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    Seismic Events Reveal Ocean Temperatures

    Decades ago, researchers proposed sending sound waves through the ocean to measure its temperature. Although the technique worked, it ran into noise pollution issues, but now it’s back, using naturally-occurring seismic events as the sound source.

    When fault lines shift, they generate seismic waves that travel through the ocean as sound. When they reach a land mass, the waves get converted back into seismic energy that’s then picked up by a receiver. Knowing the distance from the source to the receiver and the time necessary for the wave to travel, scientists can then determine the average temperature of the water based on the speed of sound.

    The technique can track temperature changes down to thousandths of a degree. Based on more than a decade of seismic data from the Indian Ocean, researchers found almost double the temperature increase measured by a different sensor network. (Image and video credit: Science; research credit: W. Wu et al.; submitted by Kam-Yung Soh)

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    The Strangeness of Sand

    Sand and other granular materials can flow, jam, and transmit forces in counterintuitive ways. This Lutetium Project video gives a nice overview of some of these bizarre properties.

    Many of sand’s odd characteristics come from the way forces move through grains that touch. Around 5:20 there’s a demo of one of these effects: the Janssen effect. Using a scale, the video shows the mass of a bunch of grains. Then, the host pours those grains into a narrow cylinder. If you watch the scale, you’ll see that it shows a smaller mass than before. That’s not because of a difference in mass between the bowl and the cylinder; the scale is calibrated to only measure the mass of the grains. In the narrow cylinder the grains appear to weigh less because part of their weight is being supported by force chains that run to the container’s walls. (Image and video credit: The Lutetium Project)

  • The Best of FYFD 2020

    The Best of FYFD 2020

    2020 was certainly a strange year, and I confess that I mostly want to congratulate all of us for making it through and then look forward to a better, happier, healthier 2021. But for tradition and posterity’s sake, here were your top FYFD posts of 2020:

    1. Juvenile catfish collectively convect for protection
    2. Gliding birds get extra lift from their tails
    3. How well do masks work?
    4. Droplets dig into hot powder
    5. Updating undergraduate heat transfer
    6. Branching light in soap bubbles
    7. Boiling water using ice water
    8. Concentric patterns on freezing and thawing ice
    9. Bouncing off superhydrophobic defects
    10. To beat surface tension, tadpoles blow bubbles

    There’s a good mix of topics here! A little bit of biophysics, some research, some phenomena, and some good, old-fashioned fluid dynamics.

    If you enjoy FYFD, please remember that it’s primarily reader-supported. You can help support the site by becoming a patron, making a one-time donation, buying some merch, or simply by sharing on social media. Happy New Year!

    (Image credits: catfish – Abyss Dive Center, owl – J. Usherwood et al., masks – It’s Okay to Be Smart, droplet – C. Kalelkar and H. Sai, boundary layer – J. Lienhard, bubble – A. Patsyk et al., boiling – S. Mould, ice – D. Spitzer, defects – The Lutetium Project, tadpoles – K. Schwenk and J. Phillips)

  • Vanishing Spirits: Gin

    Vanishing Spirits: Gin

    Photographer Ernie Button has spent years exploring the patterns left by evaporating scotch. A team of researchers found that the uniformity of scotch whisky’s stain requires three ingredients: alcohol to drive concentration gradients, surfactants to pull particulates away from the drop’s edge, and polymers to help stick particles to the glass.

    Button wondered whether other spirits might produce similar patterns, and, indeed, some do. The photos above are stains left behind by evaporated gin that’s been aged for a year in oak casks. The patterns are extremely similar in appearance to those from aged scotch whiskies, suggesting that the same fluid dynamical effects are at play here, despite the difference in liquor. But do all grain spirits make these patterns? Check back tomorrow to find out. (Image, research, and submission credit: E. Button; see also)

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    Rocket Yeast

    Usually, microbial colonies are grown on a solid substrate, but what happens when they grow on a liquid surface? That’s the question explored in this Gallery of Fluid Motion video featuring colonies of brewer’s yeast on various liquid substrates. When the viscosity of the liquid is low enough, the colony actually gets pulled apart (Image 2). This behavior is driven by a convective flow in the liquid caused by the colony’s own growth. As the yeast grow, they deplete nearby sugar, creating a density gradient that triggers convection beneath the colony. (Image, video, and research credit: S. Atis et al.)

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    Recreating Acoustics

    The cultural heritage of a site is made up of more than its appearance; its soundscape is vital, as well. Acousticians and historians work together to preserve and recreate the auditory landscape of important sites through acoustical measurements and digital reconstructions based on architecture and building materials. Thanks to projects like these, researchers can achieve feats like recreating a concert within the Notre Dame Cathedral as it was before the 2019 fire. To learn more about the technologies behind these feats, check out this Physics Today article. (Image and video credit: Ghost Orchestra; for more, see Physics Today)

  • The Structure of the Blue Whirl

    The Structure of the Blue Whirl

    Several years ago, researchers discovered a new type of flame, the blue whirl. Now computational simulations have helped them untangle the complex structure of this clean-burning flame. Their work shows that the blue whirl is made up of three types of flames, which meet to form a fourth.

    The conical base of the whirl is a fuel-rich flame in which the fuel and oxygen are initially well-mixed. Above that is a diffusion flame, where the fuel and oxygen are initially separate and the flame’s ability to burn is limited by how readily the two mix. Along the sides of the blue whirl is a third flame type, visible only as a faint wisp. Like the first flame, this one is premixed, but it contains much less fuel than oxygen. Finally, those three flames meet in the bright blue ring of the whirl, where the ratio of fuel and oxygen is just right to burn the fuel completely. (Image and research credit: J. Chung et al.; via Science News; submitted by Kam-Yung Soh)

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    The Greedy Cup in Your Washing Machine

    A Pythagorean, or “greedy” cup, is one that automatically drains itself once filled to a certain level. In other words, it’s a self-starting siphon – one that triggers only at certain fill level. And chances are you have an example of this mechanism close at hand: inside your washing machine’s soap tray. That’s why the tray has such a clearly marked maximum fill line; if you were to put more soap than that in the tray, it would automatically drain! (Image and video credit: S. Mould)

  • Curls Past the Canaries

    Curls Past the Canaries

    When winds flow past a solitary peak, like an island in the ocean, they’re disrupted into a series of counter-rotating curls. That’s what we see here stretching to the southwest of Madeira Island. The official name for this flow is a von Karman vortex street, and it can be found anywhere from a soap film to a starship. (Image credit: J. Stevens; via NASA Earth Observatory)

  • The Undisturbed Waters of Lake Kivu

    The Undisturbed Waters of Lake Kivu

    Deep in Africa lies one of the world’s strangest lakes. Lake Kivu, over 450 meters in depth, is so stratified that its layers never mix. The upper portion of Lake Kivu consists of less-dense fresh water, which sits upon deeper layers of saltier water full of dissolved carbon dioxide and methane pumped into the lake by volcanic activity.

    The lake’s lack of convection means that this deep water simply stays put for thousands of years as it collects gases that remain dissolved only thanks to the immense pressure of the water above. Should that deep water be disturbed — by an earthquake, climate changes, or simply oversaturation — the resulting eruption of carbon dioxide could be deadly for the millions of people living nearby. A similar eruption at smaller Lake Nyos in 1986 asphyxiated about 1,800 people.

    Fortunately, Lake Kivu is well-monitored, so such an upwelling should not catch observers off-guard. Learn more about Lake Kivu’s oddities over at Knowable. (Image and research credit: D. Bouffard and A. Wüest, via Knowable Magazine; submitted by Kam-Yung Soh)

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