Nanoscale physics explains why puddles stop spreading

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. yellow puddles “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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Ultra-thin, all-inorganic molecular nanowires

Nanowires are wired-shaped materials with diameters that are tens of nanometers or less. There are many types of nanowires, including semiconducting composite nanowires, metal oxide composite nanowires, and organic polymer nanowires, and they are typically used in functional materials and devices used as sensors, transistors, semiconductors, photonics devices, and solar cells. Molecular wires composed of only inorganic materials have attracted significant attention due to their stable structures, tunable chemical compositions, and tunable properties. However, there have only been a few reports regarding the development of all-inorganic molecular nanowires. Dr. Zhenxin Zhang and Prof. Wataru Ueda at the Catalysis Research Center at Hokkaido University (Prof. Ueda is currently working for Kanagawa University) and their collaborators at Hokkaido University, Hiroshima University, and Japan Synchrotron Radiation Research Institute/SPring-8 successfully created ultrathin all-inorganic molecular nanowires, composed of a repeating hexagonal molecular unit made of Mo and Te; the diameters of these wires were only 1.2 nm. These nanowires were obtained by the disassembly of the corresponding crystals through cation exchange and subsequent ultrasound treatment (, "Ultrathin inorganic molecular nanowire based on polyoxometalates"). Structure of Mo-Te Oxide Molecular Nanowire 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.
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Nanocavites on sensor chip can improve accuracy of prostate cancer diagnosis

New research has shown how a smart sensor chip, able to pick up on subtle differences in glycoprotein molecules, can improve the accuracy and efficiency of prostate cancer diagnosis. Researchers at the University of Birmingham believe that the novel technology will help improve the process of early stage diagnosis. Glycoprotein molecules, proteins that are covalently bound to one or more carbohydrate chains, perform a wide range of functions in cell surfaces, structural tissues and blood. Because of their essential role in our immune response, they are useful clinical biomarkers for detecting prostate cancer and other diseases. The team of chemical engineers and chemists, created a sensor chip with synthetic receptors along a 2D surface to identify specific, targeted glycoprotein molecules that are differentiated by their modified carbohydrate chains. In doing so, they developed a more accurate and efficient way of diagnosing prostate cancer than the current tests which rely heavily on antibodies. These antibodies are expensive to produce, subject to degeneration when exposed to environmental changes (such as high temperatures or UV light) and more importantly, have a high rate of false-positive readings. Professor Paula Mendes said, “There are two key benefits here. Crucially for the patient, it gives a much more accurate reading and reduces the number of false positive results. Furthermore, our technology is simple to produce and store, so could feasibly be kept on the shelf of a doctors’ surgery anywhere in the world. It can also be recycled for multiple uses without losing accuracy.” Most previous research on detecting glycoproteins centred on the protein of the molecule. Problematically for diagnosis, the protein part of glycoproteins does not always change if the body is diseased. The findings, published in the journal ("Selective glycoprotein detection through covalent templating and allosteric click-imprinting"), show how the rate of false readings that come with antibody based diagnosis can be reduced by the smart technology that focuses on the carbohydrate part of the molecule. The complex sugar structure in glycoprotein can be subtly different between samples from healthy and diseased patients. In order to achieve more accurate readings, the team wanted to identify the presence of disease by detecting a particular glycoprotein which has specific sugars in a specific location in the molecule. Professor Mendes added, “Biomarkers such as glycoproteins are essential in diagnostics as they do not rely on symptoms perceived by the patient, which can be ambiguous or may not appear immediately. However, the changes in the biomarkers can be incredibly small and specific and so we need technology that can discriminate between these subtle differences – where antibodies are not able to.” To engineering the sensor chip, the team developed a smart surface with nano-cavities that fit the particular target glycoprotein. To create the nano-cavities, the sugar part of the prostate cancer glycoprotein is reacted with a custom-designed molecule that contains a boron group at one end (the boron linkage forms a reversible bond to the sugar). The other end of this custom molecule is made to react with molecules that have been tethered to a gold surface. The glycoprotein is then bound to the surface via its sugar groups, before the rest of the surface is blocked with a third molecule. When the glycoprotein is removed (by breaking the reversible boron bonds) it leaves behind a perfect cast. Within that cast, there was a special area with boron-containing molecules that can recognise a specific set of sugars. Professor Mendes said, “It is essentially a lock, and the only key that will fit is the specific prostate cancer glycoprotein that we’re looking for. Other glycoproteins might be the right size, but they won’t be able to bind to the very specific arrangement of boron groups.” Dr John Fossey added, “It’s estimated that one in eight men will suffer from prostate cancer at some point in their life, so there’s a clear need for more accurate diagnosis. By focussing on the sugar, we appear to have hit the ‘sweet spot’ for doing just that. Our focus now is to take this technology and develop it into something accessible to people across the world.” The team also hope that further investment, and collaboration with commercial partners, will open the door to adapting the current technology for other diseases. Dr Fossey continued, “We believe that this could be applicable to other diagnostic challenges. Lots of diseases produce specific glycoproteins, so there are a number of possible avenues to improve the accuracy of our diagnoses.”
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Nanotechnology transforms cotton fibers into modern marvel

Juan Hinestroza and his students live in a cotton-soft nano world, where they create clothing that kills bacteria, conducts electricity, wards off malaria, captures harmful gas and weaves transistors into shirts and dresses. “Cotton is one of the most fascinating – and misunderstood materials,” said Hinestroza, associate professor of fiber science, who directs the Textiles Nanotechnology Laboratory at Cornell. “In a nanoscale world – and that is our world – we can control cellulose-based materials one atom at a time.” The Hinestroza group has turned cotton fibers into electronic components such as transistors and thermistors, so instead of adding electronics to fabrics, he converts the fabric into an electronic component. Marcia Silva da Pinto 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.”
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