surface tension

Clutching at straws © stock.adobe.com/au/EvanTravels

The science of spill prevention It doesn’t take a genius to know that if you knock a glass of water on its side, the liquid will spill out. However, if you happened to be drinking your water through a thin straw, some of the liquid will be retained within this straw — even when turned on its side.

B

On the other hand, if the diameter of the glass

carefully designed, the area of the exposed liquid

ut what is it exactly that makes this

is small enough, such as in a very thin straw, then

surface could change dramatically in response

possible? Is it down to the size of the straw, its shape,

gravity is not able to overcome the forces of surface

to small changes as it crosses the threshold

its material or something else entirely? Seven years

tension and the liquid remains within the tube. This

from staying put to spilling out. Their precise

ago, Professor Andrew Parry, Dr Carlos Rascón and

is why drinking straws, or capillaries in pens, are

calculations could have application in technologies

Professor Dirk Aarts began the calculations that

only a few millimetres across.

that have liquids present at small scales — such

would eventually provide them with the answer.

However, the researchers found that this

as biomedical diagnostics, oil recovery and inkjet

If you look closely at water in a glass, you can

apparently simple rule relating to the size of the

printing — where choosing the right tube shape

see the edges curve up slightly, so that the surface

tube opening breaks down when its cross-section

could be as important as its size.

of the water looks like a shallow bowl. This is due

is changed from being circular to being elliptical or

“We have discovered that it should be possible

to the force of surface tension, a phenomenon

triangular. In this case, it is possible that the liquid

to create minute straw shapes that would mean

that determines how the liquid surface touches the

will spill even when the tube is microscopically

that any liquid spills or empties out of the tube,

sides of the glass. The majority of the liquid in the

small.

no matter how thin it is,” said Professor Parry.

glass is not held this way, so that when the glass is

Writing in the journal Proceedings of the

tipped to the side, the force of gravity wins and all

National Academy of Sciences, the researchers

This article is a modified version of a news item published

the liquid spills out.

propose that if the size and shape of a tube are

by Imperial College London under CC BY 4.0

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