Why do puddles stop spreading?

Susanna Pilny for redOrbit.com – @PlinyTheShorter

It’s something we take for granted every day: If you spill water on the counter, it’ll spread out for a while and then stop. But the mathematical formulas we have to describe this phenomenon predict that a liquid will spread endlessly—something that any child could tell you doesn’t happen.

Now, researchers from MIT believe they have finally found an explanation.

“The classic thin-film model describes the spreading of a liquid film, but it doesn’t predict it stopping,” explained co-author Amir Pahlavan. As it turns out, the old model only dealt with activities on a macroscopic level, completely ignoring molecular-level forces that also have a part to play.

These forces are tiny, but have a major effect on how liquid behaves. “[W]hat’s actually stopping the puddle is forces that only act at the nanoscale,” said Pahlavan.

Close to the liquid’s edge, “the liquid-solid and liquid-air interfaces start feeling each other. These are the missing intermolecular forces in the macroscopic description.”

Why this is important

This might seem inconsequential, but it can have many applications down the road. Being able to calculate how fluid will behave can play a major role across multiple industries. For example, it is essential in figuring out how much oil is needed to keep a gear lubricated in a machine, or how much “mud” is needed to allow an oil rig to run smoothly.

It could also be important for building microchips, because as their design grows smaller, heat buildup becomes a more pressing issue. Liquids can be used to keep microchips cool and functioning, but only if you can figure out how cooling fluids will flow and spread across chips.

The work, which was supported by the U.S. Department of Energy, can be found in the journal Physical Review Letters.

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