FYFD

Tag: science

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    When the Mediterranean Dried Up

    Geological evidence shows that millions of years ago, the Mediterranean Sea nearly dried out. In fluid mechanics, we’d describe this problem using one of our fundamental equations: conservation of mass, also known as continuity.

    Imagine a volume containing the entire Mediterranean. To describe the amount of sea water in that volume, you need to keep track of two major quantities: how much water is flowing into the volume and how much is leaving it. For the prehistoric (as well as today’s) Mediterranean, the sources feeding the sea are 1) an inflow from the Atlantic through the Strait of Gibraltar; 2) inflows from rivers; and 3) rainfall. Water is lost primarily to evaporation.

    As explained in the video, the Mediterranean’s dry spell was heralded by tectonic changes that sealed the Strait of Gibraltar, depriving it of its largest source of inflow. At the same time, warmer temperatures and less rainfall reduced influx from rivers and the atmosphere while increasing evaporation rates. The result? Water levels in the Mediterranean dropped by hundreds of meters, creating massive salt deposits, wiping out native marine life, and allowing mass migration by land-dwelling animals. Eventually, though, the Strait re-opened, creating what might have been a massive flood. (Video and image credits: PBS Eons)

  • Captured by Waves

    Captured by Waves

    Acoustic levitation and optical tweezers both use waves — of sound and light, respectively — to trap and control particles. Water waves also have the power to move and capture objects, as shown in this award-winning poster from the 2019 Gallery of Fluid Motion. The central image shows a submerged disk, its position controlled by the arc-shaped wavemaker at work on the water’s surface. The complicated pattern of reflection and refraction of the waves we see on the surface draws the disk to a focal point and holds it there.

    On the bottom right, a composite image shows the same effect in action on a submerged triangular disk driven by a straight wavemaker. As the waves pass over the object, they’re refracted, and that change in wave motion creates a flow that pulls the object along until it settles at the wave’s focus. (Image and research credit: A. Sherif and L. Ristroph)

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    Rattlesnakes Sip Rain From Their Scales

    Getting enough water in arid climates can be tough, but Western diamondback rattlesnakes have a secret weapon: their scales. During rain, sleet, and even snow, these rattlesnakes venture out of their dens to catch precipitation on their flattened backs, which they then sip off their scales.

    Researchers found that impacting water droplets tend to bead up on rattlesnake scales, forming spherical drops that the snake can then drink. Compared to other desert-dwelling snakes, Western diamondbacks have a far more complicated microstructure to their scales, with labyrinthine microchannels that provide a sticky, hydrophobic surface for impacting drops. (Video and image credit: ACS; research credit: A. Phadnis et al.; via The Kid Should See This)

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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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    A Dance of Hydrogen Bubbles

    Hydrogen bubbles rise off zinc submerged in hydrocholoric acid in this short film from the Beauty of Science team. In high-speed video, the rise of the bubbles is stately and mesmerizing. Notice how the smallest bubbles appear as perfect spheres; for them, surface tension is strong enough to maintain that spherical shape even against the viscous drag of their buoyant rise. Larger bubbles, formed from mergers both seen and unseen, have a harder time staying round. In them, surface tension must battle gravitational forces and drag from the surrounding fluid. (Image and video credit: Beauty of Science; via Laughing Squid)

  • Tapping a Can Won’t Save Your Beer

    Tapping a Can Won’t Save Your Beer

    It happens to the best of us: sometimes our beer gets shaken up during transit. One common reaction to this is to tap the side of the can repeatedly before opening, but a new scientific study shows that tapping doesn’t affect the volume of beer lost. Danish scientists tested over 1,000 cans of beer in randomized combinations of shaken, unshaken, tapped, and untapped, and observed no difference between tapped and untapped cans.

    The foam-up upon opening takes place in shaken beer because carbon dioxide bubbles form in the pressurized beer, especially along defects in the wall where bubbles can nucleate. When the pressure is released, the carbon dioxide becomes supersaturated and comes out of solution, especially into the pre-formed bubbles, which rapidly grow and overflow. In theory, tapping could disturb those bubbles before opening, but in practice, it makes no difference. Your best bet? Give the beer time to settle before you open it. (Image credit: Q. Dombrowski; research credit: E. Sopina et al.; via Ars Technica)

  • The Physics of Al Dente

    The Physics of Al Dente

    It’s a simple weeknight routine: toss a handful of spaghetti noodles in boiling water, wait a few minutes, and enjoy with the sauce of your choice. But there’s a surprising amount of physics in the humble strand of spaghetti, and a new model focuses on the way spaghetti sags and curls as it cooks.

    Spaghetti, like most pastas, is made of semolina flour mixed with water, extruded (in commercially produced spaghetti), and then dried. Once immersed in water, the rod of pasta begins to swell and soften as water works its way slowly inward. At the same time, it will lose some of its starches to the surrounding water. If the water is hot enough, the pasta undergoes an additional process, starch gelatinization, which is responsible for cooked pasta’s characteristic texture. That perfect al dente condition occurs right as the hydration front reaches the pasta’s core.

    As all of this happens, the initially straight spaghetti strand sags, settles, and curls. Researchers found that, even with a relatively simple model that assumes spaghetti doesn’t stick to the pot, they could capture shape change of individual spaghetti strands, suggesting it’s possible to identify perfectly cooked pasta by shape alone. (Image credit: Pixabay; research credit: N. Goldberg and O. O’Reilly; via Ars Technica)

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    Swinging Jets

    In the tiny realm of microfluidics, flows are, in general, completely laminar. That makes mixing a challenge. But it turns out that pumping water steadily into multiple inlets can spontaneously generate oscillations between the jets, allowing dramatic mixing even at low Reynolds numbers. Two inlets in a parallel channel (first image) oscillate steadily over a small range of conditions, but widening the channels (second image) allows the jets to switch back and forth over a larger range. And adding additional inlets (third image) can create even more complex fluid oscillators! (Image, video, and research credit: A. Bertsch et al.)

  • River Avon

    River Avon

    One of the challenges in fluid dynamics is considering the instantaneous versus the average. Many flows — especially turbulent ones — are different at every point in space and in time. That’s a lot of data to collect and to wrap one’s head around. So often researchers will average turbulent measurements over a period of time and break that information down into two variables: an average velocity and a fluctuating one.

    What does that have to do with this image? Well, by capturing the River Avon’s flow near Pulteney Bridge as a long exposure, photographer Peter Leadbetter gives us a look at the river’s “averaged” flow. The long exposure smooths out some of the intermittent features visible in a faster picture, and instead draws our attention to the overall path of the flow and regions that may behave differently, like those near the wall in the foreground. The averaging researchers do is much the same. It will erase or obscure some features while making the large-scale patterns more obvious. (Image credit: P. Leadbetter; submitted by Ioanna S.)