Dilatancy

Definition

Dilatancy is a phenomenon in which a material increases in volume and exhibits a higher viscosity when subjected to shear stress. This behavior is commonly observed in certain non-Newtonian fluids, where the fluid thickens under stress, making it harder to flow. Dilatant materials can appear solid-like under high shear rates but return to a more liquid state when the stress is removed, highlighting the complex interplay between shear forces and material structure.

5 Must Know Facts For Your Next Test

1. Dilatancy is often observed in materials like cornstarch mixed with water, which can act as a solid under pressure but flow like a liquid when at rest.
2. The increase in viscosity for dilatant materials occurs because the particles within the fluid structure rearrange and interlock under stress, resisting flow.
3. Dilatant behavior is critical in various applications, including safety gear and impact-resistant materials that need to absorb shock and return to their original form.
4. Not all non-Newtonian fluids exhibit dilatancy; many exhibit shear-thinning behavior instead, where viscosity decreases with increased shear rates.
5. The study of dilatancy helps in understanding phenomena in various fields, including rheology, food science, and material engineering.

Review Questions

• How does dilatancy affect the flow characteristics of certain non-Newtonian fluids?
  • Dilatancy causes non-Newtonian fluids to thicken and become more viscous when subjected to shear stress, leading to unique flow characteristics. As shear rates increase, the particles within the fluid interlock more tightly, making it harder for the fluid to flow. This behavior contrasts sharply with Newtonian fluids, where viscosity remains constant regardless of shear rate. Understanding dilatancy allows us to predict how certain materials will behave under different conditions, which is essential in various applications.
• Compare and contrast dilatancy with thixotropy in non-Newtonian fluids.
  • Dilatancy and thixotropy are both behaviors exhibited by non-Newtonian fluids but have opposite effects under stress. While dilatant materials increase in viscosity when subjected to shear forces, thixotropic materials decrease in viscosity over time under constant shear. This means that dilatant fluids become harder to move as stress increases, while thixotropic fluids become easier to move. Both behaviors are crucial for understanding how these fluids will perform in practical applications, such as inks or paints.
• Evaluate the practical implications of dilatancy in engineering and material science applications.
  • Understanding dilatancy has significant implications for engineering and material science, especially when designing impact-resistant materials and safety equipment. For example, materials that exhibit dilatant behavior can absorb shocks effectively by behaving like solids upon impact while allowing for flow during regular handling. This duality can enhance product safety and performance. Moreover, knowledge of dilatant properties aids in formulating new materials tailored for specific applications, improving their functionality based on desired mechanical responses under varying conditions.

Related terms

Shear-thickening: Shear-thickening refers to the increase in viscosity of a fluid as the shear rate increases, often associated with dilatant behavior.

Non-Newtonian fluids: Non-Newtonian fluids are those whose viscosity changes with the rate of shear strain, contrasting with Newtonian fluids that maintain a constant viscosity.

Thixotropy: Thixotropy is a time-dependent shear-thinning property of certain materials, where viscosity decreases under continuous shear and recovers when the shear is removed.