FYFD

Category: Research

  • 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)

  • Wrinkles on Bubble Collapse

    Wrinkles on Bubble Collapse

    A viscous bubble wrinkles when it collapses, and scientists long assumed this behavior was caused by gravity. But a new experiment shows that the buckling is, instead, driven by surface tension.

    To test gravity’s influence on bubble collapse, the researchers popped bubbles in three orientations: the (normal) upright orientation (Images 1 and 2), upside-down (Image 3), and sideways (Image 4). In all cases, the bubble’s thin film wrinkled as it collapsed, indicating that gravity had little influence on the process. Instead the authors concluded that surface-tension-driven collapse causes the dynamic buckling of the film. (Image and research credit: A. Oratis et al.; submitted by Zander B.)

  • 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)

  • 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.)

  • Sensing Obstacles Through Flow

    Sensing Obstacles Through Flow

    Mosquitoes, bats, and even eels use non-visual means to sense their environments. For mosquitoes, part of their obstacle avoidance comes from the exquisite sensitivity of their antennae, which are able to sense subtle changes in the air flow around them as they approach a wall or the ground. Researchers used this same technique to help a quadcopter avoid crashing by adding air pressure sensors that respond to the changes in the copter’s wake as it approaches the ground. (Image and research credit: T. Nakata et al.; via Science)

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    Shear and Convection in Turbulence

    In nature, we often find turbulence mixed with convection, meaning that part of the flow is driven by temperature variation. Think thunderstorms, wildfires, or even the hot, desiccating winds of a desert. To better understand the physics of these phenomena, researchers simulated turbulence between two moving boundaries: one hot and one cold. This provides a combination of shear (from the opposing motion of the two boundaries) and convection (from the temperature-driven density differences).

    Please note that, despite the visual similarity, these simulations are not showing fire. There’s no actual combustion or chemistry here. Instead, the meandering orange streaks you see are simply warmer areas of turbulent flow, just as the blue ones are cooler areas. The shape and number of streaks are important, though, because they help researchers understand similar structures that occur in our planet’s atmosphere — and which might, under the wrong circumstances, help drive wildfires and other convective flows. (Image, research, and video credit: A. Blass et al.)

  • Floating in Levitating Liquids

    Floating in Levitating Liquids

    When it comes to stability, nature can be amazingly counter-intuitive, as in this case of flotation on the underside of a levitating liquid. First things first: how is this liquid layer levitating? To answer that, consider a simpler system: a pendulum. There are two equilibrium positions for a pendulum: hanging straight down or pointing straight up. We don’t typically observe the latter position because it’s unstable; the slightest disturbance from that perfectly vertical situation will make it fall. But it’s possible to stabilize an inverted pendulum simply by shaking it up and down. The vibration creates a dynamic stability.

    The same physics, it turns out, holds for a layer of viscous fluid. With the right vibration, the denser fluid can levitate stably over a layer of air. Inside this vibrating layer, the rules of buoyancy are a little different because the vibration modifies the effects of gravity. As a result, bubbles deep in the liquid layer sink (Image 1). The researchers used this behavior to create their levitating layer (Image 2). The shaking also serves to stabilize objects floating on the underside of the liquid layer, allowing the boat in Image 3 to float upside down! (Image and research credit: B. Apffel et al.; via NYTimes; submitted by multiple sources)

  • 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.)

  • Dead Water

    Dead Water

    In the days before motorized propulsion, sailors would sometimes find themselves slowed nearly to a stop by what they called ‘dead water‘. As discovered in laboratory experiments over a century ago by Vagn Walfrid Ekman, the dead water phenomenon occurs where a layer of fresh water exists over saltier water. The ship’s motion generates internal waves in the salty layer, which in turn causes substantial additional drag on the boat. In a related phenomenon, named for Ekman, the internal waves generated by a boat’s initial acceleration cause its speed to fluctuate.

    While these phenomena have little effect on today’s shipping, they can be relevant for swimmers in areas like harbors and fjords where fresh water meets the sea. And their effects were undoubtedly substantial for much of history. There is even speculation that dead water might have caused the defeat of Mark Antony and Cleopatra’s superior navy at the hands of Octavian’s smaller ships in the Battle of Actium. (Image credit: M. Blum; research credit: J. Fourdrinoy et al.; via Hakai Magazine; submitted by Kam-Yung Soh)

  • Collecting Animal Tears

    Collecting Animal Tears

    Like humans, most vertebrates rely on tear films to keep their eyes moist and protected from the environment. But compared to humans, some animals’ tears have superior staying power. The caiman, for example, can go up to 2 hours between blinks without their eyes drying out; in contrast, humans have to blink about 15 times per minute – and sometimes even that isn’t enough to keep our eyes moist!

    Researchers are collecting animal tears and studying their composition to better understand how their tears protect vision. Subtle changes in chemical make-up can lead to large variations in performance; just look at the many dried tear patterns in Image 2. Scientists hope that understanding other species’ tears will help us develop better treatments for our own vision problems. As someone who struggles with dry eyes at times, I’d be happy for some caiman-tear-inspired eye drops! (Image credit: A. Oriá; research credit: A. Raposo et al.; via NYTimes; submitted by Kam-Yung Soh)