing characteristic time-dependent curves. A rheopexic index or related metrics can be used to quantify the effect and compare materials. Because the behavior is history-dependent, accurate assessment requires careful control of prior shear and test conditions.

Applications and implications: Rheopexy affects processing and performance in industries such as coatings, paints, cosmetics, pharmaceuticals, and drilling fluids. Time‑dependent strengthening under shear can complicate pumping, mixing, or coating uniformity, but in some cases may help reduce sag or improve structural integrity after application. While less common than thixotropy, rheopexy is an important consideration in the design and selection of non-Newtonian fluids.