MIT scientists find what stops a puddle from spreading

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MIT scientists have solved the longstanding mystery of why puddles don't spread endlessly - and it comes down to certain forces that act at the nanoscale.
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. But the formulae scientists use to describe such a fluid flow say that the water should just keep spreading endlessly.
This mystery has now been solved by researchers at Massachusetts Institute of Technology - and while this phenomenon might seem trivial, the finding's ramifications could be significant.

Understanding such flowing fluids is essential for processes from the lubrication of gears and machinery to the potential sequestration of carbon dioxide emissions in porous underground formations, researchers said.
"The classic thin-film model describes the spreading of a liquid film, but it doesn't predict it stopping," said graduate student Amir Pahlavan.
It turns out that the problem is one of scale, he said.

"It's only at the molecular level that the forces responsible for stopping the flow begin to show up. And even though these forces are minuscule, their effect changes how the liquid behaves in a way that is obvious at a much larger scale," he said.

"Within a macroscopic view of this problem, there's nothing that stops the puddle from spreading. There's something missing here," Pahlavan said.
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.
"Properly accounting for these forces resolves the previous paradoxes," he said.
"What's striking here," Pahlavan added, is that "what's actually stopping the puddle is forces that only act at the nanoscale."

The ability to calculate how a fluid will behave can have important consequences, scientists said.
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.
Another area where the new findings could be important is in the design of microchips. As their features get smaller and smaller, controlling the buildup of heat has become a major engineering issue; some new system use liquids to dissipate that heat.

Understanding how such cooling fluids will flow and spread across the chip could be important for designing such systems, Pahlavan said.