Why pints spill but straws don't!

It is not only the size, but the shape of a tube that determines whether a liquid will spill out of it when tipped over, new research has found.
Glasses of liquid, when turned horizontally, inevitably spill. This is not necessarily the case however with very thin straws, which, when turned on their sides, can retain liquid in them.
This simple relationship was thought to be based purely on the size of the tube opening, but by investigating more closely, researchers, including those from University of Oxford in the UK, have determined that this rule of thumb does not always hold true.

The shape of the tube turns out to be important too, and if it is squashed enough - forming an elliptical shape - then no matter how thin the straw is the liquid will always spill out.
This new understanding has practical applications in technologies that have liquids present on small scales - such as biomedical diagnostics, oil recovery and inkjet printing - where choosing the right tube shape could be as important as its size, researchers said.
"If your pint glass falls over, tragedy has struck and you know you're going to spill your beer. But conversely that doesn't necessarily happen if you suck your beer into a straw and turn that horizontally," said Professor Andrew Parry of Imperial College London in the UK.

"In that case common experience tells us that if the straw is thin enough the liquid stays in. Now, we have discovered that it should be possible to create minute straw shapes that would mean that any liquid spills, or empties out of the tube, no matter how thin it is," said Parry.

The team's findings are based on the behaviour of liquids in contact with surfaces. If you look closely at water in a glass, you can see the edges curve up slightly, so that the surface of the water looks like a shallow bowl.
This is due to the force of surface tension, a phenomenon that determines how the liquid surface touches the sides of the glass.

The majority of the liquid in the glass is not held this way, so that when the glass is tipped to the side, the force of gravity wins and all the liquid spills out.
On the other hand, if the diameter of the glass is small enough, such as in a very thin straw, then gravity is not able to overcome the forces of surface tension and the liquid remains within the tube, researchers said.
This is why drinking straws, or capillaries in pens, are only a few millimetres across.
However, the researchers found that this simple rule breaks down when the shape of the tube is changed from being circular to being elliptical or triangular cross-section.

In this case it is possible that the liquid will spill even when the tube is microscopically small.
The research was published in the journal PNAS.