Cream on

Why does a 2 mm layer of cream on Tia Maria create rapidly circulating and swirling patterns?

This is a perfect experiment for the after-dinner tabletop laboratory, and as it’s one that New Scientist brought to the attention of the world, we feel particularly smug. Use a good, single pouring cream – milk is too runny, double cream too thick. You can even drink the end product.

What do I need?

- a wide-bottomed glass

- single cream

- a bottle of Tia Maria

- a spoon

What do I do? Pour 1 cm of Tia Maria into the glass and add a very thin layer of single cream, poured gently over the back of the spoon so that it spreads over the surface of the Tia Maria.

What will I see? The surface of the cream will break up into a pattern of cells as it reacts with the Tia Maria.

What’s going on? The swirling patterns are caused by convection – the bulk movement of fluids usually caused by temperature differences. However, in this case the convection is driven by a difference in concentration between the two liquids. This is called solutal convection.

The alcohol in the Tia Maria is the driving force. After the cream is added, the alcohol begins to diffuse through it. When it reaches the surface it alters the surface tension, reducing it. Areas unaffected maintain their higher surface tension and so pull the low-surface tension liquid areas towards them. As the surface liquid is pulled away, Tia Maria moves up to fill its place. This has even less surface tension than the liquid pulled away and the process becomes self-sustaining as a convection cycle is created. This will last as long as a difference in concentration between the cream and the Tia Maria remains.

Surface tension convection is known as Marangoni convection. It plays a part in drying different paints and causes patterns such as those seen in wine ‘legs’ running down the inside of a drinking glass (see ‘Drinker’s legs’ experiment on page 13). Similar effects are seen in rolling clouds or oil frying in a pan. Tia Maria is something of an oddity though, because in other systems the convection cells are circular or hexagonal.

PS: We are especially pleased to include this experiment because when the question was first asked in New Scientist nobody knew the answer, which led to a group of researchers studying the phenomenon and writing the definitive paper on the subject.

For more experiments go to www.newscientist.com/hamster

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