Rheology of waxy oils: A critical review

Waxy oil is a rheologically complex material presenting shear-rate-dependent viscosity, static and dynamic yield stresses, gel elasto-viscoplasticity, time/deformation-dependent gel structure, and irreversibility. All these properties depend not only on oil composition and temperature, as with many materials, but also on other parameters, such as shear and thermal histories. In addition, the composition can change completely the effect of a specific parameter on the rheological properties of the waxy oil. Differences in material composition or experimental procedures may explain some contradictory results in the literature. Despite this intricating behavior, the literature provides important findings regarding the rheological behavior of waxy oils. This work presents a comprehensive discussion of how the rheological properties of waxy oils are affected by composition, sample preparation, pre-test, thermal, and shear histories. We also discuss the relation between material microscopy and macroscopic behavior, focusing on the complex rheological behavior and emphasizing possible artifacts commonly encountered in measurements.