Pahlavan: Why do puddles stop spreading?


July 22, 2015

PhysicsNanoFluids

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. There’s just one problem: The formulas scientists use to describe such a fluid flow say that the water should just keep spreading endlessly. Everyone knows that’s not the case — but why?

This mystery has now been solved by researchers at MIT — 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.The new findings are reported in the journal Physical Review Letters in a paper by Ruben Juanes, an associate professor of civil and environmental engineering, graduate student Amir Pahlavan, research associate Luis Cueto-Felgueroso, and mechanical engineering professor Gareth McKinley.

“The classic thin-film model describes the spreading of a liquid film, but it doesn’t predict it stopping,” Pahlavan says. It turns out that the problem is one of scale, he says: 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. “Within a macroscopic view of this problem, there’s nothing that stops the puddle from spreading. There’s something missing here,” Pahlavan says. Read the full article at MIT NEWS

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