Saturday, July 21, 2007

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.


Anne Suzuki said...

i was searching for a jet seems that i landed on some kind of liquid jet which is i don't know what it is. :) but very interesting. =)

JimmyGiro said...

Viscosity thinning... lubrication by a thin layer... thin layer of air !?

Stand back folks, this is a job for ARMCHAIR PHYSICIST !!!

The Dutch suggestion seems to be the closest in what is probably the same type of mechanism, regardless of cooking oil or shampoo.

Both types of molecule are non-symmetrical, therefore have the potential to orient themselves. The shampoo probably is a liquid crystal to start with, whereas the cooking oil needs a push to orient itself.

The dropping column of liquid will stretch smoothly due to the lower end moving faster than the top, the surface molecules of the jet will therefore align momentarily due to sheer flow.

When the aligned jet meets the random bulk, there is a demarcation of form. In the same way that Mancunians taste the same after you put them in a food blender, despite the fact that city fans will not mix with united fans. Or thermals will appear to have 'clear defined' edges even though the difference between the gas interface is merely a small temperature and pressure gradient (or should that be small difference but large gradient?).

In order to blend, the demarcation needs to be removed. So the jet slows down front end first, and the tail end concertinas into it and undermines the orientation of the falling stretch.

Instead of using that laser for pretty fibre optic displays, they should have shone it at various reflection angles to the external surface of the jet, and measured the molecular alignment vis ellipsometry.

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