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Why do thixotropic fluids become less viscous when they are stirred for a long time?

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differential manometer, Torricellian barometer, and siphon — fluid mechanics Figure 1: Schematic representations of (A) a differential manometer, (B) a Torricellian barometer, and (C) a siphon.

Thixotropic fluids become less viscous when stirred or agitated because they exhibit a time-dependent decrease in viscosity under constant shear stress.1

In their resting state, these substances typically exist as thick, gel-like structures. When subjected to mechanical disturbance—such as shaking or stirring—the internal structure of the gel is disrupted, causing it to liquefy and flow more freely. This process is reversible; once the agitation ceases and the fluid is allowed to stand, it will gradually revert to its original, more viscous gel-like state.23

Common examples of this behavior include certain types of paint, drilling mud containing bentonite, and even some food products like mayonnaise. Unlike Newtonian fluids, which maintain a constant viscosity regardless of the rate of shear, thixotropic fluids are classified as non-Newtonian because their internal resistance to flow changes over time during the application of force.231