FYFD

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  • 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)

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    The Colors of a Thin Film

    Soap bubbles and other thin films are colorful thanks to wave interference across their tiny thickness, but you may have noticed that only some colors appear. Others, like red, seem to be missing. In this video, Dianna digs into the details of wave interference and color theory to explain why we don’t see pure colors in a bubble.

    As she points out near the end of the video, the way to make a red bubble is to shine purely red light on the bubble, but even then, you’ll see stripes on it related to the light’s wavelength. Scientists actually use this property to measure the thickness of tiny air gaps between a droplet and a surface. (Image and video credit: Physics Girl)

  • Jets Beneath Leidenfrost Drops

    Jets Beneath Leidenfrost Drops

    When a droplet impacts, it’s not unusual for converging ripples to form an upward jet, like the one seen here. But under the right circumstances, jets can form downward, too. This study looks at the ultrafast jets that can form beneath an impacting Leidenfrost drop.

    These Leidenfrost drops are striking a surface much hotter than their boiling point, so a large vapor cavity forms quickly beneath them. Using x-ray imaging, the researchers were able to capture the dynamics of this cavity’s formation and collapse (Image 2). The field of view in the animation shows only a portion of the drop’s cavity, so Image 3 may help you orient relative to the drop at large.

    Initially, we see the center of the droplet hitting the surface, followed by the fast growth of a vapor cavity. Rippling capillary waves converge on top of the cavity, creating a pinch-off. From there, a bubble rises up while a fast jet shoots downward. (Image credit: water jet – A. Min, others – S. Lee et al.; research credit: S. Lee et al.)

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    Slow Motion Speech

    Sneezing, coughing, and speaking all produce a spray of droplets capable of spreading COVID-19 and other respiratory illnesses. This Slow Mo Guys video is the latest demonstration in a long line of evidence for why wearing masks in public is such an important part of ending our current public health crisis. Also, I think we can all agree: that sneeze footage is gross. (Image and video credit: The Slow Mo Guys)

  • Synchronizing Microfluidic Drops

    Synchronizing Microfluidic Drops

    In nature, synchronization occurs when oscillators interact. A group of metronomes shifting to tick in unison is a classic example. Here, the system is a microfluidic T-junction and the oscillators are the liquid interfaces along the narrower inlet channels. Systems like this one have long been used to create alternating droplets (Image 1), corresponding to out-of-phase synchronization. But a new paper shows that the same system can perform in-phase synchronization (Image 2), too, generating droplets at the same time.

    For any synchronization to occur, the main channel must be narrow enough for the two side channels to influence one another. Once that’s the case, the out-of-phase synchronization happens at a relatively high flow rate, and lowering the flow rate causes the system to transition to in-phase synchronization. (Image and research credit: E. Um et al.; submitted by Joonwoo J.)

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    Coalescing Drops

    This year’s Nikon Small World in Motion competition was won by fluid dynamics! The first place video shows droplets on a superhydrophobic surface coalescing. The droplets are a mixture of water and ethanol. Their initial merger creates a ripple of waves that’s followed by a ghostly vortex ring that jets into the interior. Previous research on coalescence during impact shows jets driven by surface tension but the jet here doesn’t appear to be confined to the surface. (Image and video credit: K. Rabbi and X. Yan; via Nature; submitted by Kam-Yung Soh)

    Droplets on a superhydrophobic surface coalescing.

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    Storm Eyes and Mushrooms in a Drop

    In industry, drying droplets often have many components: a liquid solvent, solid nanoparticles, and dissolved polymers. The concentration of that last component — the polymers — can have a big effect on the way the droplet dries, as seen in the video above.

    Without polymers, the droplet dries similarly to a coffee ring stain. But at moderate concentration, we see something very different. The droplet forms an eye in the middle, similar to a hurricane’s, and the edges of the droplet sprout mushroom-shaped plumes that grow and merge with one another along the edge. With even larger polymer concentrations, the mushrooms sweep their way inward, leaving a feathery stain behind. (Video, image, and research credit: J. Zhao et al.)

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    Leidenfrost on Water

    When a skillet is hot enough, water droplets will skitter across the surface almost frictionlessly thanks to the Leidenfrost effect. The incredibly high temperature of the surface relative the the liquid’s boiling point causes part of the drop to vaporize, enveloping the remainder of the liquid in a protective vapor cocoon. 

    We see this effect for more than just solid surfaces, though. This video demonstrates how pouring liquid nitrogen on a pool of water creates plenty of Leidenfrost weirdness as well. It looks as though the initial pour freezes some condensation to dust or other particles, which then stream outwards on a cloud of vapor. Larger droplets of liquid nitrogen actually manage to hold together on the pool’s surface. Their vapor keeps them from touching the water, but that flow also jostles them, creating a ring of ripples around the jiggling drop. (Video and image credit: Science Marshal)

    Animation of a droplet of liquid nitrogen skittering on water

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    “Colors”

    Paint, soap, bleach, oil, and oat milk combine to create the gorgeous colorscapes of Thomas Blanchard’s short film “Colors”. Watch as droplets burst and waves of color flow past. It’s a lovely break from whatever you’re dealing with at the moment, and at less than 3 minutes long, you can spare the time! (Image and video credit: T. Blanchard)

  • Oil Drops and Filter Feeders

    Oil Drops and Filter Feeders

    Natural oils provide critical nutrients to filter feeders like zooplankton and barnacles. These creatures capture oil droplets on bristle-like appendages such as cilia and setae. But this droplet-catching turns into a disadvantage during petroleum spills, when capturing and ingesting oil can be lethal. A recent study looks at the fluid dynamics of oil droplet capture for these tiny creatures.

    The authors found that filter feeders capture a range of droplets regardless of size and oil viscosity. But not all droplets stay attached long enough to get consumed, and the larger a droplet is, the lower the flow velocity necessary to detach it from the animal. That suggests a method of limiting uptake of spilled petroleum into the marine food chain: use surfactants to break up the oil into droplets large enough that they’ll detach from filter feeders before getting eaten. (Image credit: D. Pelusi; research credit: F. Letendre et al.; submitted by Christopher C.)

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