Short Answer:

Viscosity is defined as the property of a fluid that resists its motion or flow. It represents the internal friction between adjacent layers of fluid moving at different velocities. A fluid with high viscosity (like honey) flows slowly, while a fluid with low viscosity (like water) flows easily.

In simple words, viscosity measures how thick or sticky a fluid is. It depends on the nature of the fluid and temperature. As temperature increases, the viscosity of liquids decreases, while for gases, it increases. It plays an important role in fluid mechanics, lubrication, and hydraulic systems.

Detailed Explanation :

Viscosity

Viscosity is one of the most important physical properties of fluids that describes their resistance to deformation and flow. When a fluid moves, one layer slides over another. The friction or resistance that opposes this relative motion between the layers is called viscosity. It is a measure of the internal resistance offered by the fluid to the applied shear stress.

Viscosity exists due to cohesive forces between molecules and the momentum transfer between layers moving at different speeds. Fluids with strong intermolecular attraction or higher internal friction have higher viscosity. For example, honey is more viscous than water because it has stronger molecular bonds and resists flow more strongly.

Concept of Viscosity

When a fluid is in motion, its layers move at different velocities. The layer near a solid boundary moves slowly, while the layer farther away moves faster. This velocity difference creates a shear stress between the layers.

According to Newton’s law of viscosity,

where,

τ = Shear stress between fluid layers (N/m² or Pa)
μ = Dynamic (absolute) viscosity (N·s/m² or Pa·s)
du/dy = Velocity gradient or rate of shear strain (s⁻¹)

This law shows that shear stress in a fluid is directly proportional to the velocity gradient. The constant of proportionality is called dynamic viscosity (μ).

Types of Viscosity

Viscosity can be expressed in two main forms:

Dynamic (or Absolute) Viscosity (μ):
It measures the internal resistance of a fluid to motion when an external force is applied. It is the proportionality constant in Newton’s law of viscosity.
SI Unit: Pascal-second (Pa·s) or N·s/m²

Example: The dynamic viscosity of water at 20°C is approximately 1.002 × 10⁻³ Pa·s.

Kinematic Viscosity (ν):
It is the ratio of dynamic viscosity to the density of the fluid.

SI Unit: m²/s
Kinematic viscosity represents how easily a fluid flows under the influence of gravity and is useful in fluid flow calculations.

Newtonian and Non-Newtonian Fluids

Based on how shear stress relates to the velocity gradient, fluids can be classified as:

Newtonian Fluids:
These fluids obey Newton’s law of viscosity. Their shear stress is directly proportional to the rate of shear strain.
Examples: Water, air, kerosene, and thin oils.
Non-Newtonian Fluids:
These fluids do not obey Newton’s law of viscosity. The relationship between shear stress and velocity gradient is not linear.
Examples: Blood, toothpaste, paints, and polymer solutions.

In non-Newtonian fluids, viscosity changes with the rate of shear, while in Newtonian fluids, it remains constant.

Factors Affecting Viscosity

Temperature:
- For liquids, viscosity decreases with an increase in temperature because intermolecular forces weaken, allowing molecules to move more freely.
- For gases, viscosity increases with an increase in temperature because molecular motion and momentum transfer increase.
Pressure:
- In liquids, viscosity increases slightly with pressure since molecules are forced closer together.
- In gases, the effect of pressure on viscosity is very small.
Chemical Composition:
The viscosity depends on molecular structure. Fluids with larger or more complex molecules have higher viscosity.
Impurities:
The presence of impurities can either increase or decrease viscosity, depending on the nature of the solute and solvent.

Importance of Viscosity in Engineering

Viscosity plays a key role in mechanical, civil, and chemical engineering. It affects how fluids flow, transfer energy, and provide lubrication.

Lubrication:
In machines, oil is used as a lubricant. The correct viscosity ensures smooth motion and reduces wear between moving parts. Too low viscosity causes insufficient lubrication, while too high viscosity increases frictional losses.
Hydraulic Systems:
Viscosity determines how easily hydraulic oil can flow through pipes and valves. Proper viscosity ensures efficient power transmission and reduces energy loss.
Fluid Flow Calculations:
In fluid mechanics, viscosity helps determine whether a flow is laminar (smooth) or turbulent (irregular). This is identified using the Reynolds number, which depends on viscosity.
Heat Transfer:
In heat exchangers, viscosity affects the flow rate and heat transfer efficiency of fluids.
Design of Pumps and Pipes:
The resistance to flow in pipes depends on viscosity. Engineers must account for it to calculate pressure drops and select suitable pumps.

Examples of Viscosity Values

Water (20°C): 1.002 × 10⁻³ Pa·s
Air (20°C): 1.8 × 10⁻⁵ Pa·s
Mercury: 1.55 × 10⁻³ Pa·s
Engine oil (40°C): 0.25 Pa·s
Honey: 2 to 10 Pa·s

These values show how different fluids resist flow depending on their viscosity. Thicker fluids like honey have higher viscosity, while thinner fluids like air have very low viscosity.

Effect of Viscosity in Daily Life

Viscosity is not only important in engineering but also in daily life. It explains why honey flows slowly, how paints spread on walls, and why blood circulation is smoother when viscosity is normal. In industries, viscosity control ensures product quality in lubricants, food items, and chemicals.

Conclusion

In conclusion, viscosity is the property of a fluid that resists motion between its layers. It represents the internal friction within the fluid and affects its flow behavior. It depends mainly on temperature, pressure, and molecular structure. Viscosity is a key factor in fluid mechanics, lubrication, and hydraulic systems, influencing flow efficiency, energy loss, and mechanical performance. Understanding viscosity helps engineers design systems that use fluids safely and efficiently.