Strain shifts under stress-controlled oscillatory shearing in theoretical, experimental, and structural perspectives:...

Rheological measurements in which the applied stress or strain is oscillated are widely used to interrogate viscoelastic properties due to the independent control over the time scale and length scale afforded by changes in amplitude and frequency. Taking a nontraditional approach, we treat stress-controlled oscillatory tests as creep tests with transiently varying stress and apply an analysis typically used for steady creep and recovery experiments. Defining zero strain as the state prior to external shearing, it is shown that strain responses to small-amplitude oscillatory stressing are naturally shifted from the starting point by an amount proportional to the phase of the applied stress. The phenomenology is experimentally observed with entangled polymerlike micelles and polyethylene oxide solutions. A theory of strain shifting in the steady alternating state is provided based on recovery rheology, where differences between total strain and recoverable strains are acknowledged. User-controlled variables, such as the amplitude of the stress, the angular frequency, and the phase of the stress, as well a lone material parameter, the zero-shear viscosity, are shown to dictate the amount of shifting. A rapid and efficient approach of determining the zero-shear viscosity is, therefore, presented. We investigate the microstructural evolution via in situ small-angle neutron scattering when strain shifting appears. The microscopic orientation is shown to correlate to the recoverable strain independent of the shifting. Additional measurements are carried out on collagen, pluronic-hyaluronic acid, alginate gels, and polystyrene melts to show the generic nature of the strain shift phenomenon. In addition, we demonstrate that the strain-shift knowledge can be applied to determine the horizontal shift factor in time-temperature superposition, free of any numerical fitting procedures.