aquafresh stripes

Science of a different stripe

There are some questions that have plagued humanity for thousands of years. Why are we here? How did we get here? Are we alone in the universe?

...And how do they get the stripes into toothpaste?

Well, we can at least answer that last one. Aquafresh is our red, white and blue-striped toothpaste, which is available in 127 countries and used by 260 million people globally. Anyone who used the toothpaste as a child is likely to have wondered at some point how those stripes get into the tubes and how is it that they come out of it so uniformly. The answer to the first part is simple – the second, not so much.

Getting the stripes into the tubes

Each tube of toothpaste is filled from the bottom using a filling machine that funnels the toothpaste, just like the machines that fill ice cream cones.

Aquafresh
Aquafresh

We get the stripes in by drawing on different colours of paste, and merging these into a divided nozzle, that keeps the colours separate but dispenses them at an even and consistent flow into the tube. If you could cut open a cross section of this nozzle (without getting messy) it would look like this:

So far, so simple. Now for the tricky part - how do those stripes keep their position and consistency when you squeeze the tube?

The science of stripes

The key is that the different stripes all have the same ‘rheology’. This means that under different pressures they retain the same thickness and they flow in the same way, so they stay as stripes in the paste.

When at rest inside the tube on your bathroom shelf, the stripes are thick. But when you squeeze the tube, the toothpaste ‘yields’, and they become thinner as they flow out of the nozzle. But because they are all of the same matched rheology, they flow at the same speed and consistency and they retain their position in the paste. This is true whether being squeezed through the filler in the factory or out of the tube at your home.

But now comes the really clever part. The toothpaste also has something known as ‘thixotropic rheology’. This means that when you remove the pressure, (i.e. once it is on your toothbrush), the stripes regain their thickness, enabling the product to look like a three-striped cylinder. Then when you brush, the product thins in your mouth making it easier to disperse and rinse away.

So there you have it. When it comes to the stripes in Aquafresh, the secret is all in the rheology. As for the rest of life’s big questions well...unfortunately there you’re on your own.