Decoupling elastic and viscous effects in thixotropy and a cautionary tale for interpretation of the dynamic moduli

The topic of thixotropy has historically received much attention due to its importance in a wide range of complex fluids and their applications. However, a thorough understanding of the phenomenon and how to model it remain outstanding challenges. In this work, we examine two materials that exhibit phenomenology often referred to as thixotropic through the lens of stress-controlled recovery rheology. When subjected to an oscillatory shear stress, the materials, an aqueous surfactant system that structurally forms multilamellar vesicles as well as a frequently studied fumed silica suspension, show a transient increase in the resulting strain amplitude. We use both creep and oscillatory tests in conjunction with recovery rheology to measure the elastic and viscous contributions to flow and deformation and find that the elastic contributions remain constant, even at larger amplitudes where nonlinear responses are induced. We conclude that the observed behavior is, therefore, strictly a viscous phenomenon, in contrast with common modeling efforts that describe both the viscous and elastic behaviors as being transient. We additionally examine how typical use of the dynamic moduli can give a misleading description of the material’s behavior, whereas examination of the energetic contributions provides a description consistent with the recovery measurements.