Why puddles stop spreading

Researchers at MIT may have solved the mystery of why, when water is spilled, puddles form with defined edges rather than continuing to spread as established scientific formulas predict.

While this phenomenon might seem trivial, the finding's ramifications could affect a host of situations where the ability to calculate how a fluid will behave can have important consequences. For example, 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.

"The classic thin-film model describes the spreading of a liquid film, but it doesn't predict it stopping," Pahlavan, a graduate student at MIT, said. "Within a macroscopic view of this problem, there's nothing that stops the puddle from spreading."

Classical descriptions of spreading have a number of inconsistencies: For example, they require an infinite force to get a puddle to start spreading. According to the researchers, it is only at the molecular level that the forces responsible for stopping the flow begin to show. 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 says.

Pahlavan added: "What's actually stopping the puddle is forces that only act at the nanoscale." Even though these forces are minuscule, their effect changes how the liquid behaves in a way that the researchers say is obvious at a much larger scale.

The findings are reported in the journal 'Physical Review Letters' in a paper by graduate student Amir Pahlavan and his team.

Tom Austin-Morgan

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