The term thixotropy consists of the Greek words "thixis" (touch) and "trepein" (to turn). It means change or transition due to mechanical load.
In rheology thixotropic behavior is defined as time-dependent behavior. It means a reduction of the structural strength during a constant shear load phase and a more or less rapid but complete regeneration of the structure during the subsequent rest phase. This cycle of structural decomposition and regeneration is a completely reversible process.
A substance which does not regenerate its structure, even after an infinitely long rest phase, is not thixotropic. An example for a substance which is not thixotropic is French-style set yogurt. After stirring, the yogurt remains considerably thinner compared to the initial state so that you can observe a permanent change in the structure.
The opposite behavior may also be seen: the structural strength of a sample may increase during a constant shear load phase and decrease during the subsequent rest phase. This behavior is called rheopexy and is also a completely reversible process.
Often the term thixotropy is incorrectly used for describing flow processes. For example when observing the structural decomposition or time-dependent flow behavior and not the regeneration (e.g. in the "free flow test" of coatings and inks, when the flow path of a sample on an inclined plate is measured for different periods of the time-at-rest phase before raising the plate).
In contrast to thixotropy, shear thinning is a behavior in which the viscosity decreases with an increasing shear load.
When Is It Useful to Determine the Thixotropy?
Whenever the behavior of a sample should be investigated or a process should be simulated, thixotropic behavior is of interest, e.g. the behavior after a coating or filling process. During production and processing, there are often sudden changes between different levels of shear load (e.g. pumping, spraying).
Important quality factors for paints and coatings are the structural regeneration over time, which in turn influences the surface leveling and sagging behavior. If the structural regeneration occurs too quickly this leads to poor surface leveling. If the regeneration is too slow, the coating or paint will sag and result in an insufficient layer thickness.
For food products consisting of several ingredients, such as salad dressing, a quick structural regeneration is important to prevent separation after filling into the bottle. For ketchup lovers, thixotropy is important to ensure that enough sauce remains on the chips and does not flow off.
Immediately after the filling process rapid structural regeneration also makes sure that less material drips out of the filling nozzle and less mess is caused by dripping.
Thixotropy is often a decisive criteria for the end user when evaluating the product positively or negatively.
Figure 1. Upwards (1st interval in the measuring procedure) and downwards (3rd interval) flow curves of a lotion for determining the thixotropy via the hysteresis area. In the second interval a constant (high) shear rate is applied.
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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.
For more information on this source, please visit Anton Paar GmbH.