Time-dependent viscosity

Time-dependent viscosity is a property of one class of non-Newtonian fluids in which the apparent viscosity of the fluid changes with time as the fluid continues to undergo shear.

Most commonly, in a non-Newtonian fluid, the viscosity (the measure of a fluid's ability to resist gradual deformation by shear or tensile stresses) of the fluids is dependent on shear rate or shear rate history (time). These shear-thickening fluids are divided into two groups: time dependent and time-independent viscosity. In the case of time dependent viscosity, the apparent viscosity of a fluid changes with time as the fluid is continuously sheared. They can be termed as memory materials. If the apparent viscosity decreases with time, the fluid is called thixotropic and if it increases with time, it is called rheopectic. Thixotropic behaviour is the result of a break down in the microstructure of the material as shearing continues. This happens when the shear is exceeded of a limit. It leads to non-linear stress-strain behaviour. Thixotropy can be associated with the effect of bubbles. Examples of these types of fluids are gelatine, shortening, cream, paints, yogurt, xanthan gum solutions, aqueous iron oxide gels, gelatine gels, pectin gels, synovial fluid, hydrogenated castor oil, some clays (including bentonite, and montmorillonite), carbon black suspension in molten tire rubber, some drilling muds, many paints, many floc suspensions, and many colloidal suspensions. In the case of rheopatic fluids, the structure builds as shearing continues. Rheopectic behaviour may be described as time-dependent dilatant behaviour. This type of behaviour is much less common but can occur in highly concentrated starch solutions over long periods of time. Shear induced crystallization may be responsible for rheopatic behaviour. Other examples are gypsum pastes and printer inks.

Thixotropic fluids are fluids that show a shear thinning property. Certain gels or fluids that are thick (viscous) under static conditions will flow (become thin, less viscous) over time when shaken, agitated, or otherwise stressed. They then take a fixed time to return to a more viscous state. In more technical language: some non-Newtonian pseudoplastic fluids show a time-dependent change in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity. A thixotropic fluid is a fluid which takes a finite time to attain equilibrium viscosity when introduced to a step change in shear rate. Some thixotropic fluids return to a gel state almost instantly, such as ketchup, and are called pseudoplastic fluids. Others such as yogurt take much longer and can become nearly solid. Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated.

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