Antithixotropy

Antithixotropy, or anti-thixotropy, is a time-dependent rheological phenomenon observed in certain gels, pastes, and concentrated suspensions. It is characterized by a gradual increase in apparent viscosity (or modulus) with time when the material is subjected to a constant shear rate or stress. This behavior is the opposite of thixotropy, where viscosity decreases under sustained shear; under rest a thixotropic material tends to recover its viscosity.

In antithixotropic systems, once shear is removed, the viscosity often relaxes back toward its original value, though the recovery can be slow and incomplete depending on the material and conditions. The mechanism is typically related to shear-induced structuring, networking, or aggregation that strengthens under flow. Microstructural changes may include formation or strengthening of transient networks, particle clusters, or selective orientation that raises resistance to flow. The exact processes are material-specific and can involve reversible bonding, associative polymers, or colloidal interactions.

Measurement and interpretation of antithixotropy rely on time-resolved rheological tests. Step-shear or constant-shear-rate experiments, creep tests, and oscillatory measurements can reveal a viscosity or modulus that increases with time under sustained shear. The terminology is not entirely uniform in the literature; some authors explicitly distinguish rheopectic behavior from other forms of time-dependent thickening, while others use antithixotropy as a general label for rheopectic systems that oppose classic thixotropic behavior.

Examples of observed antithixotropic behavior appear in certain gels, concentrated clays, silica suspensions, and some polymer solutions where time under shear promotes network formation and thickening.