The main difference between thixotropic and rheopectic fluids lies in how their viscosity changes with stress over time: Thixotropic fluids are liquids or gases whose viscosity decreases upon applying stress over a known time period. These fluids become progressively less viscous when shaken or agitated. Examples of thixotropic fluids include certain gelatin solutions and some clay suspensions.
Rheopectic fluids are fluids that become more viscous under constant stress over time. These fluids exhibit the opposite behavior of thixotropic fluids, as their viscosity increases with stress over time. Some common examples of rheopectic fluids include gypsum pastes, printer ink, and lubricants. Both thixotropic and rheopectic fluids are considered non-Newtonian fluids, as they do not follow Newton's law of viscosity, which states that the stress-shear rate relation is linear. These types of fluids are relatively rare compared to Newtonian fluids, but they have unique properties that make them useful in various applications.