Viscosity & Rheology

Thixotropy and Rheopexy

In addition to shear-dependent behavior, certain fluids exhibit time-dependent changes in viscosity. These phenomena, known as thixotropy and rheopexy, describe how a fluid’s viscosity changes over time when subjected to constant shear. Understanding these behaviors is critical in industries where product performance is influenced by the duration of applied force, such as in paints, coatings, cosmetics, and lubricants.

What is Thixotropy?

Thixotropy is a time-dependent shear-thinning behavior. This means that a fluid's viscosity decreases over time when a constant shear force is applied. Once the shearing stops, the fluid gradually returns to its original, thicker state.

Example:

Paint: When paint is applied with a brush or roller, it becomes thinner, allowing for smooth application. Once on the surface, as the shearing action stops, the paint thickens again, preventing drips and runs.
Heavy printing inks: These materials thin out during application, allowing them to spread easily, but regain viscosity over time to hold their shape.

In a manufacturing process, thixotropic behavior can be beneficial, especially when materials need to be easily pumped or spread during application but require thickness or structure afterward.

Real-world applications:

Cosmetics: Lotions and creams that spread easily during application but regain thickness to form a protective layer.
Food products: Sauces that pour easily but settle into a stable consistency on the plate.

What is Rheopexy?

Rheopexy, on the other hand, is the opposite of thixotropy. It describes fluids that increase in viscosity over time when subjected to constant shear. Once the shearing force is removed, the fluid gradually returns to a less viscous state.

Example:

Gypsum slurries: These slurries thicken as they are stirred or mixed over time, making them easier to mold or shape during construction.

Although rheopexic behavior is less common than thixotropy, it is crucial in applications that require materials to build structure under continuous shear, such as in certain chemical or industrial processes.

Real-world applications:

Greases: In mechanical applications, certain greases stiffen when subjected to prolonged shearing, helping to form a stronger lubricant layer for protection.

Visualizing Time-Dependent Behavior

Both thixotropy and rheopexy can be represented graphically by plotting shear stress against shear rate over time:

Thixotropic fluids will show a decrease in viscosity over time when shear is applied, forming a "hysteresis loop" when the shear rate is increased and then decreased. This demonstrates how the fluid becomes thinner with time under shear and thickens once the shear is removed.
Rheopectic fluids will show an increase in viscosity with constant shear over time, demonstrating how the fluid builds structure under stress.

The Importance of Time-Dependent Behavior

Time-dependent flow behaviors like thixotropy and rheopexy are crucial in applications where a fluid’s performance must adapt to different processing or use conditions. In industries like coatings, cosmetics, and lubricants, these behaviors impact product application, stability, and longevity.

For example, thixotropic fluids can be applied smoothly but remain stable and thick once at rest, making them ideal for paints, adhesives, and personal care products. Rheopectic fluids, although less common, are useful in applications where increasing viscosity over time provides benefits, such as in specific greases and slurries.

At AMETEK Brookfield, we provide advanced instrumentation to measure and analyze time-dependent flow behaviors like thixotropy and rheopexy. By understanding these behaviors, manufacturers can optimize product formulations, enhance processing efficiency, and ensure superior product performance.

Understanding thixotropy and rheopexy is essential for anyone working with complex fluids where time-dependent viscosity changes affect performance and quality. Brookfield offers the expertise and tools to help you master these important flow characteristics for success in your industry.