FYFD

Category: Phenomena

  • Wrapping Up

    Wrapping Up

    It’s often at the intersection of topics that we can learn something new and fascinating. The latest video from The Lutetium Project shows examples of this at the intersection of solid mechanics and fluid dynamics with a look at elastocapillarity. Breaking that word down, that’s where elasticity – that stretchy quality associated with solids – meets capillarity – the surface-tension-dominated behavior of a fluid. In particular, they explore some of the mind-boggling and surprising interactions that happen between drops, bubbles, and thin flexible fibers smaller than the width of a human hair. Check out the full video below. (Images credit: K. Dalnoki-Veress et al.; video credit: The Lutetium Project)

  • Vibrated to Bits

    Vibrated to Bits

    Sound and vibration can be powerful tools for controlling liquids. In this animation, a water/glycerin drop violently bursts into a cloud of droplets when it is vibrated vertically 1000 times per second by a piezoelectric actuator. This vibration shakes the drop with accelerations of 150 g. Initially, the amplitude is small enough to simply create ripples around the drop’s circumference. As it increases, the drop deforms more at the edges and starts to eject droplets there. When the vibration hits a critical amplitude, the entire drop explodes into droplets. The technique is called vibration-induced droplet bursting, and its near-instantaneous ability to atomize drops makes it a candidate for applications like spray cooling microprocessors or spray coating a solid surface. (Video credit: B. Vukasinovic, source)

  • Turbine Wakes in the Sea

    Turbine Wakes in the Sea

    What we we build always has an impact on the environment around us. The white dots you see in the image above are an array of offshore wind turbines, standing in waters 20 to 25 meters deep. The brownish lines extending from each turbine show the underwater wakes of the turbines, colored by the sediment they’ve picked up. As with trees in a snowstorm, the currents flowing past the base of the turbine likely form a horseshoe vortex that lifts up the sediment into the wake. Because the tides in this area reverse direction every six hours, these sediment plumes can appear quite dynamic in satellite imagery, frequently changing strength and direction. (Image credit: NASA Earth Observatory)

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    Pascal’s Barrel

    Pascal’s Law tells us that pressure in a fluid depends on the height and density of the fluid. This is something that you’ve experienced firsthand if you’ve ever tried to dive in deep water. The deeper into the water you swim, the greater the pressure you feel, especially in your ears. Go deep enough and the pressure difference between your inner ear and the water becomes outright painful.

    In the video demonstration above, you’ll see how a tall, thin tube containing only 1 liter of water is able to shatter a 50-liter container of water. Not only does this show just how powerful height is in creating pressure in a fluid, but it shows how a fluid can be used to transmit pressure over a distance – one of the fundamental principles of hydraulics! (Video credit: K. Visnjic et al.; submitted by Frederik B.)

  • Washington Ice Disk

    Washington Ice Disk

    Winter weather in northern latitudes sometimes brings with it unusual phenomena like this ice disk spinning in the Middle Fork Snoqualmie River in Washington state. Photographer Kaylyn Messer ventured out to capture photos and videos of the event over the weekend. There are a couple theories as to how such disks form, but swirling river eddies are a key ingredient. One theory posits that chunks of ice forming on the river get caught up by the spinning eddy and slowly freeze together to form the disk. Another theory proposes that the disks occur when an existing chunk of ice breaks away, gets caught in the spinning eddy and slowly has its edges ground down into a circle. Personally, I lean toward the former explanation, though there is likely grinding at the edges either way. See more about this ice circle over at Messer’s blog.  (Image credit: K. Messer; GIF by @itscolossal; via Colossal)

  • Sedimentary Swirls

    Sedimentary Swirls

    Sediment swirls in Bear Lake caught the eye of an astronaut aboard the International Space Station last year. Bear Lake is situated in the Rocky Mountains, on the Idaho-Utah border. The eddies in the center of the lake are each about 3 km across and are likely the result of inflow from the lake’s tributaries. Silt and sediment picked up by the rivers and streams gets deposited into Bear Lake, revealing the turbulent mixing of tributary waters with those already in the lake. (Image credit: NASA; via NASA Earth Observatory)

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    Why Ice is Slippery

    Ice is slippery. This is a fundamental fact we humans have dealt with so often that we rarely take the time to ask why. Other solids aren’t inherently slippery, so what is it that makes ice so? Remarkably, scientists only began to ask this question and propose theories within the past couple hundred years. One common suggestion is that the high pressure of an ice skate on ice locally melts the ice, creating a thin liquid layer a skater glides across. But this does not explain why ice is slippery for shoes or tires, nor why it’s possible to ice skate at more than a few degrees below freezing. Several other effects may be in play, such as frictional heating or the peculiar molecular forces between water molecules. Current research suggests that ice has a thin liquid layer tens or hundreds of nanometers thick that causes its slippery nature. For a great review of the subject, see Robert Rosenberg’s Physics Today article. (Video credit: SciShow)

  • Superhydrophobic Coatings

    Superhydrophobic Coatings

    Superhydrophobic–or water repellent–materials are much sought after. Their remarkable ability to shed water is actually mechanical in nature–not chemical. Surfaces with a highly textured microstructure, like a lotus leaf or a butterfly wing, shed water naturally because air trapped between the high points prevents the water from contacting most of the solid surface. The result is that a drop sitting on the surface will have a very high contact angle and be nearly spherical. Instead of wetting the surface and spreading out, it can slide right off, as seen in the animations above. Here researchers have treated the coins and the right half of the cardboard with a spray-on coating that creates superhydrophobic microscale roughness. Similar coatings are commercially available, but such coatings are delicate and lose their hydrophobicity over time as the microstructure breaks down. (Image credits: Australian National University, source)

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    Growing Snowflakes

    Watching a snowflake grow seems almost magical–the six-sided shape, the symmetry, the way every arm of it grows simultaneously. But it’s science that guides the snowflake, not magic. Snowflakes are ice crystals; their six-sided shape comes from how water molecules fit together. The elaborate structures and branches in a snowflake are the result of the exact temperature and humidity conditions when that part of the snowflake formed. The crystals look symmetric and seem to grow identical arms simultaneously because the temperature and humidity conditions are the same around the tiny forming crystals. And the old adage that no two snowflakes are alike doesn’t hold either. If you can control the conditions well enough, you can grow identical-twin snowflakes! (Video credit: K. Libbrecht)

  • Resonating with the Windows Down

    Resonating with the Windows Down

    Ever roll down your window a bit while driving and immediately hear a terrible, rhythmic noise? That awful whum-whum-whum is–oddly enough–an example of the same physics that allows you to make an open bottle whistle by blowing over it. Fluid dynamicists call it Helmholtz resonance. Air flowing over the bottle neck or around the car makes the air inside the container vibrate with a frequency that depends on the bottle or car’s characteristics. That vibration generates noise that we hear as a hum or whistle for a bottle or a lower frequency whum-whum for a car window.

    The images above show flow past different open windows on a car. Air flow remains relatively steady past the side-view mirror and front window of a modern car, so the noise from opening the front window is not usually too bad. But flow separation and reconnection near the rear window of a car creates very unsteady airflow there which exacerbates this resonance issue. This is why lowering the rear window usually causes more noise. Fortunately, the solution is relatively simple: open more than one window and it disrupts the resonance! (Image credit: Car and Driver; submitted by Simon H.)