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3.5. Viscosity

How To Invent (Almost) Anything > 3. A Simple Science Lens > 3.5. Viscosity

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Imagine pushing a brick across a treacle-covered kitchen table. It would not be an easy slide. But what if you replaced the treacle with engine oil? The brick’s journey would now be much smoother. So what is going on? How does a lubricant act to reduce the work of brick-pushing?

One thing a lubricant does is to keep apart by a small distance the surface molecules of the brick and the table. By keeping them apart we can reduce the bounce needed (they ride smoothly on the wave of liquid), reduce the need to fracture on the surface, reduce the elastic bending of bits that protrude, and reduce the electrostatic forces.

The difference between the oil and the treacle is how easily the molecules within the lubricant move relative to one another (because the electrostatic forces are really weak). This kind of ‘internal friction’ is viscosity of the liquid. In a really good lubricant, the molecules bond better to the brick and the table than they do to one another.

Now imagine something that has really weak attraction between the molecules (they could even be repel one another). This sounds very much like a gas. If you can keep the bodies apart, gas is an excellent lubricator, as Sir Christopher Cockerell, the inventor of the hovercraft, well knew.

Think about a jar of jam. Inside you have a wide blade and you want to stir the jam. If the opening in the pot of jam is much larger than the size of the blade when you stir, it is easy for the molecules to move as they have lots of places to go. They can move to the biggest gaps. When the space available is limited they are harder to push around. The same is true if you want to move people around in a crowded room! So when you have a very smooth surface against another very smooth surface the viscosity can go up a lot because the molecules do not have much room to move around. Thinking creatively means using principles of friction and viscosity in situations where others would not, but which will lead to real insights.

Other sections in this chapter:

• 3.1. Bounce
• 3.2. Elasticity
• 3.3. Fracturing
• 3.4. Electrostatic Action
• 3.5. Viscosity
• 3.6. Gravity
• 3.7. Invention 'So What'?

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