Step 1: Understanding the Concept:
The question asks which instrument is used to observe the structural breakdown that happens in a thixotropic material as it is sheared.
Step 2: Key Formula or Approach:
Thixotropy is a time-dependent property: shear stress at a fixed shear rate falls as shearing continues, because the internal network of the material is being broken down, and it climbs back up once shearing stops and the structure rebuilds. Studying this needs an instrument that can apply a controlled shear rate and record the resulting shear stress continuously.
Step 3: Detailed Explanation:
A rotational viscometer does exactly this. Running it up through increasing shear rates and back down again traces out two different curves, an up-curve and a down-curve, and the gap between them, called the hysteresis loop, is a direct measure of how much structural breakdown occurred during shearing. The area enclosed by this loop is often used as a numerical indicator of the degree of thixotropy. Instruments built for measuring flow rate in pipes, such as rotameters or orifice meters, or for tracing plane areas, such as planimeters, have no way to apply and record shear stress on a sample, so they cannot be used for this purpose.
Step 4: Final Answer:
A viscometer is the instrument used to study the structural breakdown of a thixotropic material.