FYFD

Category: Phenomena

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    Bouncing Off Defects

    The splash of a drop impacting a surface depends on many factors — among them droplet speed and size, air pressure, and surface characteristics. In this award-winning video from the 2019 Gallery of Fluid Motion, we see how the geometry of a superhydrophobic surface can alter a splash.

    When a drop falls on a protruding superhydrophobic surface, like the apex of a cone, it can be pierced from the inside, completely changing how the droplet rebounds and breaks up. The variations the video walks us through are all relatively simple, but resulting splashes may surprise you nevertheless. (Image and video credit: The Lutetium Project)

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    Superman’s Hair Gel

    I love a good tongue-in-cheek physical analysis of superheroes. This estimate of the drag force experienced by Superman’s hair when outracing a plane or speeding bullet was done by Cornell students. According to their calculations, Superman’s hair (or his hair gel) must withstand nearly 80,000 Newtons of force. That’s a bit less than the typical force experienced by a restrained passenger in a car crash at highway speeds.

    In grad school, my labmates and I held a spirited debate about the difference in drag Superman would experience when flying at hypersonic speeds depending on whether he had one or both arms extended in front of him. Sadly, we never found the chance to test our hypotheses in the wind tunnel. (Image and video credit: R. Geltman et al.)

    Superman races to the rescue.
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    Spinning Ink Out of Markers

    I have to say I’m grateful that my classmates in school never discovered the mess-generating superpower of felt-tipped markers. As the Slow Mo Guys demonstrate here, when you spin or fling these markers, ink will stream out of them. That’s due, in part, to the air vents present near the tips. Markers (and other pens) have those to equalize the pressure between the outside and the ink reservoir; otherwise, the ink won’t flow to the felt tip as it should. Is anyone else surprised by the sheer volume of liquid ink apparently contained in these pens? (Image and video credit: The Slow Mo Guys)

  • Siberia’s Rivers

    Siberia’s Rivers

    Each winter the Kolyma River in Siberia freezes to a depth of several meters. But by June the river thaws and discharges its annual 136 cubic kilometers of  water into the Arctic. The dark color of the river comes from the sediment and organic material it carries. The Kolyma is the world’s largest river underlain with continuous permafrost. Parts of the river system’s permafrost date back to the Pleistocene more than 12,000 years ago. Since much of its organic matter comes from its permafrost, researchers expect the amount of organic material in the Kolyma’s discharge to increase as the permafrost degrades in our warming climate. (Image credit: NASA Earth Observatory)

  • Behind the Bubbly

    Behind the Bubbly

    Carbonation and the fizzy bubbles that come with it are surprisingly popular among humans. Through fermentation or artificial introduction, carbon dioxide gas gets dissolved into a liquid under high pressure. Then, when the pressure is released to atmospheric levels, that gas comes out of solution, forming tiny bubbles that eventually grow large enough to rise buoyantly to the surface. There they will either pop – releasing carbon dioxide gas and aromatics – or form a layer of foam – like in beer – that insulates the liquid and makes it harder to spill. (Image credit: D. Cook; see also R. Zenit and J. Rodríguez–Rodríguez; via Jennifer O.)

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    Waltzing Defects

    Liquid crystals are a peculiar state of matter with both liquid and crystalline properties. In this video, a microfluidic device breaks water into droplets surrounded by a shell of liquid crystal. Because the molecular structure of the liquid crystals is helical and cannot pack neatly in a spherical shell, there are visible defects in the liquid crystal shells. Given time, those defects can merge as the liquid crystal shell thickens. (Image and video credit: The Lutetium Project)

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    Inside the Fire Lab

    Fire plays an important role in nature, one with which humanity must live without controlling fully. After several disastrous historic wildfires in the American West, the U.S. Forest Service established its own fire lab, where research foresters can study flames firsthand. This video takes us inside the Fire Lab for a look at the facilities and people responsible for helping us better understand this fundamental force of nature. (Video and image credit: Gizmodo + Atlas Obscura)

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    An Introduction to the Reynolds Number

    For those who’d like an overview of the mathematics involved in fluid dynamics, Numberphile has a lovely introduction, given by our friend Tom Crawford. The governing equations in fluid dynamics, the Navier-Stokes equations, are quite complicated, but that’s just been inspiration for scientists and mathematicians to come up with clever ways to simplify them. And, ultimately, that’s what the Reynolds number is — a way to help us judge which forces, and therefore which mathematical terms, are the most important in a given problem. (Video credit: Numberphile; submitted by COMPLETE)

    GIF displaying various examples of Reynolds number from marbles in treacle (Re ~0.001) through a cruise ship (Re ~ 1 billion)
  • Flow on Commercial Wings

    Flow on Commercial Wings

    Even in an era of supercomputers, there is a place for quick and dirty methods of flow visualization. Here we see a model of a swept wing like those seen on many commercial airliners. It was painted with a layer of fluorescent oil, then placed in a wind tunnel and subjected to flow. As air blows across the model, it moves the oil, leaving behind streaks that show how air near the surface moves. 

    We can see, for example, that near the fuselage, the air flows mostly front to back across the wing. That’s what we expect, especially for a wing generating lift. But further out on the wing, the flow moves mostly along the wing, not across it. There’s also a distinctive line running just a short ways behind the leading edge on this outer section of wing. It looks as though air flowing over the wing separated at this point, leaving disordered and unhelpful flow behind. It’s likely that the model was tested at an angle of attack where the outer section of the wing was beginning to stall. (Image credit: ARA)

  • Inside the Earth’s Mantle

    Inside the Earth’s Mantle

    Plate tectonics is a relatively young scientific theory, only gaining traction among geologists in the late 60s and early 70s. One key tenet of the theory is subduction where plates meet and one is forced down into the mantle, like in this illustration of the subduction zone near Japan. In early incarnations of the theory what happens to that subducting slab of rock once it’s in the mantle were ignored. But over the decades, geologists have built maps of the interior of our planet through the seismic waves they record. What they’ve found is that the continental chunks that break off and sink can have long-lasting effects.

    Beneath the Earth’s crust, the mantle behaves like an extremely slow-moving fluid under incredibly high temperatures and pressures. It can take tens of millions of years, but those broken slabs sink through the mantle, dragging fluid with them. This creates a large-scale flow known as a mantle wind, which can have far-reaching effects at the Earth’s surface. Through modeling and simulation, geologists have found these deep mantle flows may explain why mountain ranges like the Himalayas and Andes didn’t grow until millions of years after their plates collided and why earthquakes sometimes occur far from plate boundaries. For more, check out this great article from Ars Technica. (Image credit: British Geological Survey; via Ars Technica; submitted by Kam-Yung Soh)