Liquids bounce again
[I can’t resist putting up this little item, written for news@nature, because it is a classic kitchen experiment you can do yourself – the recipe is below.]

Jumping jets move from the bathroom to the kitchen

After bouncing shampoo, physicists now bring you bouncing cooking oil. A team in Texas has found that the trampolining of a liquid jet falling onto a bath of the same liquid is more common than expected.

Last year, a group in the Netherlands studied this bouncing effect for a jet of shampoo. The bounce, which was first reported over 40 years ago, happens because of the peculiar nature of shampoo, which gets thinner (less viscous) as it flows. A jet of it hitting a liquid surface is therefore lubricated by a thin layer at the interface, enabling it to bounce off rather than merge.

But the liquids now studied by Matthew Thrasher and colleagues at the University of Texas at Austin don’t have this property – they are viscous, but have ‘normal’ flow behaviour, like water.

The researchers directed a jet of oil vertically onto the surface of a tank of the same oil. They found that the jet could undergo both a ‘leaping’ rebound and a bizarre ‘flat’ bounce in which it sprang horizontally across the liquid surface.

The bounce here is due to a thin layer of air that separates the two liquid surfaces, the researchers say.

They point out that the effect can easily be recreated in a kitchen experiment with cooking oil. Just fill a glass pie dish with about 4 cm of oil and pour onto it a thin stream from a cup about 3-6 cm above the surface. While pouring, move the stream in a circle about once every 2 seconds (or rotate the dish on a Lazy Susan). The bounce can be encouraged by passing a small rod like a chopstick through the stream every now and then.