Ever wonder why when water is spilled onto a tabletop, the puddle eventually stops spreading?

Researchers from the Massachusetts Institute of Technology have solved this question, and say it could be significant for the wider understanding of liquids.

When you spill a bit of water onto a tabletop, the puddle spreads and then stops—leaving a well-defined area of water The forces responsible for stopping the flow begin to show up at molecular level. Image credit: Wikipediawith a sharp boundary.

According to the researchers, understanding these types of flowing fluids is critical for processes from the lubrication of gears and machinery to the potential sequestration of carbon dioxide emissions in porous underground formations.

These findings were published 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," says Pahlavan.

They say the problem is one of scale, but it turns out it is only at the molecular level that the forces responsible for stopping the flow begin to show up.

"Within a macroscopic view of this problem, there's nothing that stops the puddle from spreading. There's something missing here," Pahlavan says.

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 says.

The team says this discovery could be important in the design of microchips, and learning about how heat builds up on their features.

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

This initial analysis dealt only with perfectly smooth surfaces. However in continuing this research, Juanes says the next step will be to extend the analysis to include fluid flows over rough surfaces.

"This work puts us in a position to be able to better describe multiphase flows in complex geometries like rough fractures and porous media," says Juanes.

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