What is the Difference Between Thixotropic and Rheopectic Fluids

The main difference between thixotropic and rheopectic fluids is that thixotropic fluids become less viscous when subjected to shear stress, whereas rheotropic fluids become more viscous when subjected to shear stress.

Both thixotropic fluids and rheopectic fluids are non-Newtonian fluids, meaning their viscosity or flow behaviour cannot be described by a simple linear relationship between shear stress and shear rate.

Key Areas Covered

1. What are Thixotropic Fluids
     – Definition, Behaviour, Applications
2. What are Rheopectic Fluids
     – Definition, Behaviour, Applications 
3. Difference Between Thixotropic and Rheopectic Fluids
     – Comparison of Key Differences

Key Terms

Thixotropic Fluids, Rheopectic Fluids

Difference Between Thixotropic and Rheopectic Fluids - Comparison Summary

What are Thixotropic Fluids

Thixotropic fluids are materials that exhibit a time-dependent decrease in viscosity under constant shear stress. This means when a thixotropic fluid is stirred or agitated, it becomes less viscous and flows more easily. But when the shear stress is removed, the fluid gradually returns to its original high viscosity state. These types of liquids exhibit this behaviour due to the presence of large, long, chain-like molecules that become entangled with one another when the fluid is at rest. When it undergoes shear stress, the molecules begin to unravel and slide past each other, which in turn reduces the viscosity of the fluid. When the shear stress is removed, molecules slowly entangle again and return the fluid to its original state.

Compare Thixotropic and Rheopectic Fluids - What's the difference?

Figure 1: Shear Stress vs Time for Constant Strain Rate in Thixotropic and Rheopectic Fluids compared to a Newtonian Fluid

Thixotropic fluids have a variety of applications, including the fields of construction, electronics, and automotive. In the oil and gas industries, thixotropic drilling fluids help in drill cutting to the surface and support the wellbore. Another use of thixotropic materials is in the cosmetics industry. These fluids are used in the production of creams, lotions, and gels. The smooth and easy-to-spread texture provided by these types of fluids helps improve the product’s overall appearance. Thixotropic fluids also help the food industry to make products like sauces and desserts to give a smooth texture to food products.

What are Rheopectic Fluids

Rheopectic fluids are a type of non-Newtonian fluid that exhibits an increase in viscosity when subjected to shear stress over time. When agitated, the fluid becomes more viscous. When the shear stress is removed, the fluid gradually returns to its original lower viscosity state.

Rheopectic fluids can be found in natural and synthetic materials. One example of rheopectic fluid is corn starch mixed with water. Another example of a rheopectic fluid is Silly Putty. Silly Putty is a silicon-based polymer that exhibits rheopectic behaviour. When Silly Putty is moulded or squeezed, it becomes more viscous. When the pressure is removed, Sily Putty returns to its original lower viscosity state.

Thixotropic vs Rheopectic Fluids

Figure 2: Silly Putty

Rheopectic fluids are used in a wide range of consumer and industrial applications based on the properties they exhibit. In the printing industry, these types of fluids act as inks in screen printing and flexography. When these inks are in contact with a printing surface, they have a higher viscosity. It allows them to form a more precise and consistent image. These types of fluids are also useful in polishing. They have a higher viscosity when they are in contact with the surface being polished. They are also useful in the pharmaceutical and cosmetics industries.

Difference Between Thixotropic and Rheopectic Fluids

Definition

Thixotropic fluids become less viscous when subjected to shear stress, whereas rheotropic fluids become more viscous when subjected to shear stress.

Behaviour

Thixotropic fluids have a shear thinning behaviour, meaning that their viscosity decreases as the shear rate increases, whereas rheopectic fluids have a shear thickening behaviour, meaning that their viscosity increases as the shear rate increases.

Applications

Thixotropic fluids are used in paint, adhesives, drilling fluids, sealants, cosmetics, and food products, whereas rheopectic fluids are used in printing, polishing, and suspension applications, including inks, polishing materials, and pharmaceuticals.

Conclusion

Both thixotropic fluids and rheopectic fluids are non-Newtonian fluids. The main difference between thixotropic and rheopectic fluids is that thixotropic fluids become less viscous when they undergo shear stress, whereas rheotropic fluids become more viscous when they undergo shear stress.

Reference:

1. “Basics of thixotropy.” Anton Paar Wiki.
2. “Thixotropic and Rheopectic Fluids.” Fluid Dynamics.

Image Courtesy:

1. “Thixotropic-rheopectic” By DirectEON (talk) – self-made (CC BY-SA 3.0) via Commons Wikimedia
2. “UFV Science Rocks (14698165796)” By University of the Fraser Valley (CC BY 2.0) via Commons Wikimedia

About the Author: Hasini A

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