Viscosity of common fluids by poise

Viscosity is a fundamental property of fluids that describes their resistance to flow. It is a measure of the internal friction between adjacent layers of a liquid or gas in motion. In simpler terms, viscosity determines how “thick” or “thin” a fluid is — for instance, honey has a much higher viscosity than water.
The unit of viscosity in the CGS (centimetre–gram–second) system is the poise (P), named after the French physicist Jean Léonard Marie Poiseuille. One poise is defined as the viscosity of a fluid in which a shearing stress of one dyne per square centimetre maintains a velocity gradient of one centimetre per second per centimetre.
1 poise (P)=1 dyne\cdotps/cm21 \, \text{poise (P)} = 1 \, \text{dyne·s/cm}^21poise (P)=1dyne\cdotps/cm2
For practical purposes, viscosity is often expressed in centipoise (cP), where:
1 P=100 cP1 \, \text{P} = 100 \, \text{cP}1P=100cP

Types of Viscosity

There are two primary forms of viscosity:

• Dynamic (absolute) viscosity: The internal resistance of a fluid to flow when an external force is applied. It is measured in poise (P) or pascal-seconds (Pa·s) in the SI system.
• Kinematic viscosity: The ratio of dynamic viscosity to the fluid’s density, measured in stokes (St) or centistokes (cSt).

Conversion Between Units

The SI unit of dynamic viscosity is the pascal-second (Pa·s). The relationship between poise and pascal-second is given as:
1 P=0.1 Pa\cdotps1 \, \text{P} = 0.1 \, \text{Pa·s}1P=0.1Pa\cdotps 1 cP=0.001 Pa\cdotps1 \, \text{cP} = 0.001 \, \text{Pa·s}1cP=0.001Pa\cdotps

Viscosity of Common Fluids

The following table lists the approximate dynamic viscosities of various common fluids at or near room temperature (25°C), expressed in poise (P) and centipoise (cP).

Temperature Dependence of Viscosity

The viscosity of liquids decreases with an increase in temperature, whereas the viscosity of gases increases with temperature. This occurs because:

• In liquids, heating reduces intermolecular forces, making it easier for molecules to slide past each other.
• In gases, higher temperatures increase molecular collisions, leading to greater resistance to flow.

For example:

• Water’s viscosity drops from 1.002 cP at 20°C to 0.653 cP at 40°C.
• Air’s viscosity increases from 0.017 cP at 0°C to 0.019 cP at 40°C.

Measurement of Viscosity

Several instruments are used to measure viscosity, depending on the nature of the fluid and accuracy required:

• Capillary Viscometer (Ostwald or Ubbelohde type): Based on Poiseuille’s law; suitable for Newtonian liquids.
• Rotational Viscometer: Measures torque required to rotate an object in the fluid.
• Falling Sphere Viscometer: Determines viscosity by timing the fall of a sphere through the fluid.
• Cup and Bob Viscometer: Commonly used for paints and oils.

Viscosity and Fluid Classification

Fluids can be broadly classified according to their viscosity behaviour:

• Newtonian Fluids: Show constant viscosity independent of shear rate (e.g., water, air, ethanol).
• Non-Newtonian Fluids: Show variable viscosity with changing shear rate (e.g., blood, ketchup, toothpaste, mud).

Importance and Applications

Viscosity plays a crucial role in multiple scientific and engineering disciplines:

• Mechanical Engineering: Lubrication design, engine oils, and hydraulics depend on viscosity control.
• Chemical Engineering: Determines flow rates in pipes, reactors, and process systems.
• Medicine and Biology: Blood viscosity affects circulation and diagnostics.
• Food Industry: Viscosity influences texture and mouthfeel in products like sauces and syrups.
• Meteorology and Environmental Science: Viscosity of air and water influences drag, flow, and heat transfer in natural systems.