Credit: Amir Pahlavan
Credit: Amir Pahlavan

A recent discovery about how much puddles spread on a surface could improve our understanding of liquid-solid interfaces and multiphase flows in complex geometries such as rough fractures and porous media, according to researchers at Massachusetts Institute of Technology.

When you inevitably spill coffee or a glass of water on your desk, liquid will seem to go everywhere but each individual drop will form a tiny puddle on the smooth surfaces. Why don't those tiny puddles simply keep on spreading across the surface, why is the puddle bounded by a sharp, well-defined boundary? After all, the spreading process is one that involves an attempt by the system to minimize its energy through a complex interplay between gravity, capillary action and viscous forces. As such, one might imagine that science already had the answer to this problem, but seemingly the formulas that describe fluid flow suggest that water should just keep spreading endlessly.

Now, MIT's Ruben Juanes, Amir Pahlavan, Luis Cueto-Felgueroso and Gareth McKinley think they may have solved the mystery. Their findings could have implications for understanding fluid flow, lubrication, "fracking" and even the sequestration of atmospheric carbon dioxide into porous reservoirs underground.

Credit: Jose-Luis Olivares-MIT and Amir Pahlavan
Credit: Jose-Luis Olivares-MIT and Amir Pahlavan

Pahlavan explains that, "The classic thin-film model describes the spreading of a liquid film, but it doesn't predict it stopping." It is only at the molecular level that the tiny forces ultimately responsible for stopping the flow become manifest. "Within a macroscopic view of this problem, there's nothing that stops the puddle from spreading. There's something missing here," Pahlavan adds. Close to the edge, the forces between liquid-solid and liquid-air interfaces become apparent leading to a modified surface tension. Thus, taking these forces into account can resolve the paradox that, theoretically at least, initiating puddle spread somehow requires an infinite force. Logically, this cannot be true, so forces at the nanoscale must have a critical role, suggests Pahlavan.

"You start with something very simple, like the spread of a puddle, but you get at something very fundamental about intermolecular forces," Juanes says. "The same process, the same physics, will be at play in many complex flows."

The team's initial analysis considered only perfectly smooth surfaces, so the next step is to investigate more realistic conditions, such as those seen in a microchip fabrication plant or an oil well, where fluid flow across and through complex and porous surfaces is important.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".