Topics
Viscosity and Flow of Fluids
Fluid Layers and Internal Friction
Viscosity and Terminal Velocity
What is Viscosity?
Viscosity is a measure of a fluid’s resistance to flow.
A fluid with high viscosity resists motion and flows relatively slowly.
A fluid with low viscosity flows more easily.
For example:
- honey has higher viscosity than water
- glycerine has higher viscosity than water
- air has very low viscosity compared with most liquids
Viscosity applies to both liquids and gases.
Why Does Viscosity Occur?
When a fluid flows, neighbouring layers of the fluid move relative to one another. Interactions between particles resist this relative motion. This internal resistance produces viscosity.
- In liquids, intermolecular attractions play an important role.
- In gases, viscosity is mainly related to the transfer of momentum between rapidly moving gas molecules.
Viscosity and Flow of Fluids
Viscosity of fluids determine their flow speed.
High-Viscosity Fluids
High-viscosity fluids resist flow strongly.
Examples include:
- honey
- glycerine
- thick lubricating oils
- syrup
These fluids generally flow more slowly under the same conditions.
A highly viscous liquid may also take longer to spread or pour.
Low-Viscosity Fluids
Low-viscosity fluids flow more easily.
Examples include:
- water
- alcohol
- petrol
- air
Their particles or fluid layers can move past one another with less resistance.
A low-viscosity fluid therefore responds more readily to an applied force.
Fluid Layers and Internal Friction
A flowing fluid can be imagined as consisting of many layers.
Suppose liquid flows through a pipe.
The layer touching the pipe wall moves more slowly because of interaction with the wall.
Layers farther from the wall can move faster.
Neighbouring layers resist sliding past one another.
This resistance is sometimes described as internal friction.
Viscosity measures the strength of this resistance.
Comparing Viscosity
Viscosity can be compared by observing how quickly fluids move under similar conditions.
For example, if equal quantities of water, oil and honey are poured down identical slopes:
- water generally moves fastest
- oil moves more slowly
- honey moves slowest
This provides a simple qualitative comparison of viscosity.
For accurate measurements, specialised instruments are used.
Dynamic Viscosity
The resistance of one layer of fluid sliding past another is described by dynamic viscosity.
Dynamic viscosity is commonly represented by the Greek letter η.
The SI unit of dynamic viscosity is:
pascal-second (Pa·s)
Another commonly encountered unit is the poise, although SI units are preferred in Physics.
Kinematic Viscosity
Another useful quantity is kinematic viscosity.
It relates dynamic viscosity to the density of the fluid:
where:
- 𝝂 (nu) is kinematic viscosity
- η (eta) is dynamic viscosity
- ρ(rho) is density
The SI unit of kinematic viscosity is:
m2/s
Kinematic viscosity is particularly useful in fluid engineering.
Viscosity and Temperature
Temperature affects the viscosity of a fluid.
Viscosity and Temperature in Liquids
For most liquids, viscosity decreases as temperature increases.
When a liquid is heated, its particles move more energetically.
The effect of intermolecular attraction becomes less effective in resisting flow.
As a result, the liquid flows more easily.
This is why:
- warm honey pours more easily than cold honey
- heated oil usually flows more readily than cold oil
Viscosity and Temperature in Gases
Gases behave differently.
For gases, viscosity generally increases as temperature increases.
At higher temperatures, gas molecules move faster and transfer momentum more effectively between neighbouring layers.
So the effect of temperature on viscosity is opposite for liquids and gases.
Viscosity and Pressure
Pressure can also affect viscosity.
For many liquids under ordinary conditions, moderate changes in pressure produce relatively small changes in viscosity.
At very high pressures, however, viscosity can increase significantly.
For gases, changes in pressure can also influence viscosity, particularly under specialised conditions.
For most everyday applications, temperature usually has the more noticeable effect.
Measuring Viscosity
Viscosity can be measured using instruments called viscometers.
Different viscometers are designed for different fluids and applications.
Measurements may be based on:
- how quickly a liquid flows through a narrow tube
- how fast an object falls through a liquid
- the resistance experienced by a rotating component
- the time taken for a fixed volume of liquid to flow
The method chosen depends on the type of fluid and the required accuracy.
Falling-Ball Method
One way to compare or measure viscosity is to observe a ball falling through a liquid.
A ball falls relatively quickly through a low-viscosity liquid.
It falls more slowly through a high-viscosity liquid because the fluid provides greater resistance to motion.
This method demonstrates clearly how viscosity opposes movement through a fluid.
Viscous Drag
When an object moves through a fluid, the fluid resists its motion.
Part of this resistance is related to viscosity.
This resisting force is called viscous drag.
The amount of drag depends on factors including:
- viscosity of the fluid
- speed of the object
- size and shape of the object
- nature of the flow
Viscous drag is important in swimming, vehicle design and movement through liquids and gases.
Stokes’ Law
For a small sphere moving slowly through a viscous fluid under suitable conditions, the viscous drag can be described by Stokes’ Law:
F=6πηrv
where:
- F is viscous drag force
- η (eta) is dynamic viscosity
- r is radius of the sphere
- v is speed of the sphere relative to the fluid
Stokes’ Law applies only under particular flow conditions, but it provides an important connection between viscosity and drag.
Viscosity and Terminal Velocity
An object falling through a fluid experiences several forces.
These may include:
- weight acting downward
- buoyant force acting upward
- viscous drag acting upward against the motion
As the object speeds up, viscous drag increases.
Eventually, the upward forces may balance the object’s weight.
The net force then becomes zero and the object continues at a constant speed called terminal velocity.
This links viscosity with buoyancy and the motion of objects through fluids.
Viscosity and Flow Through Pipes
Viscosity strongly affects the flow of fluids through pipes.
A highly viscous fluid requires a greater pressure difference to produce the same flow rate as a less viscous fluid.
This is important in:
- water supply systems
- oil pipelines
- chemical processing
- medical tubing
- industrial machinery
Pipe diameter also has a major influence on flow.
Laminar Flow
In laminar flow, fluid moves in smooth, orderly layers.
Adjacent layers slide past one another with relatively little mixing.
Laminar flow is more likely when:
- fluid speed is low
- viscosity is relatively high
- the passage is narrow
The flow of a viscous liquid slowly through a narrow tube is often approximately laminar.
Turbulent Flow
In turbulent flow, fluid motion becomes irregular.
The fluid develops eddies, swirling motion and mixing.
Turbulence is more likely when:
- fluid speed is high
- viscosity is relatively low
- the dimensions of the flow passage are large
Turbulent flow can increase resistance and energy loss.
Reynolds Number
The tendency of fluid flow to be laminar or turbulent can be described using the Reynolds number.
It depends on factors including:
- fluid density
- fluid speed
- characteristic size of the flow
- viscosity
A low Reynolds number generally favours laminar flow.
A high Reynolds number makes turbulent flow more likely.
The Reynolds number is widely used in fluid mechanics and engineering.
Viscosity and Lubrication
Lubricants reduce friction between moving surfaces.
A suitable lubricant forms a film that separates the surfaces and reduces direct contact.
Viscosity is therefore an important property of lubricating oils.
If an oil is too thin, it may not provide sufficient separation.
If it is too viscous, it may create excessive resistance to movement.
The correct viscosity must therefore be chosen for the operating conditions.
Viscosity of Engine Oil
Engine oils are designed to work over a range of temperatures.
When an engine is cold, the oil must still flow sufficiently well to reach moving components.
When the engine becomes hot, the oil must remain viscous enough to maintain an effective lubricating film.
This is why the temperature dependence of viscosity is important in engine design and maintenance.
Viscosity in the Human Body
Viscosity is important in biological fluids.
Blood, for example, must flow through vessels ranging from large arteries to extremely small capillaries.
Its viscosity affects the resistance to blood flow.
The behaviour of blood is more complicated than that of a simple fluid, but viscosity remains an important physical property.
Other biological fluids also depend on appropriate viscosity for normal function.
Viscosity and Blood Flow
For a given blood vessel, greater fluid viscosity tends to increase resistance to flow.
The heart must generate pressure differences to move blood through the circulatory system.
Blood flow therefore connects several important ideas about fluids:
- pressure
- viscosity
- fluid flow
Biological systems are complex, but the underlying Physics helps explain why fluid properties matter.
Effects of Viscosity
The effects of viscosity can be observed in many areas.
Viscosity in Food
Viscosity strongly affects the texture and behaviour of foods.
Examples include:
- honey
- sauces
- syrups
- yoghurt
- cooking oils
- chocolate
- beverages
Food manufacturers measure and control viscosity to maintain consistent texture, processing and quality.
Temperature can be especially important because many food liquids become easier to pour when heated.
Viscosity in Paints and Inks
Paint and ink must have suitable viscosity.
If paint is too viscous, it may be difficult to spread.
If it is too thin, it may run or fail to form the required coating.
Ink viscosity affects:
- printing
- droplet formation
- spreading
- flow through equipment
This connects directly with the effects of Surface Tension.
Viscosity in Medicine
Many medicines are liquids or suspensions whose flow properties must be controlled.
Viscosity is important in:
- syrups
- eye drops
- injections
- creams and gels
- pharmaceutical manufacturing
The correct viscosity can affect how a product is delivered, spreads and remains in contact with a surface.
Viscosity in Industry
Industrial processes often involve fluids that must be pumped, mixed, poured, sprayed or transported.
Viscosity affects the energy required for these operations.
Industries in which viscosity is important include:
- petroleum
- chemicals
- food processing
- paints and coatings
- pharmaceuticals
- cosmetics
- manufacturing
Controlling viscosity can improve both product quality and process efficiency.
Viscosity in Nature
Viscosity affects many natural processes.
Examples include:
- movement of water in rivers
- movement of air in the atmosphere
- flow of magma
- movement of fluids through soil
- circulation within living organisms
Magma, for example, can vary greatly in viscosity. This affects how easily it flows and influences volcanic behaviour.
Viscosity in Everyday Life
We observe viscosity frequently without necessarily using the term.
Examples include:
- pouring water
- squeezing shampoo from a bottle
- spreading paint
- pouring honey
- using cooking oil
- applying creams
- lubricating a bicycle chain
Whenever we notice that one fluid flows more easily than another, viscosity is involved.
Applications of Viscosity
Viscosity is important in:
- lubrication
- pipelines
- engines
- medicine
- food processing
- paints and inks
- cosmetics
- chemical industries
- biological systems
- transportation
Choosing and controlling viscosity is essential wherever fluids must move in a predictable way.
Properties of Fluids That Define Their Behaviour
Several properties help us understand how fluids behave.
Density compares mass with volume.
Pressure describes how force is distributed over an area.
Buoyancy is the upward force produced by pressure differences within a fluid.
Surface tension arises from molecular interactions at the surface of a liquid.
Viscosity describes a fluid’s resistance to flow.
Together, these ideas explain many familiar fluid behaviours, from floating and sinking to droplet formation and the different ways liquids and gases flow.
Key Points
- Viscosity is a fluid’s resistance to flow.
- High-viscosity fluids flow less readily than low-viscosity fluids.
- Viscosity arises from interactions between neighbouring regions of a fluid.
- Dynamic viscosity is measured in pascal-seconds (Pa·s).
- Kinematic viscosity relates dynamic viscosity to density.
- For most liquids, viscosity decreases as temperature increases.
- For gases, viscosity generally increases as temperature increases.
- Viscometers are used to measure viscosity.
- Viscosity contributes to the drag experienced by objects moving through fluids.
- Stokes’ Law describes viscous drag on a small sphere under suitable conditions.
- Viscous drag can lead to terminal velocity.
- Viscosity strongly affects flow through pipes.
- Laminar flow is smooth and orderly, while turbulent flow is irregular.
- Reynolds number helps indicate whether flow is likely to be laminar or turbulent.
- Lubricants must have suitable viscosity for their operating conditions.
- Viscosity is important in blood flow, food, medicines, paints, inks, industry and natural processes.
- Viscosity completes the connected sequence from Density → Pressure → Buoyancy → Surface Tension → Viscosity.
Glossary of Key Terms
Recap of the Key Terms in Viscosity
- Dynamic Viscosity: A measure of a fluid’s resistance to one layer sliding past another.
- Fluid: A substance that can flow, such as a liquid or a gas.
- Internal Friction: The resistance between neighbouring layers of a fluid as they move relative to one another.
- Kinematic Viscosity: The ratio of dynamic viscosity to the density of a fluid.
- Laminar Flow: Smooth, orderly fluid flow in which neighbouring layers move with little mixing.
- Lubricant: A substance used between moving surfaces to reduce friction and wear.
- Reynolds Number: A dimensionless quantity used to help predict whether fluid flow is likely to be laminar or turbulent.
- Stokes’ Law: A relation describing the viscous drag on a small sphere moving slowly through a fluid under suitable conditions.
- Terminal Velocity: The constant speed reached by an object moving through a fluid when the forces acting on it become balanced.
- Turbulent Flow: Irregular fluid flow involving mixing, eddies and swirling motion.
- Viscometer: An instrument used to measure the viscosity of a fluid.
- Viscosity: A measure of a fluid’s resistance to flow.
- Viscous Drag: A resisting force caused by viscosity when an object moves through a fluid.
- High-Viscosity Fluid: A fluid that resists flow strongly and flows relatively slowly.
- Low-Viscosity Fluid: A fluid that offers less resistance to flow and moves more easily.
Questions and Answers
Recap the concepts you have learnt. Try to answer the questions. You can find the answer to any question by clicking on the icon.
What is viscosity?
Viscosity is a measure of a fluid’s resistance to flow.
What does high viscosity mean?
High viscosity means that a fluid strongly resists flow and moves relatively slowly.
What does low viscosity mean?
Low viscosity means that a fluid offers less resistance to flow and moves more easily.
Give two examples of high-viscosity fluids.
Honey and glycerine are examples of high-viscosity fluids.
Give two examples of low-viscosity fluids.
Water and air are examples of low-viscosity fluids.
Does viscosity apply only to liquids?
No. Viscosity applies to both liquids and gases.
Why does viscosity occur?
Viscosity occurs because neighbouring regions or layers of a fluid resist moving past one another.
What is meant by internal friction in a fluid?
Internal friction is the resistance between neighbouring fluid layers moving relative to one another.
How can fluid flow be imagined in layers?
A flowing fluid can be considered as layers moving at different speeds relative to one another.
Why does fluid close to the wall of a pipe move more slowly?
Interaction between the fluid and the pipe wall slows the fluid near the surface.
What is dynamic viscosity?
Dynamic viscosity measures the resistance of one layer of fluid sliding past another.
What symbol is commonly used for dynamic viscosity?
Dynamic viscosity is commonly represented by the Greek letter η.
What is the SI unit of dynamic viscosity?
The SI unit is the pascal-second, written as Pa·s.
What is kinematic viscosity?
Kinematic viscosity is dynamic viscosity divided by the density of the fluid.
What is the SI unit of kinematic viscosity?
The SI unit of kinematic viscosity is square metre per second, written as m²/s.
How does temperature affect the viscosity of most liquids?
The viscosity of most liquids decreases as temperature increases.
Why does warm honey flow more easily than cold honey?
Heating reduces the viscosity of honey, allowing it to flow more easily.
How does temperature affect the viscosity of gases?
The viscosity of gases generally increases as temperature increases.
Does pressure affect viscosity?
Yes. Pressure can affect viscosity, although temperature is often more important under ordinary conditions.
What instrument is used to measure viscosity?
An instrument called a viscometer is used to measure viscosity.
How can viscosity be compared using a falling ball?
A ball falls more slowly through a more viscous fluid and faster through a less viscous fluid.
What is viscous drag?
Viscous drag is a resisting force caused by viscosity when an object moves through a fluid.
What factors affect viscous drag?
Viscous drag depends on factors such as fluid viscosity, object speed, size and shape, and the nature of the flow.
What does Stokes’ Law describe?
Stokes’ Law describes the viscous drag on a small sphere moving slowly through a fluid under suitable conditions.
What factors appear in Stokes’ Law?
The viscous drag depends on fluid viscosity, the radius of the sphere and its speed relative to the fluid.
What is terminal velocity?
Terminal velocity is the constant speed reached when the forces acting on a moving object become balanced.
What forces can act on an object falling through a fluid?
Weight, buoyant force and viscous drag can act on the object.
How does viscosity affect flow through pipes?
A more viscous fluid generally requires a greater pressure difference to produce the same flow rate.
What is laminar flow?
Laminar flow is smooth, orderly flow in which fluid moves in layers with little mixing.
What is turbulent flow?
Turbulent flow is irregular fluid motion involving mixing, eddies and swirling.
When is laminar flow more likely to occur?
Laminar flow is more likely at lower speeds, with higher viscosity and in narrower passages.
When is turbulent flow more likely to occur?
Turbulent flow is more likely at higher speeds, with lower viscosity and in larger flow passages.
What is the Reynolds number?
The Reynolds number is a dimensionless quantity used to help predict whether flow is likely to be laminar or turbulent.
What does a low Reynolds number usually indicate?
A low Reynolds number generally indicates that laminar flow is more likely.
What does a high Reynolds number usually indicate?
A high Reynolds number generally indicates that turbulent flow is more likely.
Why is viscosity important in lubrication?
A lubricant must have suitable viscosity to form a protective film while still allowing moving parts to operate efficiently.
Why is viscosity important in engine oil?
Engine oil must flow when the engine is cold while remaining viscous enough to provide lubrication when the engine becomes hot.
Why is viscosity important in blood flow?
Blood viscosity affects the resistance to flow through blood vessels.
Why is viscosity important in paints and inks?
It affects how easily they spread, flow through equipment and form suitable coatings or droplets.
Why is viscosity important in everyday life and industry?
Viscosity affects lubrication, pipelines, food, medicines, paints, biological fluids, manufacturing and many other situations involving fluid flow.