Rheopectic

Rheopectic is a term used in rheology to describe fluids or soft solids whose apparent viscosity increases with time under the influence of a constant shear rate. This behavior is the opposite of thixotropy, where viscosity decreases with time under shear. In rheopectic materials, the internal microstructure evolves during shear, forming or strengthening a network that makes flow progressively more difficult as shear continues.

Rheopectic behavior is observed in some concentrated suspensions, gels, and complex fluids such as certain paints, printer inks, lubricants, and drilling fluids. It tends to appear in systems with strong particle interactions or polymer networks, where shear induces bonding, flocculation, or alignment that persists after shear is applied. The degree of rheopexy often increases with solids content, time under shear, and specific shear rate ranges, and can be sensitive to temperature and composition.

Practically, rheopectic materials can complicate processing and application. During coating, pumping, or extrusion, viscosity may rise with time, reducing flow and causing sag or inconsistent coating thickness. To manage rheopexy, manufacturers may apply brief high-shear preconditioning to break down the structure, or adjust formulation and temperature to control the effect. Rheopectic behavior is typically characterized using a rheometer by exposing the material to a constant shear rate and recording viscosity as a function of time.