Strain shift measured from stress-controlled oscillatory shear: Evidence for a continuous yielding transition and new techniques to determine recovery rheology measures.

The article focuses on the rheological behavior of yield stress fluids, demonstrating through stress-controlled oscillatory shear experiments that these fluids exhibit flow below their traditionally defined yield stress, challenging the classical Oldroyd–Prager formalism which predicts no flow below this threshold. Using the concept of strain shift—the nonzero offset about which strain oscillates under applied stress—the study quantifies unrecoverable strain as an indicator of flow occurring at all stress amplitudes over finite timescales. Experimental results on Carbopol yield stress fluids are compared with predictions from the Herschel–Bulkley–Saramito (HB–Saramito) model, which adheres to the Oldroyd–Prager formalism, and the Kamani–Donley–Rogers (KDR) model, which allows for continuous transitions and flow below the yield stress. The work further establishes a direct relationship between strain shift and the fluid component of the dynamic loss modulus, enabling determination of flow-related rheological metrics without iterative recovery tests, thereby providing a more comprehensive and time-efficient method for characterizing complex fluid behavior under transient conditions.