Step 1: Understanding the Concept:
The question asks what change in viscosity is associated with thixotropic behavior.
Step 2: Key Formula or Approach:
Thixotropy is a time-dependent, reversible change in a material's consistency. The key word is time-dependent: unlike pseudoplastic flow, where viscosity falls instantly and reversibly with shear rate, a thixotropic material's viscosity keeps falling the longer shear is applied, and it takes real time, not just removing the stress, to recover.
Step 3: Detailed Explanation:
This happens because many thixotropic systems, such as gels and certain suspensions, contain a loose internal network built from weak bonds between particles or polymer chains. Shearing breaks these bonds progressively, so the network weakens and viscosity drops as shearing continues. When the shear is removed, the bonds slowly reform and the network, along with the higher viscosity, is rebuilt over time. Formulators use this property deliberately: a thixotropic gel can be made runny enough to pour or spread by shaking or spreading it, then thicken back up once it sits still, which keeps it from running off a vertical surface or settling out during storage.
Step 4: Final Answer:
Thixotropic behavior is associated with a decrease in viscosity under shear.