Dilatant vs. Shear Thickening

Attribute	Dilatant	Shear Thickening
Definition	Material that becomes thicker and more resistant to flow under shear stress	Material that exhibits an increase in viscosity with increasing shear rate
Behavior	Shows a decrease in viscosity under shear stress	Shows an increase in viscosity under shear stress
Examples	Cornstarch and water mixture	Silica suspensions in water
Applications	Body armor, impact protection materials	Shear thickening fluids for damping and impact protection

Introduction

When it comes to materials science, understanding the behavior of fluids under different conditions is crucial. Two phenomena that are often studied are dilatant and shear thickening. While both involve an increase in viscosity under shear stress, there are key differences between the two. In this article, we will explore the attributes of dilatant and shear thickening and compare their characteristics.

Definition

Dilatant fluids, also known as shear thickening fluids, are materials that become thicker and more viscous when subjected to shear stress. This means that as the fluid is agitated or stirred, its viscosity increases, making it harder to deform. Shear thickening, on the other hand, is a specific type of dilatant behavior where the viscosity of the fluid increases abruptly at a certain shear rate. This sudden change in viscosity is what distinguishes shear thickening from other types of dilatant behavior.

Behavior Under Stress

One of the key differences between dilatant and shear thickening fluids is their behavior under stress. Dilatant fluids exhibit a gradual increase in viscosity as shear stress is applied, leading to a more uniform thickening of the fluid. In contrast, shear thickening fluids experience a sudden spike in viscosity at a critical shear rate, causing a rapid thickening of the fluid. This abrupt change in viscosity is often referred to as a "shear thickening transition."

Applications

Both dilatant and shear thickening fluids have a wide range of applications in various industries. Dilatant fluids are commonly used in applications where controlled viscosity is required, such as in the manufacturing of paints, inks, and cosmetics. Shear thickening fluids, on the other hand, are often used in impact-resistant materials, such as body armor and protective gear. The ability of shear thickening fluids to rapidly increase in viscosity under stress makes them ideal for absorbing and dissipating energy in high-impact situations.

Structural Differences

Another important distinction between dilatant and shear thickening fluids lies in their structural properties. Dilatant fluids typically consist of suspended particles in a liquid matrix, with the particles forming a network that hinders flow under shear stress. Shear thickening fluids, on the other hand, often exhibit a more complex microstructure, with interactions between particles leading to the sudden increase in viscosity at the shear thickening transition. Understanding the structural differences between dilatant and shear thickening fluids is crucial for designing materials with specific rheological properties.

Rheological Properties

When it comes to rheological properties, dilatant and shear thickening fluids exhibit distinct behaviors. Dilatant fluids typically have a shear-thinning behavior at low shear rates, meaning their viscosity decreases as shear stress is applied. In contrast, shear thickening fluids display a shear-thickening behavior, where their viscosity increases with shear rate until the shear thickening transition occurs. This difference in rheological properties is what allows shear thickening fluids to provide enhanced protection against impact and penetration.

Conclusion

In conclusion, while dilatant and shear thickening fluids both involve an increase in viscosity under shear stress, they exhibit distinct behaviors and have different applications. Dilatant fluids show a gradual increase in viscosity, while shear thickening fluids experience a sudden spike in viscosity at a critical shear rate. Understanding the structural and rheological properties of these fluids is essential for harnessing their unique characteristics in various industries. By studying the attributes of dilatant and shear thickening, researchers can develop innovative materials with tailored rheological properties for specific applications.