thixotropic

Thixotropy is a time-dependent shear-thinning property observed in certain gels, pastes, and suspensions. In a thixotropic material, viscosity decreases under sustained or increasing shear, and after the shear is removed, the internal structure gradually rebuilds and viscosity recovers toward its original value. The extent and speed of recovery depend on composition, temperature, and rest time. This behavior is distinct from simple shear thinning that may not recover its structure, and from rheopecty, where viscosity increases with continued shear.

The effect arises from reversible changes in the material’s microstructure. Under shear, a network of bonds, entanglements, or particle contacts breaks apart, reducing viscosity. When the shear is stopped, bonds reform and the network gradually rebuilds, increasing viscosity again. The time-dependent nature means the material’s flow can depend on its shear history.

Common thixotropic materials include paints and varnishes, cement and drilling muds, and various cosmetics. In foods, products such as tomato ketchup, yogurt, and mayonnaise often exhibit thixotropy, aiding their handling and mouthfeel by allowing easy pouring or spreading while helping to maintain structure at rest.

In practice, thixotropy is characterized using rheological tests that compare up- and down-shear-rate paths or by thixotropic recovery tests. The area between the curves or the measured recovery time provides a quantitative index of thixotropy. Understanding thixotropy is important for formulation, processing, and end-use performance in many industrial and consumer products.