Viscosity

In science, viscosity refers to a fluid’s internal resistance to flow. It describes how easily a liquid or gas moves. For example, honey flows much slower than water, meaning honey has a higher viscosity. Understanding viscosity helps explain the movement of fluids in nature, industries, and biological systems.

1.0What is Viscosity?

Viscosity Meaning

The viscosity meaning can be understood as the measure of a fluid’s “thickness” or internal friction. It shows how strongly the molecules of a fluid resist movement when a force is applied.

• Fluids with low viscosity (like water or air) flow easily.
• Fluids with high viscosity (like honey or oil) flow slowly.

Viscosity plays a key role in engineering, meteorology, medicine, and even everyday activities like pouring syrup or lubricating a machine.

Everyday Examples of Viscosity

• Low-viscosity fluids: Water, alcohol, petrol, air (flow quickly and smoothly).
• High-viscosity fluids: Honey, engine oil, glycerin, and tar (flow slowly and stickily).
  Viscosity influences countless real-world situations, such as the pouring of syrup, movement of blood in veins, and performance of lubricants in machinery.

2.0Scientific Explanation of Viscosity

Molecular Interaction in Fluids

Viscosity arises due to internal friction between adjacent layers of fluid moving at different speeds.

• In a flowing liquid, molecules in one layer drag the molecules in the neighboring layer through intermolecular forces.
• This frictional resistance is the viscous force, responsible for energy loss in fluid motion.

The stronger the intermolecular attraction, the greater the viscosity.

Viscous Force and Internal Friction

Consider a liquid flowing between two parallel plates — the lower plate is stationary, and the upper plate moves with velocity v.

• Layers closer to the stationary plate move slower, while those near the moving plate move faster.
• The difference in velocity between layers causes shear stress, which resists the flow.
  This resistance is quantified by viscosity (η).

3.0Types of Viscosity

Dynamic (Absolute) Viscosity

Dynamic viscosity (also called absolute viscosity) measures the internal resistance offered by a fluid when one layer moves over another.
It’s given by the equation:

$[\eta =\frac{F\cdot d}{A\cdot v}]$

Where:

• F = Force applied,
• d = Distance between layers,
• A = Area of the layer,
• v = Velocity difference.

A high value of η indicates the fluid is thick (like tar), while a low value indicates it’s thin (like air).

Kinematic Viscosity

• Kinematic viscosity relates the dynamic viscosity of a fluid to its density.
  It is represented by the symbol ν (nu) and given as:

$\nu =\frac{\eta }{\rho }$

Where ρ is the density of the fluid.
Kinematic viscosity is commonly used in engineering, particularly in the design of hydraulic systems and lubricants.

Newtonian and Non-Newtonian Fluids

• Newtonian Fluids: Fluids like water, air, and alcohol, where viscosity remains constant regardless of the applied force.
• Non-Newtonian Fluids: Substances such as ketchup, toothpaste, or paint, where viscosity changes depending on the applied stress.
• Example: Ketchup becomes thinner when shaken — a property known as shear-thinning.

4.0Units and Formula of Viscosity

Mathematical Expression

According to Newton’s Law of Viscosity: Viscosity formula $F=\eta A\frac{(v_1-v_2)}{x}$

Where:

• F = Force required to maintain flow,
• A = Area of layers,
• v₁ - v₂ = Velocity difference between layers,
• x = Distance separating layers.

Viscosity Unit and Symbols

Viscosity Symbols

The symbol used for viscosity is the Greek letter eta (η). For kinematic viscosity, the symbol is nu (ν).

SI and CGS Units

• SI Unit: Pascal-second (Pa·s) or N·s/m²
• CGS Unit: Poise (P)
  1 Poise = 0.1 Pa·s

For convenience, small values are expressed in centipoise (cP), where 1 cP = 0.001 Pa·s.
For example, water at 25°C has a viscosity of approximately 1 cP.

5.0Newtonian vs. Non-Newtonian Fluids

Not all fluids behave the same way when force is applied to them. This leads to a major classification in fluid mechanics.

Newtonian Fluids

For these fluids, viscosity remains constant regardless of the shear rate (how fast they are moving or how hard you push them). The relationship between shear stress and shear rate is linear.

• Examples: Water, air, alcohol, gasoline.
• Behavior: If you double the force pushing water through a pipe, it flows twice as fast. Its "thickness" does not change.

Non-Newtonian Fluids

For these fluids, viscosity changes depending on the shear rate. They can become thinner or thicker when force is applied.

• Shear-Thinning (Pseudoplastic): The viscosity decreases as the shear rate increases.
• Example: Ketchup or Paint. Ketchup sits thick in the bottle, but once you shake it (apply shear stress), it becomes runnier and flows out.
• Shear-Thickening (Dilatant): The viscosity increases as shear rate increases.
• Example: Oobleck (cornstarch and water). If you stir it slowly, it acts like a liquid. If you punch it, it acts like a solid.

6.0Factors Affecting Viscosity

Temperature

Temperature has the most significant effect on viscosity.

• For liquids: Viscosity decreases with increasing temperature because molecules move faster and overcome internal friction.
• For gases: Viscosity increases with temperature as molecular collisions become more frequent.

Example: Honey flows more easily when heated, while air becomes thicker at high temperatures.

Pressure

Pressure has a minor effect on liquids but a noticeable effect on gases.

• Increasing pressure usually increases viscosity in gases due to denser molecular packing.
• In liquids, since molecules are already close together, pressure has little effect.

Nature and Composition of Fluid

• Strong intermolecular forces (e.g., in glycerin) lead to high viscosity.
• Weaker forces (e.g., in alcohol) result in low viscosity.
  Also, impurities or dissolved substances can alter viscosity. For example, adding sugar to water increases its viscosity.

7.0Viscosity Measurement

Capillary Flow Method (Poiseuille’s Law)

Jean Poiseuille developed an equation to describe the flow of liquid through a narrow tube (capillary).
The volume of liquid V flowing per second through a tube of radius r and length l under pressure difference P is given by:

$V=\frac{\pi P{r}^4}{8\eta l}$

By measuring V, P, r, and l, the viscosity η can be calculated.
This principle is used in laboratory viscometers.

Falling Sphere Viscometer

A small sphere is allowed to fall through a liquid. Its steady speed is measured, and viscosity is determined using Stokes’ Law:

$\eta =\frac{2r^2({\rho }_s-{\rho }_f)g}{9v}$

Where:

• r = radius of the sphere
• ρₛ, ρ_f = densities of sphere and fluid
• v = terminal velocity
• g = acceleration due to gravity

Rotational Viscometers

These devices measure viscosity by rotating a spindle or disk inside the fluid. The torque required to turn the spindle is proportional to the fluid’s viscosity. Used widely in industries like oil, paint, and food production.

8.0Applications of Viscosity in Daily Life and Industry

Automobiles and Lubricants

Engine oils and lubricants are carefully chosen for their viscosity.

• High viscosity oils protect engine parts under high pressure.
• Low viscosity oils are preferred in cold weather for smoother movement.
  Viscosity ensures reduced friction, less wear and tear, and better fuel efficiency.

Medicine, Food, and Cosmetics

• In pharmaceuticals, viscosity controls the flow and stability of syrups, gels, and creams.
• In food processing, it determines the texture of sauces, soups, and beverages.
• In cosmetics, proper viscosity ensures even spreading of lotions and creams.

Engineering and Environmental Applications

• Viscosity is critical in fluid mechanics, pipeline design, and aerodynamics.
• It also plays a role in understanding lava flow, ocean currents, and blood circulation in biological systems.