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 ("Thin films in partial wetting: internal selection of contact-line dynamics") 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. 
“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. (Image: Jose-Luis Olivares/MIT and Amir Pahlavan) “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. 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 intermolecular forces,” 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. Howard Stone, a professor of mechanical and aerospace engineering at Princeton University who was not involved in this work, says, “The authors have produced a nice result … which is relevant to many wetting situations. They introduce a mathematical formalism involving non-hydrodynamic interactions between the liquid and solid and use this within a thin-film description common in the literature. Then, they obtain several new insights. I am confident the paper will interest many in the community.” 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.”
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“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. (Image: Jose-Luis Olivares/MIT and Amir Pahlavan) “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. 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 intermolecular forces,” 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. Howard Stone, a professor of mechanical and aerospace engineering at Princeton University who was not involved in this work, says, “The authors have produced a nice result … which is relevant to many wetting situations. They introduce a mathematical formalism involving non-hydrodynamic interactions between the liquid and solid and use this within a thin-film description common in the literature. Then, they obtain several new insights. I am confident the paper will interest many in the community.” 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.”
This diagram shows structure of Mo-Te oxide nanowire. (a) Polyhedral representation and (b) ball-and stick representation of a hexagonal unit of [TeIVMoVI6O21]2-, (c) a single molecular wire of Mo-Te oxide. The bridge oxygen atoms that connect the hexagonal units are highlighted in yellow. (d) Assembly of single molecular wires into crystalline Mo-Te oxide. Mo: blue, Te (Se): brown, O: red. (click on image to enlarge) Furthermore, the researchers have shown that the ultrathin molecular wire-based material exhibits high activity as an acid catalyst, and the band gap of the molecular wire-based crystal is easily tuned via heat treatment. It is expected that the metal oxide molecular wire-based materials will open up new fields of research in heterogeneous catalysts, thermochromic materials, and semiconductors, as well as other related fields. "This is a very rare isolated molecular nanowire based on transition metal-oxygen octahedra, and is an attractive catalyst due to the large surface area," said Professor Masahiro Sadakane, a coauthor of this study, from Hiroshima University.
Marcia Silva da Pinto, post-doctoral researcher, works on growing metal organic frameworks onto cotton samples to create a filtration system capable of capturing toxic gas, as Juan Hinestroza looks on. “Creating transistors and other components using cotton fibers brings a new perspective to the seamless integration of electronics and textiles, enabling the creation of unique wearable electronic devices,” Hinestroza said. Taking advantage of cotton’s irregular topography, Hinestroza and his students added conformal coatings of gold nanoparticles, as well as semiconductive and conductive polymers to tailor the behavior of natural cotton fibers. “The layers were so thin that the flexibility of the cotton fibers is always preserved,” Hinestroza said, “Fibers are everywhere from your underwear, pajamas, toothbrushes, tires, shoes, car seats, air filtration systems and even your clothes.” Abbey Liebman ’10 created a dress using conductive cotton threads capable of charging an iPhone. With ultrathin solar panels for trim and a USB charger tucked into the waist, the Southwest-inspired garment captured enough sunshine to charge cell phones and other handheld devices – allowing the wearer to stay plugged in. The technology may be embedded into shirts to measure heart rate or analyze sweat, sewn into pillows to monitor brain signals or applied to interactive textiles with heating and cooling capabilities. “Previous technologies have achieved similar functionalities, but those fibers became rigid or heavy, unlike our yarns, which are friendly to further processing, such as weaving, sewing and knitting,” Hinestroza said. Synthesizing nanoparticles and attaching them to cotton not only creates color on fiber surfaces without the use of dyes, but the new surfaces can efficiently kill 99.9 percent of bacteria, which could help in warding colds, flu and other diseases. Two of Hinestroza’s students created a hooded bodysuit embedded with insecticides – using metal organic framework molecules, or MOFs – to fend off malarial mosquitoes. Malaria kills more than 600,000 people annually in Africa. While insecticide-treated nets are common in African homes, the anti-malarial garment can be worn during the day to provide extra protection and does not dissipate like skin-based repellants. Other students have used MOFs to create a mask and hood capable of trapping toxic gases in a selective manner. MOFs, which are clustered crystalline compounds, can be manipulated at the nano level to build nanoscale cages that are the exact same size as the gas they are trying to capture. “We wanted to harness the power of these molecules to absorb gases and incorporate these MOFs into fibers, which allows us to make very efficient filtration systems,” he explains. Hinestroza always looks for new ways to employ cotton as a canvas for creating infinite modern uses. “We want to transform traditional natural fibers into true engineering materials that are multifunctional and that can be customized to any demand,” he said. “We are chemists, we are material scientists, we want to create materials that will perform many functions, but have it remain flexible and as comfortable as a t-shirt or an old pair of jeans.”