Rheopexic

Rheopexic is an adjective used in rheology to describe a time-dependent flow behavior in which a material’s viscosity or resistance to flow increases with time under constant shear or stress. This phenomenon is part of the broader class of time-dependent rheological behaviors and is the opposite of thixotropy, where viscosity decreases with time under shear. Materials that display this behavior are often described as rheopectic, and the term rheopexic is used to emphasize the progressive strengthening of flow resistance over time.

Etymology and terminology commonly relate rheopexy to the prefix rheo-, meaning flow, and to suffixes used in rheology that denote time-dependent changes. In the literature, rheopectic and rheopexic usage may vary, but both refer to materials whose structure or interactions become more resistant to flow as shear continues.

Mechanisms underlying rheopexy are material-specific but generally involve time-dependent formation or strengthening of internal networks under shear. Examples include the progressive formation of particle floc networks, shear-induced aggregation, or restructuring of gels and suspensions that raise viscosity or elastic modulus with elapsed time.

Measurement and interpretation rely on rheological testing. At a fixed shear rate, a rheopexic fluid shows a rise in viscosity over time, and often an increase in storage modulus in oscillatory tests. This behavior is contrasted with thixotropic materials, which show decreasing viscosity under the same conditions. Rheopexy can complicate processing and pumping, but understanding it aids formulation, process control, and equipment design.