thixotropes

Thixotropy is a time-dependent rheological property of certain gels, suspensions, and pastes in which the viscosity or structure decreases under constant shear and gradually recovers when the shear is removed. In contrast to simple shear thinning, thixotropy involves a reversible change in microstructure that evolves with time under flow. Rheopexy is the opposite behavior, where viscosity increases with time under shear.

The mechanism of thixotropy involves the breakdown of a weak, interconnected network or flocculated structure under applied shear, followed by gradual rebuilding when the material rests. Factors such as particle interactions, concentration, temperature, and particle size influence the rate and extent of structure disruption and reformation. The result is a transient, history-dependent viscosity that can be altered by adjusting shear conditions and rest times.

Testing for thixotropy typically uses rheological measurements that capture time-dependent changes in viscosity or modulus. Rheometers perform shear rate or shear stress sweeps with rest periods to observe viscosity decay during flow and recovery after flow stops. A thixotropic fluid often displays a hysteresis loop in a flow curve, and the area of that loop quantifies the degree of thixotropy. Thixotropy is sometimes described with a thixotropic index or a kinetic model that relates structure breakdown and rebuild to time and temperature.

Common thixotropic materials include paints and coatings, ketchup, mayonnaise, yogurt, cosmetic creams, and drilling fluids. In industry, thixotropy influences processes such as pumping, spraying, and extrusion, where a low viscosity under flow facilitates handling, while a high viscosity at rest helps maintain shape and stability.