thixotropy

Thixotropy is a property of certain non-Newtonian fluids in which the apparent viscosity decreases with time under constant shear and recovers when the shear is removed. The effect is reversible: after shear stops, the material gradually rebuilds its internal structure and viscosity returns toward its original value. The extent and rate of thixotropy depend on temperature, composition, particle interactions, and the microstructure of the material. A related concept is rheopexy (anti-thixotropy), where viscosity increases with time under shear.

Mechanism and characteristics are tied to the microstructure of the fluid. In many thixotropic systems, a weak network or gel-like structure—formed by particles, polymers, or colloids—breaks down under shear, causing the viscosity to drop. When the shear is removed, bonds reform and the structure rebuilds, causing viscosity to recover. The rebuilding can take from seconds to hours, depending on the system. Common thixotropic materials include clay suspensions, paints and coatings, certain inks, gels of cellulose and starch, many food products, and some cosmetics.

Measurement and applications: Thixotropy is typically assessed with rheometry by applying shear steps and observing viscosity over time, often presenting a hysteresis loop when flow is initiated and then reversed. The area of that loop or a defined thixotropic index provides a measure of the effect. Practical implications include easier pumping and spreading of thixotropic fluids, while maintaining stability at rest. Applications span paints, coatings, drilling fluids, cosmetics, and food products such as ketchup or yogurt, where a balance between flow under stress and shape stability at rest is desired.