Is It a Liquid or a Solid? The Strange Science of Non-Newtonian Fluids
Most students learn that matter can be divided into three familiar states: solids, liquids and gases.
A solid keeps its shape. A liquid flows and takes the shape of its container. A gas expands to fill the available space.
That classification is useful, but nature is rarely quite so tidy.
Some materials appear to behave like liquids when they are handled gently, yet become surprisingly solid when they are struck, squeezed or moved rapidly. A simple mixture of cornflour and water can flow through your fingers one moment and resist a sharp impact the next.
It raises a fascinating question:
How can the same material behave like a liquid and a solid without changing temperature or chemical composition?
The answer introduces us to the strange world of non-Newtonian fluids.
A Liquid That Does Not Follow the Normal Rules
Water, cooking oil and many other familiar liquids are described as Newtonian fluids.
In a Newtonian fluid, the viscosity remains approximately constant at a particular temperature.
Viscosity is a measure of how strongly a fluid resists flowing. Water has a relatively low viscosity, so it flows easily. Glycerol and golden syrup have much higher viscosities, so they flow much more slowly.
However, although glycerol is considerably more viscous than water, its viscosity does not suddenly change simply because we stir it faster or apply a greater force.
Cornflour mixed with water behaves differently.
When it is moved slowly, it flows. When it experiences a sudden force, it becomes much more resistant to movement. Its apparent viscosity increases.
This makes it a non-Newtonian fluid.
More precisely, a cornflour-and-water mixture is an example of a shear-thickening suspension. The faster we try to deform it, the more strongly it resists.
What Is Actually Inside the Mixture?
Cornflour does not dissolve in water in the same way that sugar or salt does. Instead, tiny solid particles remain suspended throughout the liquid.
When the mixture is handled gently, the particles have time to move around one another. Water acts as a lubricant between them, allowing the mixture to flow.
A sudden impact changes the situation.
The particles are forced together so quickly that they cannot rearrange themselves easily. They form temporary networks and become jammed against one another. The mixture then strongly resists further movement.
It can feel solid, but it has not undergone a permanent change of state. The effect only continues while the force is being applied.
Release the pressure, and the particles can begin moving again. The material returns to its flowing, liquid-like behaviour.
This is why a ball made from cornflour mixture appears solid while it is being squeezed but collapses into a puddle as soon as it is left alone.
Making a Non-Newtonian Fluid
The experiment is remarkably simple.
You will need:
Cornflour
Water
A large bowl or tray
A spoon
Food colouring, if required
Begin with approximately two parts cornflour to one part water. Add the water gradually because different brands of cornflour may require slightly different quantities.
Mix slowly until the material flows when gently tilted but strongly resists rapid stirring.
The mixture should not be watery. If it splashes easily, add more cornflour. If it remains dry and crumbly, add a small amount of water.
Once the consistency is correct, the investigation can begin.
Demonstration One: Slow Finger, Fast Finger
Place a finger gently onto the surface of the mixture and push down slowly.
Your finger should gradually sink into it.
Remove your finger and then strike the surface quickly with the flat of your hand or tap it sharply with one finger. The surface suddenly feels firm.
The mixture has not had time to flow away from the force. Its particles have become temporarily jammed together.
This is one of the clearest demonstrations because the only variable being changed is the rate at which the force is applied.
The hand is the same. The mixture is the same. The temperature is the same.
Only the speed of the movement has changed.
Yet the behaviour of the material is completely different.
Demonstration Two: Make a Temporary Solid Ball
Pick up some of the mixture and roll it rapidly between your hands.
While you continue applying pressure, it can be shaped into a surprisingly firm ball.
Now stop rolling and open your hands.
The ball immediately loses its shape and flows between your fingers.
Students often find this especially memorable because they can feel the transformation. It is not simply something they are being told about or shown on a diagram.
They experience the change directly.
The material has not chemically reacted, frozen or dried. It appears solid only because continuous force keeps the particles jammed together.
Demonstration Three: Can You Pull Your Hand Out?
Place your fingers into a deeper container of the mixture and try to remove them quickly.
The mixture grips surprisingly firmly.
Now relax and withdraw your fingers slowly. They come out much more easily.
This helps explain why panicking and making rapid movements in mud or other dense suspensions can sometimes make movement more difficult. Slow, controlled movement gives particles and liquid time to rearrange.
However, it is important not to suggest that all mud and quicksand behave exactly like cornflour mixture. Non-Newtonian fluids form a broad family, and different materials respond to force in different ways.
Some become thicker when moved rapidly. Others become thinner.
Demonstration Four: Dancing Cornflour
One of the most spectacular demonstrations involves placing the mixture above a loudspeaker.
The loudspeaker must first be protected with a secure waterproof membrane or covered tray. The mixture should never be poured directly onto the speaker cone.
When a low-frequency sound is played, the speaker vibrates rapidly. These vibrations continually accelerate and compress parts of the mixture.
At suitable frequencies and amplitudes, strange moving columns, folds and finger-like shapes can appear. The material seems to crawl or dance across the surface.
The sound waves are supplying repeated forces. Where the force is greatest, the mixture temporarily stiffens. As the force changes, it begins to flow again.
The resulting patterns can look almost alive.
This demonstration links several areas of science:
sound waves;
frequency;
vibration;
forces;
particle behaviour;
energy transfer;
properties of materials.
It is a particularly good example of how topics normally taught separately are actually connected.
Comparing Different Fluids
A useful investigation is to compare cornflour mixture with water, cooking oil and glycerol.
Pour equal amounts into separate transparent containers and observe how they behave when tilted, stirred or allowed to flow down a ramp.
Water flows quickly because it has a low viscosity.
Cooking oil usually flows more slowly.
Glycerol flows much more slowly because it has a higher viscosity.
However, these liquids do not suddenly become solid when struck. Their viscosities remain relatively predictable under normal classroom conditions.
The cornflour mixture is different because its resistance depends strongly on how rapidly the force is applied.
Students could investigate:
how long each fluid takes to travel down a ramp;
how quickly a ball bearing falls through each fluid;
how stirring speed affects resistance;
how changing the cornflour-to-water ratio affects behaviour;
whether temperature changes the results.
This turns a dramatic demonstration into a genuine scientific investigation involving variables, measurements and evidence.
Not All Non-Newtonian Fluids Become Thicker
The term “non-Newtonian” does not simply mean “a liquid that becomes solid when struck”.
It refers to any fluid whose viscosity does not remain constant under different flow conditions.
Cornflour and water are shear-thickening: they become more resistant when moved rapidly.
Other materials are shear-thinning. Their apparent viscosity decreases when they are stirred, spread or squeezed.
Paint is a familiar example. It needs to be thick enough not to run down the wall after application, but it must also spread easily under a brush or roller.
Tomato ketchup can also become easier to pour after it has been shaken. Toothpaste flows when squeezed but remains on the toothbrush when the pressure is removed.
Some materials require a minimum force before they begin to flow at all. This is why toothpaste can stay inside an open tube until it is squeezed.
The behaviour of non-Newtonian fluids is therefore much broader than the cornflour experiment suggests.
Why Does This Matter Outside the Classroom?
Non-Newtonian fluids are not simply scientific curiosities. Their behaviour is important in engineering, medicine, manufacturing, geology and product design.
Impact-Resistant Materials
Shear-thickening fluids have been investigated for use in protective equipment.
A flexible material is usually more comfortable to wear than a rigid plate. However, a material that temporarily stiffens during an impact could combine flexibility with additional protection.
Researchers have therefore studied fabrics containing shear-thickening fluids for possible use in protective clothing, sports equipment and body-armour systems.
The principle is similar to the cornflour experiment: flexible during ordinary movement, but much more resistant during a sudden impact.
Paints and Printing Inks
Paint needs carefully controlled flow properties.
It must move easily under a brush, roller or spray nozzle, but it should resist dripping once it reaches the wall.
Printing inks must also flow through machinery in a controlled way and then remain in position on the paper or packaging.
Understanding non-Newtonian behaviour allows manufacturers to design products that are easy to apply but remain stable afterwards.
Food Manufacturing
Many foods are non-Newtonian.
Yoghurt, sauces, chocolate, mayonnaise, cream, dough and ketchup all have complicated flow properties.
Manufacturers need to know how these materials will behave while being mixed, pumped, poured, transported and packaged.
A sauce may need to move easily through a factory pipe but remain thick enough to stay on the food when served.
Cosmetics
Shampoo, moisturiser, toothpaste, foundation and other cosmetics need very specific textures.
A cream should spread smoothly across the skin but should not run out of its container. Toothpaste should flow when squeezed but hold its shape on the toothbrush.
These properties are created by controlling the material’s non-Newtonian behaviour.
Geology and Natural Flows
Mud, wet sediment, lava and debris flows can behave in complex ways.
Their movement depends on particle size, water content, pressure, temperature and the forces acting on them.
Understanding these flows is important when studying landslides, volcanic eruptions, river sediment and unstable ground.
However, geological materials do not all behave like cornflour. Some become easier to move once they start flowing, while others can stiffen or jam under particular conditions.
Blood and Biological Fluids
Blood is also non-Newtonian.
It is not simply water with red colouring. It contains cells, proteins and many dissolved substances. Its apparent viscosity changes depending on the size of the blood vessel and the rate of flow.
This has important consequences for circulation and for the design of medical equipment such as pumps, artificial heart valves and blood-flow monitoring systems.
The mucus found in the respiratory and digestive systems also has specialised flow properties. It must be able to move while remaining thick enough to trap particles and protect delicate tissues.
What Looks Like a Simple Mess Is Actually Serious Science
Cornflour and water can easily be dismissed as a messy classroom activity.
In reality, it introduces some profound scientific ideas.
It shows that matter cannot always be placed into simple categories. It demonstrates that the properties of a material may depend not only on what it is made from, but also on how forces are applied to it.
It also encourages students to question the language they use.
Is the mixture really becoming a solid?
Not quite.
It is behaving like a solid for a short period because its internal particles have become jammed together. Once the force disappears, the structure breaks down and the material flows again.
That distinction is important. Scientific explanations should describe what is happening rather than merely repeat what something looks like.
Learning Through Touch, Movement and Surprise
In my experience, students remember science particularly well when an experiment challenges something they thought they already understood.
Most students believe they know the difference between a liquid and a solid. They have used both since early childhood.
Then they meet a substance that refuses to fit neatly into either category.
They press it gently and their finger sinks.
They strike it and it feels hard.
They squeeze it into a ball and then watch it melt through their hands without any change in temperature.
That moment of surprise creates curiosity. Curiosity creates questions, and those questions provide an opportunity for deeper scientific thinking.
At Philip M Russell Ltd, practical demonstrations are not treated as decorations added to a lesson. They are used to create the experience that the explanation must account for.
The student sees something unexpected, proposes an idea, tests it and then improves the explanation.
That is much closer to the way science actually works than simply copying a definition from a worksheet.
Practical Safety and Clearing Up
Cornflour mixture is generally straightforward to handle, but sensible precautions are still needed.
Use a tray to contain spills and protect nearby electrical equipment.
Do not pour large quantities down a sink. The particles can settle and contribute to blockages. Allow the mixture to dry before placing it in household waste, or scrape it into a suitable container for disposal.
If food colouring is used, remember that it may stain clothing and surfaces.
For the loudspeaker demonstration, keep the mixture completely separated from the electrical components by using a strong waterproof membrane or shallow sealed tray. Begin with a low volume and increase it gradually.
The Strangeness Is the Point
Non-Newtonian fluids show us that scientific categories are models rather than unbreakable rules.
Water behaves in a familiar and predictable way, so it is tempting to assume that every liquid must behave similarly.
Cornflour and water demonstrate that this is not true.
A material can flow gently through our fingers, resist a sudden blow and then collapse back into a puddle. Its behaviour depends on the forces acting upon its microscopic particles.
The experiment is inexpensive, memorable and easy to perform, but the ideas behind it connect to advanced materials, medicine, food production, cosmetics, engineering and geology.
The next time someone asks whether cornflour mixture is a liquid or a solid, perhaps the best scientific answer is:
It depends on what you do to it.
That may sound like an evasive answer, but it captures an important truth about science.
The natural world is often far more interesting than the simple categories we first use to describe it.
