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Author: Emily Butler

Have you ever wondered why when trying to pour honey it is so much more difficult than pouring water? This is because honey has a much greater viscosity than water.

Viscosity is the measure of a fluids resistance to flowing, which you can think of as its thickness.

Fluids have varying viscosities and we can compare these by seeing how easily fluids flow down slopes. In this YouTube video by the The Sci Guy they demonstrate how fluids with a higher viscosity, which we can think of as thicker fluids, move much slower down the slope due to their higher resistance to motion when compared with the less viscous fluids. Honey took almost 100 times longer to get to the same place on the slope when compared with water. This is because water had the highest flow rate (fastest) and lowest viscosity (thinnest).

We can make sense of this by thinking of liquids as being a collection of molecules held together by cohesive forces. The viscosity of the fluid is defined by how strong these cohesive forces are. In a highly viscous fluid, like honey for example, these cohesive forces are much strong, making it harder for the molecules to move past each other, reducing the flow rate of the fluid. However, in a less viscous fluid, like water, these cohesive forces are much weaker, allowing molecules to move past each other much more freely, resulting in a higher flow rate. Heating the honey decreases the viscosity and reduces the forces allowing the liquid honey to flow more freely with a lower viscosity, however cooling the honey increases these forces, thickening the honey and resulting in a much slower moving fluid with a much higher viscosity.

So, why do we care about the viscosity of a fluid? The viscosity of a fluid has an impact upon how that fluid may behave and interact with surrounding objects.  If a fluid is highly viscous, it is harder for an object to move through when compared to a fluid which is less viscous. For example, if you tried to drop a marble into a jar of honey it would take a while for it to sink to the bottom, whereas if you dropped it into a glass of water it would sink almost immediately. An illustration of this can be seen below. It is much easier for the marble to move through the water due to its low viscosity. This is because fluids with a lower viscosity produce a lower resistance to movement of the object through the fluid.

We can visualise the impact of the viscosity of the fluid on an object within a flow by using this link to visualise a wind tunnel.

You can draw a shape in the tunnel, a simple circle for example and use this to compare the behaviour of the fluid after the object for varying levels of viscosity. The graph underneath allows you to look at the lift and drag on the object. Increasing the viscosity shows a much greater drag, since the higher viscosity means the object is experiencing a higher resistance to the flow.

This picture below is an example of a fluid with a low viscosity.

In contrast, this is an example of a fluid with a much higher viscosity.

As you can see the one with a much higher viscosity has much less fluctuations in the flow, the flow is changing much less frequently.  Have a go at creating your own on aerodoogle!

Newtonian and Non-Newtonian Fluids

Not only do different fluids have different viscosities, in some cases a fluid’s viscosity can change. A fluid, which undergoes changing viscosity, can change its behaviour in time. The viscosity of a fluid can change depending upon various factors, for example, some fluids have a lower viscosity when they are heated. When honey is warmed up it is easier to pour than honey at room temperature.

Fluids which change their viscosity when a force is applied to them are a special group of fluids, referred to as non-Newtonian fluids.  A force acting upon a fluid is also described as applying stress to the fluid. Water does not have this property, and is instead known as a Newtonian fluid.

You have probably encountered non-Newtonian fluids in your day-to-day life and not even realised it! Usually, it is a property of the fluid we need to overcome. Say you want to get some tomato sauce out of the bottle. You know that there is some in there but when you turn it upside down nothing comes out. So, you shake the bottle or hit it on the base. This process leads to the sauce becoming thinner allowing you to squirt some out. The stress force that has been applied to the fluid when you hit the bottle has encouraged the viscosity to decrease.

Other examples of non-Newtonian fluids are toothpaste, blood, and paint. Can you think of any more fluids that have this strange property?

Can a fluids viscosity lead it to defy the laws of physics?

The viscosity of a fluid is also interesting since it can help us to demonstrate phenomena that seem impossible, allowing us to answer questions like, can you un-mix a fluid? Is it possible to walk on water? Can fluid flow upwards?

Well yes, for a very thick viscous fluid, in some cases, it is in fact possible to unmix the fluid!  Watch this 'unmixing' phenomenon in this can be done in this YouTube video!

Radzi deomstrates in a Youtube video that it is in fact possible to walk on a liquid making use of the non-Newtonian fluid, custard, which gets thicker when force is applied. The resistance to flow can become so high that it acts as a solid and you are in fact able to walk on it! If you are interested, you could see this phenomenon at home my mixing cornflour and water to create a non-Newtonian Fluid. If you move your fingers slowly through the fluid, it feels like it has a low viscosity. However, if you punch it or squeeze it it seems much thicker.