Why do puddles stop spreading? Simple everyday phenomenon was unexplained by physics—until now

July 8, 2015
“You start with something very simple, like the spread of a puddle, but you get at something very fundamental about intermolecular forces,” Ruben Juanes says. Credit: Jose-Luis Olivares/MIT and Amir Pahlavan

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 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.

Credit: Amir Pahlavan

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

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 says. "These are the missing intermolecular forces in the macroscopic description." Properly accounting for these forces resolves the previous paradoxes, he says.

"What's striking here," Pahlavan adds, is that "what's actually stopping the puddle is forces that only act at the nanoscale." This illustrates very nicely how nanoscale physics affect our daily experiences, he says.

Whether someone's spilled milk stops on the tabletop or makes a mess all over the floor may seem like an issue of little importance, except to the person who might get soaked, or have to mop up the spill. But the principles involved affect a host of other situations where the ability to calculate how a fluid will behave can have important consequences. 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.

Many more complex flows of fluids also come down to the same underlying principles, Juanes says—for example, carbon sequestration, the process of removing carbon dioxide from fossil-fuel emissions and injecting it into underground formations, such as porous rock. Understanding how the injected fluid will spread through pores in rock, perhaps displacing water, is essential in predicting how stable such injections may be.

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

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 says.

This initial analysis dealt only with perfectly smooth surfaces. In pursuing the research, Juanes says, a next step will be to extend the analysis to include fluid flows over rough surfaces—which more closely approximate the conditions, for example, of fluids in underground porous formations. "This work puts us in a position to be able to better describe multiphase flows in complex geometries like rough fractures and porous media."

Explore further: Mixing fluids efficiently in confined spaces: Let the fingers do the working

More information: Thin films in partial wetting: Internal selection of contact-line dynamics, Phys. Rev. Lett. journals.aps.org/prl/accepted/ … 64075d4bced72a095c9f

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daqman
5 / 5 (1) Jul 08, 2015
This terrible article doesn't actually explain anything at all! It starts with a question, has a lot of fluff about why it's an important question. The title implies the answer to the question is now known but doesn't explain in simple terms what the answer is.
docile
Jul 08, 2015
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adam_russell_9615
5 / 5 (2) Jul 08, 2015
Surface tension seems to me to be an extremely common sense answer. Why the purported mystery?
Doug_Huffman
not rated yet Jul 08, 2015
Surface tension seems to me to be an extremely common sense answer. Why the purported mystery?
Click-bait. Question Mark and The Mysterions in the lede is always click-bait.
antialias_physorg
5 / 5 (4) Jul 08, 2015
Surface tension seems to me to be an extremely common sense answer. Why the purported mystery?

Because it's not the answer. It's the interfacial tension of the liquid with the underlying solid (as the linked article explains...if you want the full article just google for the title and you'll find the preprint version).

'Common sense' is an incredibly unreliable guide when it comes to microscopic (and very alrge) scales....and not surprisingly so, as it isn't evolved for those scales.
TyroneTork
not rated yet Jul 08, 2015
This research is certain to help out all the clueless scientists working on things like superhydrophobic substances. It's a wonder they got ad far as they did without understanding puddles.
docile
Jul 08, 2015
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docile
Jul 08, 2015
This comment has been removed by a moderator.
docile
Jul 08, 2015
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Mimath224
not rated yet Jul 08, 2015
@antialias_physorg Going back many years I attended a lecture where 'puddles' was part of another problem (which I can't now remember). The chemist talking said that puddles were complicated in that there were many 'forces' that affected spread. Started off whith a simple model showing that what look like smooth surfaces weren't smooth at smaller scales and so water may or may not enter small scale depressions depending if air was trapped or not. I know it's an over simplification but I think it was given that forces at the smaller scale would tend to 'anchor' the puddle. Must admit, I haven't updated myself on this so maybe I should after all this time, Ha!
mreda14
not rated yet Jul 08, 2015
I agree with the comment of docile. easy to prove. Spell water ( low viscosity) and honey ( high viscosity) on the table. Honey will not spread. Viscosity and thus friction is very high. It also depend on the contact angle of the spreading fluid with respect to the surface.In superhydrophobic surface such as the surface of lotus leaf. When rain fall down on the leaf, the water rolls down and pick up dirt. This self cleaning action due to the large contact angle. and so water will not stop from spreading.
wjr321
not rated yet Jul 09, 2015
A small question. Has anyone here read Berg's Wettability? Or has anyone followed surface physics much? These folks have solved a problem that, as far as I can see, was solved long ago.

The pinning of a sessile drop is well understood. The pinning depends upon a number of things -- initial conditions deviating form STP for one -- and the exact behavior will depend upon whatever is suspended in that drop as well. The so called "coffee stain" effect resulting from Marangoni forces has a great deal of input to this. So, the interaction of the surface and the fluid, while a little complex, is known.

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