What stops puddles from spreading

Researchers from Massachusetts Institute of Technology (MIT) say they now have answers to an unexplained puzzle - why puddle does not spread endlessly as is expected of water.

When puddles form "the liquid-solid and liquid-air interfaces start feeling each other" and these are the missing intermolecular forces that could explain why puddles stop, according to the study reported in the journal Physical Review Letters.

When you spill a bit of water onto a tabletop, the puddle spreads -- and then stops, leaving a well-defined area of water with a sharp boundary.

The formulas scientists use to describe such a fluid flow say that the water should just keep spreading endlessly. But everyone knows that is not the case.

"The classic thin-film model describes the spreading of a liquid film, but it doesn't predict it stopping," said one of the researchers Amir Pahlavan.

It turns out that the problem is one of scale, he said adding, "it is only at the molecular level that the forces responsible for stopping the flow begin to show up."

Whether someone's spilled milk stops on the tabletop or makes a mess all over the floor may seem like an issue of little importance, except to the person who might get soaked, or have to mop up the spill.

The principles involved affect a host of other situations where the ability to calculate how a fluid will behave can have important consequences.

For example, understanding these effects can be essential to figuring out how much oil is needed to keep a gear train from running dry, or how much drilling "mud" is needed to keep an oil rig working smoothly. Both processes involve flows of thin films of liquid.

According to the researchers, classical descriptions of spreading have a number of inconsistencies: For example, they require an infinite force to get a puddle to start spreading.

But close to a puddle's edge, "the liquid-solid and liquid-air interfaces start feeling each other," Pahlavan said.

"These are the missing intermolecular forces in the macroscopic description, accounting for these forces resolves the previous paradoxes, he said.