Joe Flow 09 - Thixotropy and Structure Recovery
Under constant shear stress, most samples show a decrease in structural strength. To recover, they need a certain period of time.
What is thixotropy? - And what is not?
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.
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