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  • Holiday Fluids: Snowflakes

    Holiday Fluids: Snowflakes

    Just about everyone wishes for a White Christmas, but even when that happens, it’s rare to get a good look at the beauty of individual snowflakes. Alexey Kljatov’s macro photography of snowflakes is simply stunning and highlights the incredible variety of forms snowflakes take. A snowflake forms when a water droplet freezes onto dust or other particles and grows as more water vapor freezes onto the initial crystal. The symmetry of the snowflakes, as with any crystal, comes from the internal order of its water molecules. The shape and features that form vary due to the local temperature and humidity level while vapor is freezing onto the crystal. Check out this handy graph showing which shapes form for various situations. Since snowflakes can encounter wildly different conditions on their path to the ground, it’s rare or next-to-impossible to find any two alike. Join us all this week at FYFD as we look at holiday-themed fluid dynamics. (Photo credit: A. Kljatov)

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    Holiday Fluids

    BYU Splash Lab–those breakers of bottles, skippers of rocks, spinners of eggs, students of soap films, masters of splashes, and all-around cool fluid dynamicists–have some fluids-themed, high-speed holiday greetings. Likewike, here at FYFD we’ll be spending the next week celebrating the physics and fluid dynamics of the winter holiday season! In the meantime, you can whet your appetite by brushing up on your cookie dunking techniques, watching how icicles form, and enjoying a good beverage. Stay tuned and happy holidays from FYFD! (Video credit: BYU Splash Lab/BYU News)

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    Huddling Penguins and Traffic Jams

    Male emperor penguins have the unenviable task of incubating their eggs in temperatures as cold as -50 deg Celsius and winds of up to 200 km/h. To stay warm, the penguins form huddles of up to thousands of individuals. Observations in the wild show that these huddles move in a stop-and-go fashion, with changes propagating through the penguins like waves. Researchers adapted a model used for heavy traffic flow to describe the penguins’ motion. They found that motions like those found in observed penguin huddles could be initiated by slight movements of any penguin in the model huddle, regardless of its position; in other words, the huddle has no leader. They also found that the wave that travels through the penguins can align the huddle to uniform density or help two huddles merge. To learn more, check out the researchers’ video or their paper. (Video credit: D. Zitterbart et al./New Scientist; via J. Ouellette)

  • Reader Question: What is Surface Tension?

    Reader Question: What is Surface Tension?

    Last week reader thesnazz asked:

    Is there a difference between surface tension and viscosity, or are they two manifestations of the same process and/or principles? If you know a given fluid’s surface tension, can you predict its viscosity, and vice versa?

    I’m tackling this one in parts, and you can click here to read about viscosity.

    Surface tension’s intermolecular origins are a bit clearer than those of viscosity. Essentially, within the interior of a water drop, you can imagine water molecules all hanging out with other water molecules. They tug on one another, but because they are surrounded on all sides by other water molecules, the net force of all these interactions on any molecule is zero. Not so at the surface of the drop. The surface is also called an interface; it’s a place where the fluid ends and something else–another fluid or perhaps a solid–begins. For a water molecule at that interface, the forces exerted by neighboring molecules are not balanced to zero. Instead, the imbalance causes the water molecules to be tugged inward. We call this effect surface tension.

    Because surface tension is an interfacial effect, it is not completely dependent on the fluid alone. For example, a drop of water sitting on a solid surface can take a variety of shapes depending on the properties of the solid (see also hydrophobicity) and the surrounding air as well as those of the water. This is only one of many manifestations of surface tension. Wikipedia has a pretty good overview of some others, if you’d like to learn more. Like viscosity, surface tension is usually measured rather than calculated from first principles.

    In the end, both surface tension and viscosity have molecular origins, but they are two very different and independent properties. Viscosity is inherent to a fluid, whereas surface tension depends on the fluid and its neighboring substance. Both quantities are more easily measured than calculated. Thanks again to thesnazz for a great question! As always, you can ask questions or submit post ideas here on Tumblr or via Twitter or email. (Image credit: Wikimedia)

  • Reader Question: What is Viscosity?

    Reader Question: What is Viscosity?

    Reader thesnazz asks:

    Is there a difference between surface tension and viscosity, or are they two manifestations of the same process and/or principles? If you know a given fluid’s surface tension, can you predict its viscosity, and vice versa?

    This is a good question! To answer it, let’s think about where surface tension and viscosity come from. Like many concepts in fluid dynamics, these quantities describe for a whole fluid the properties that arise from interactions between molecules.

    To prevent this becoming overly long, I’m going to tackle this over a couple posts. Today, I’ll talk about viscosity.

    One way to describe a fluid’s viscosity is as a measure of its resistance to deformation. Another way to think of it is how easily momentum is transmitted from one part of the fluid to another. The diagram above is the classic representation. A layer of fluid is sandwiched between two flat plates. If the top plate moves, friction requires that the fluid particles in contact with the plate get dragged along. This shears the fluid just below that and drags it along, but not quite as much. Those fluid particles do the same to their neighbors and so on down to the stationary second plate, where the fluid is at rest.

    Viscosity is the property that determines how much those neighboring fluid particles move; the more viscous the fluid, the more the neighboring bits of fluid resist getting pulled along. This is a property that’s inherent to a fluid. It comes from how the molecules of the fluid interact with one another, but there are no simple expressions to calculate the viscosity of a liquid or a gas from the individual interactions of its molecules. Instead we experimentally measure viscosity values and use empirical formulas to approximate how viscosity changes with temperature and other effects. (Image credit: Wikimedia)

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    Fluctuating Ferrofluids

    https://youtu.be/MU7wiveVCbg

    Ferrofluids–liquids seeded with magnetically sensitive ferrous nanoparticles–demonstrate some beautiful and bizarre behaviors when exposed to magnetic fields. This video shows the reaction of a pool of ferrofluid to the magnetic field generated by an alternating current through a simple wire coil. At 1 Hz, the fluid response is not unlike the normal-field instability–the characteristic spikes–the fluid develops when exposed to a permanent magnet. But because field is fluctuating, the spikes pop out and fade again. At 10 Hz, the behavior gets even more interesting. As the frequency of the magnetic field’s oscillation increases, the time the fluid has to respond to changes in the magnetic field decreases. Eventually, one can imagine a point where the magnetic field oscillates faster than the molecules in the fluid can rearrange themselves to respond. It’s unclear if such a mismatch in timescales is the cause of the increasing violence of the ferrofluid’s response in the later clips or whether this results from an unmentioned change to the current through the coil. For something even wilder, check out Nick’s video of the ferrofluid’s response to music. (Video credit: N. Moore)

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    Vibrating Paint

    Paint is probably the Internet’s second favorite non-Newtonian fluid to vibrate on a speaker–after oobleck, of course. And the Slow Mo Guys’ take on it does not disappoint: it’s bursting (literally?) with great fluid dynamics. It all starts at 1:53 when the less dense green paint starts dimpling due to the Faraday instability. Notice how the dimples and jets of fluid are all roughly equally spaced. When the vibration surpasses the green paint’s critical amplitude, jets sprout all over, ejecting droplets as they bounce. At 3:15, watch as a tiny yellow jet collapses into a cavity before the cavity’s collapse and the vibration combine to propel a jet much further outward. The macro shots are brilliant as well; watch for ligaments of paint breaking into droplets due to the surface-tension-driven Plateau-Rayleigh instability. (Video credit: The Slow Mo Guys)

  • Pitcher Plant Fluid Dynamics

    Pitcher Plant Fluid Dynamics

    Carnivorous pitcher plants owe much of their efficacy to the viscoelasticity of their digestive fluid. A viscoelastic fluid’s resistance to deformation has two components: the usual viscous component that resists shearing and an elastic component, often derived from the presence of polymers, that resists stretching – kind of like a liquid rubber band. It’s the latter effect that’s important when it comes to the pitcher plant trapping insects. When a fly or ant falls into the liquid within the plant, it will flail and try to swim, thereby straining the fluid. In part © of the image above, you can see how long fluid filaments stretch as the fly moves; this is because the digestive fluid’s extensional viscosity, the elastic component, is 10,000 times larger than its shear viscosity, the usual viscous component, for motions like the fly’s. This viscoelastic fluid is so effective at trapping insects that, as seen in part (b) above, it has to be diluted by more than 95% before insects can escape it! (Image credit: L. Gaume and Y. Forterre)

  • Fluids Round-up – 7 December 2013

    Fluids Round-up – 7 December 2013

    Fluids round-up time! I missed out last weekend because of the holidays, so this is a long list of links. There’s a lot of really great stuff here, including some neat fluidsy geophysics and astronomy.

    (Photo credit: E. Whittaker)

  • Lenticular Clouds Over Ice

    Lenticular Clouds Over Ice

    Lenticular clouds, like the one shown above, often attract attention due to their unusual shape. These stationary, lens-shaped clouds can form near mountains and other topography that force air to travel up and over an obstacle. This causes a series of atmospheric gravity waves, like ripples in the sky. If the temperature at the wave crest drops below the dew point, then moisture condenses into a cloud. As the air continues on into a warmer trough, the droplets can evaporate again, leaving a stationary lenticular cloud over the crest. This particular lenticular cloud was captured by Michael Studinger during Operation IceBridge in Antarctica. The line of ice in the foreground is a pressure ridge of sea ice formed when ice floes collided. (Photo credit: M. Studinger; via NASA Earth Observatory)

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