Large Amplitude Oscillatory Experiments Research Articles

Dynamic rheological properties of a fumed silica grease

The thermo-rheological characteristics of a fumed silica lubricating grease in linear and nonlinear oscillatory experiments have been investigated. The material rheological behavior represents a soft solid being thermo-rheologically complex. There is an abnormal temperature dependency in the range of − 10 to 10 °C which is related to the phase transition of the base oil. The dynamic moduli data in linear viscoelastic envelop (LVE) have been modeled using mode-coupling theory (MCT) in the whole temperature range. Two main relaxation mechanisms can be identified through linear and nonlinear viscoelastic properties related to interaction of the primary particle and its neighbor particles as well as a slow relaxation process which represents the escape of this particle from its “cage”. Finally, it is demonstrated that the dominant yielding process in large amplitude oscillatory experiments can be explained based on either particle cage rupture (consistent with MCT framework) or particle “hopping” out of its cage proposed in soft glassy rheology (SGR) model. It will be discussed that the governing mechanism depends on the applied frequency.

Superposition rheometry of a wormlike micellar fluid

Oscillatory measurements are often used to explore the nonlinear response of materials, with recently a strong focus on using large amplitude oscillatory experiments. However, the superposition of an oscillatory motion onto a steady-state shear flow is a method where the kinematic history experienced by the sample is simpler. Such a superposed oscillation can be applied either orthogonal or parallel to the main flow direction. Both superposed deformation modes can now be achieved on rotational rheometers equipped with a force-rebalanced transducer, the orthogonal mode requiring a minor modification to the control loop of the normal force. In the present work, the nonlinear properties of a wormlike micellar (WLM) solution are studied. The results are compared with the predictions of the Giesekus model, which is chosen both for its capability to describe the WLM response and for being one of the simplest continuum models that incorporate an anisotropic microstructure. From the fluid response in the homogeneous flow regime, a rate-dependent relaxation time and a rate-dependent plateau modulus can be derived. The latter provides insight into the structural anisotropy during flow at short length scales, which in this case is isotropic. Further analysis of the superposition moduli can be used to separate and quantify the effects of flow on the reptation and breaking of the chains. In the shear-banding regime, the orthogonal moduli show a weaker dependence on shear rate compared to the predictions of the Giesekus model, yet they remain sensitive to changes in the shear-banded state.

Finite amplitude oscillations of viscoelastic fluids

Finite amplitude oscillatory shear experiments are used to study the mechanical behavior of viscoelastic materials. Four rheological relations are derived which can be used to determine both linear and nonlinear rheological properties of simple fluids from data obtained using large amplitude oscillatory experiments. Two consistency conditions are derived for a fluid which obeys the K-BKZ constitutive equation, and these relations are used in conjunction with data from a finite amplitude oscillatory experiment to check the validity of the K-BKZ equation.