What is Viscosity?
Viscosity is a measure of a fluid's resistance to flow and deformation. A fluid with high viscosity (like honey) flows slowly, while a fluid with low viscosity (like water) flows easily. Viscosity is one of the most important properties in fluid mechanics, affecting everything from pipe flow to lubrication to weather patterns.
There are two types of viscosity: dynamic (absolute) viscosity measures a fluid's internal resistance to flow, while kinematic viscosity is the ratio of dynamic viscosity to fluid density. They are related by the simple equation: kinematic viscosity = dynamic viscosity / density.
Viscosity Formulas
Viscosity of Common Fluids (at 20°C)
| Fluid | Dynamic (μ, Pa·s) | Kinematic (ν, cSt) |
|---|---|---|
| Air | 1.81 × 10&sup5; | 15.1 |
| Water | 1.002 × 10³ | 1.004 |
| Olive Oil | 0.081 | ~84 |
| Honey | 2 - 10 | ~1500 - 7000 |
| Motor Oil (SAE 30) | 0.2 - 0.3 | ~230 - 340 |
| Glycerin | 1.5 | ~1190 |
Frequently Asked Questions
What is the difference between dynamic and kinematic viscosity?
Dynamic (absolute) viscosity measures a fluid's internal resistance to flow under an applied force (units: Pa-s or cP). Kinematic viscosity divides dynamic viscosity by the fluid density, representing the fluid's resistance to flow under gravity (units: m^2/s or cSt). Kinematic viscosity is often more practical because many flow situations are gravity-driven.
How does temperature affect viscosity?
For liquids, viscosity decreases significantly with increasing temperature (molecules move faster and have less interaction). For gases, viscosity increases with temperature (more molecular collisions). This is why engine oil flows more easily when warm and why SAE ratings include winter (W) grades.
What are Newtonian and non-Newtonian fluids?
Newtonian fluids (water, air, most oils) have a constant viscosity regardless of the shear rate applied. Non-Newtonian fluids change viscosity with shear rate: shear-thinning fluids (like paint, blood) become less viscous under stress, while shear-thickening fluids (like cornstarch in water) become more viscous.