Table of Contents
What Are Poise and Stokes?
Viscosity describes a fluid's resistance to flow. There are two types: dynamic (absolute) viscosity measured in Poise (P) and kinematic viscosity measured in Stokes (St). Dynamic viscosity represents the internal friction when layers of fluid slide past each other, while kinematic viscosity is the ratio of dynamic viscosity to fluid density.
The Poise unit is named after Jean Poiseuille, a French physicist who studied blood flow. One Poise equals 1 g/(cm·s). The Stokes unit is named after Sir George Gabriel Stokes. One Stokes equals 1 cm²/s. In practice, engineers commonly use centiPoise (cP) and centiStokes (cSt) since most fluids have viscosities in these ranges. Water at 20°C has approximately 1 cP dynamic viscosity and 1 cSt kinematic viscosity, making these convenient reference points.
Conversion Formula
Where ν is kinematic viscosity, μ is dynamic viscosity, and ρ is fluid density. Divide dynamic viscosity by density for kinematic viscosity, or multiply kinematic viscosity by density for dynamic viscosity.
Common Fluid Viscosities
| Fluid (20°C) | Dynamic (cP) | Density (g/cm³) | Kinematic (cSt) |
|---|---|---|---|
| Water | 1.002 | 0.998 | 1.004 |
| Ethanol | 1.20 | 0.789 | 1.52 |
| Motor Oil (SAE 30) | 250 | 0.88 | 284 |
| Glycerin | 1500 | 1.261 | 1190 |
| Mercury | 1.55 | 13.546 | 0.114 |
| Air | 0.0182 | 0.00120 | 15.17 |
Applications
- Lubrication engineering: Selecting the correct lubricant viscosity grade for bearings and gears.
- Chemical processing: Designing pipes, pumps, and mixing equipment for viscous fluids.
- Food industry: Quality control of syrups, sauces, and dairy products.
- Petroleum industry: Characterizing crude oil and fuel specifications.
- Biomedical engineering: Blood viscosity measurements for diagnostics.
Frequently Asked Questions
What is the difference between dynamic and kinematic viscosity?
Dynamic viscosity measures a fluid's internal resistance to flow under an applied force. Kinematic viscosity is dynamic viscosity divided by density, representing the fluid's resistance to flow under gravity. A dense fluid can have high dynamic viscosity but relatively low kinematic viscosity.
Why do engineers prefer centiPoise and centiStokes?
Water at 20°C has a dynamic viscosity of approximately 1 cP and kinematic viscosity of approximately 1 cSt, making these units convenient reference points. Most industrial fluids fall in ranges easily expressed in cP and cSt without excessive decimal places.
How does temperature affect viscosity?
For liquids, viscosity decreases as temperature increases because thermal energy reduces intermolecular forces. For gases, viscosity increases with temperature because faster-moving molecules transfer more momentum between layers. This temperature dependence is critical for engineering calculations.