Category Archives: Fluids and rheology

Challenge raised to role of water in behavior of Earth’s mantle

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Nature: The ability of rocks to flow is behind a range of phenomena, including plate tectonics and the convection of the mantle. To better understand rock flow, Hongzhan Fei of the University of Bayreuth in Germany and his colleagues examined individual crystals of olivine, one of the common minerals in Earth’s mantle, under pressures and temperatures similar to those at depths of 100 km to 200 km. They found that silicon was the mineral’s slowest moving atom, which the team believes is the limiting factor in the rocks’ ability to flow. The researchers found that increasing the water content did not significantly increase the silicon atoms’ movement rate, contrary to some previous studies. And it still is not clear whether silicon is actually the determining factor in rock viscosity.

Pebble formation on Mars indicates ancient river

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BBC: Pebbles imaged by NASA’s Curiosity rover in Mars’s Gale crater were probably formed by flowing water some 3.5 billion years ago, researchers have concluded. Ranging in size from 2 mm to 40 mm in diameter, the stones are too big to have been blown there by wind. Their different colors indicate they came from different locations, the smoothness of their surfaces suggests abrasion by moving water, and the way they’re stacked reveals examples of imbrication, or the toppled-domino formation indicative of past river activity. The data support earlier satellite observations of the planet’s surface that show a network of valleys and channels, which could have been carved by water. Over the next weeks, as Curiosity retraces its path, scientists hope to get even better photos using its MAHLI camera, designed to capture close-up, high-resolution images of rocks and soil.

Sensor uses carbon nanotubes to test saliva for glucose

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MIT Technology Review: A new way to test people’s glucose levels should make that process less painful for people with type 2 diabetes. Current tests rely on blood samples drawn from finger pricks. Now Mitchell Lerner of the University of Pennsylvania in Philadelphia and colleagues have developed a carbon nanotube–based transistor that can detect glucose levels in a variety of body fluids, including saliva. The nanotubes are coated with molecules of pyrene-1-boronic acid, which makes then highly sensitive for glucose detection. When exposed to glucose, the nanotube transistor’s current-voltage curve changes, and that change can be measured to indicate the glucose concentration. Although the technology has been around for a while, what the research team did was find a way to make the tubes quickly and cheaply. The system is less useful for type 1 patients, who have to give themselves daily injections of insulin. Because it takes at least 30 minutes for the glucose to show up in saliva, the device cannot quickly give an accurate reading of the blood glucose levels.

Non-Newtonian fluid shatters like glass

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New Scientist: Mixing cornstarch and water in the proper proportions creates oobleck, a non-Newtonian fluid named for a substance in a Dr. Seuss book. Non-Newtonian fluids behave as fluids when poured or poked gently with a finger. However, if struck with more force, they behave as a solid. Why oobleck behaves the way it does was explained last year: When subjected to significant pressure, the water in the material moves away from the contact point much more quickly than the starch, leaving a solid material behind. As recently reported in Physical Review Letters, Matthieu Roché, formerly of Princeton University, and his colleagues have now studied how a thin layer of the material behaves when struck by a falling object. They spread a layer of oobleck over a sheet of plexiglass onto which they dropped a 300-g metal rod from varying heights. The researchers expected that the material would tear like a soft metal. Instead, it fractured like glass or plaster, forming pointy-tipped cracks, before flowing back to normal. However, they also found that if the oobleck exceeded a certain thickness, it did not crack but instead cushioned the impact because the material underneath remained fluid. Better understanding of how non-Newtonian fluids react to varying kinds of impacts may provide insights into how they can be used in ballistic vests or vehicle suspension systems.

Microfluidic device can diagnose cancer from blood samples

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MIT Technology Review: The standard method for cancer detection is a biopsy, which can be invasive and expensive. Now, Mehmet Toner of Harvard Medical School and his colleagues have developed a device that can identify the presence of almost any sort of cancer cell in the blood stream. The microfluidic device builds on a similar earlier device that Toner’s team developed, which had to be specifically prepared to detect certain cancers and took four to five hours to complete a diagnosis. The new device is faster and works by removing all the noncancerous cells from a blood sample in a single process. White blood cells are tagged with magnetic beads covered in antibodies that recognize the white blood cells. The red blood cells, plasma, and unused magnetic beads are filtered out by microfluidic chambers, and then the white blood cells are removed by a magnetic field. What’s left is easy to examine for the presence of cancer cells. The device will be very useful for identifying the presence of circulating cancer cells, but it is unknown if it will be useful for early identification of cancer, because early-stage cancers aren’t known to produce a large number of circulating cells.

First fluid knots created in the lab

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New Scientist: A knot, in mathematical parlance, is an entanglement that cannot be untied because it has no ends. The simplest knots are the trefoil—a loop that crosses itself three times—and the Hopf link—two rings that cross once. The idea that knots could occur in fluids dates back to Lord Kelvin‘s theory of the nature of atoms, but none have been observed or created until now. Dustin Kleckner and William Irvine of the University of Chicago created plastic versions of the trefoil knot and Hopf link and dragged them quickly through water that was filled with microscopic bubbles. The plastic knots were shaped so that they would collect the bubbles and cause them to flow along the shape of the knot. The resulting flow of bubbles within the water is the first known example of fluid knots. By imaging the knots with lasers, Kleckner and Irvine were able to record them as they moved through the water, rotated, and eventually dissipated. Mathematical proofs have shown that in an ideal liquid, fluid knots, like knots made of string, would never unravel. However, Kleckner and Irvine are not certain whether their knots preserved their knottedness. They say that their next step is to create even more complex knots, and they believe that better understanding of fluid knots will help better simulate other vortices, such as those created by aircraft wings.

Physics may determine size of a tree’s leaves

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Science: The vascular system of trees may determine their maximum height and leaf size, according to a recent study published in Physical Review Letters. Kaare Jensen of Harvard University and Maciej Zwieniecki of the University of California, Davis, have observed that leaf size varies more in shorter trees than in taller ones. In their study of angiosperms, which include maples and oaks, they looked at how sap produced in the leaves travels through a network of pipelike cells to the trunk and roots. They determined that a tree’s leaves have to be big enough to get the sap flowing fast enough to overcome the resistance in the trunk. However, once the tree reaches a certain height, its resistance becomes so large that leaf size ceases to matter. Although many have found the theory plausible, others argue that it is too simple and fails to take into account the many variables in such systems.

How European forecasters predicted Sandy’s path before their US counterparts

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Ars Technica: A week before Superstorm Sandy struck the coast of New Jersey, the European Centre for Medium-Range Weather Forecasts (ECMWF) predicted the storm would indeed make landfall, whereas the US National Weather Service (NWS) had the storm veering off into the Atlantic Ocean. The NWS model fell into line with the ECMWF model three days later and enabled the NWS to issue timely, life-saving warnings. Still, the apparent shortcoming of the NWS model has highlighted a gap in investment between the US and Europe, writes Scott Johnson for Ars Technica. Not only does the ECMWF model run on a faster supercomputer than the NWS model does, but the European model has finer spatial and temporal resolution. Both advantages enhance accuracy. Comparisons aside, the ECMWF and NWS models rely on data gathered by Earth-observing satellites. If that fleet is not replenished, the accuracy of weather prediction will suffer. Indeed, after Sandy had dissipated, the ECMWF reran its model with data available five days before the storm’s landfall but omitted data from NASA’s polar-orbiting satellites. Without those data, the ECMWF model failed to predict Sandy’s destructive path.

New plasma process for industrial coating

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R&D Magazine: Coating technology is key to the success of many products, such as scratch-proof displays for smartphones and antibacterial surfaces in refrigerators. Until recently, manufacturers relied on one of two methods: wet chemical processes or vacuum plasma processes. Vacuum processes are expensive and limited to smaller components; wet chemical processes require high energy consumption and can be damaging to the environment. A third method is now being developed at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials in Bremen, Germany. Jörg Ihde and Uwe Lommatzsch have developed a plasma nozzle, no bigger than a typical spray can, that allows the coating to be applied very precisely and only where it is needed. The pair were awarded one of this year’s Joseph von Fraunhofer prizes for their innovative technique.

Novel cloaking device could shield ocean-faring vessels

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Science: A device to protect, or cloak, such objects as oil-drilling rigs and ships floating on the ocean surface is being developed by a researcher at the University of California, Berkeley. Mohammad-Reza Alam, who has published his results in Physical Review Letters, used computer simulations to test his theory. Because ocean water tends to stratify into a colder, denser layer below and a warmer, lighter layer above, waves propagate either along the surface or along the interface between the two layers. Interfacial waves have much shorter wavelengths and lower speed than surface waves, so Alam theorized that before a surface wave reaches a floating object, he could change the wave into an interfacial one, which would pass below the object, by introducing a patch of ripples of a certain wavelength on the sea floor. A second, identical patch of ripples on the other side of the object would turn the interfacial wave back into a surface wave. Although the ocean is much more complicated than the simulations, Alam’s novel approach offers a new twist on cloaking and could inspire a whole new direction of research.