Viscosity: Understanding Fluid Resistance
Viscosity is the property of a fluid that determines its resistance to shear or flow. It behaves similarly to friction in solids, arising from the interactions between fluid layers. When a fluid flows between two plates—one stationary and the other moving—the fluid layers exert forces on each other depending on their velocities. The upper layer moves with a velocity v, while the layer in contact with the stationary plate has a velocity of 0, which results in a velocity gradient across the fluid. This gradient leads to shearing stress, with the coefficient of viscosity (η) defined as the ratio of this stress to the strain rate.
Key Points:
- Definition: Viscosity is defined by the equation:
(9.16)
- Units and Measurements: The SI unit of viscosity is the pascal-second (Pa·s), and it can also be expressed in poise (P). The relationship between shear stress and shear rate fosters various applications, such as calculating forces on submerged bodies using Stokes' law.
- Factors Affecting Viscosity: Viscosity of liquids decreases with increasing temperature, while gases experience increasing viscosity with rising temperature, due to changes in molecular activity.
- Practical Implications: Understanding viscosity is essential for various fields such as engineering, medicine, and environmental science, as it plays a crucial role in fluid dynamics and behavior in real-world scenarios, including bloodstream dynamics and oil flow in pipelines.