FYFD

Tag: buoyancy

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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)

  • 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.)

  • Understanding Wildfire

    Understanding Wildfire

    Wildfires are an ongoing challenge in the western United States, where droughts and warmer conditions have combined with a century of fire suppression to form perfect conditions for monstrous fires. It’s long been understood that ambient winds can drive spreading fire, but the connection between wildfire and wind is more complicated than this.

    The heat of a fire drives buoyant air to rise, creating tower-like updrafts in a flame front. We see this both in the shape of the grass fire above, and in the wind vectors of a simulated grass fire in the lower image. Between those towers are troughs where cooler ambient wind is drawn in to replace the rising air. How a fire spreads will depend on the speed, direction, and temperature of these winds. A hot wind fed by the fire’s heat will raise the temperature of fuel in unburned areas, bringing it closer to ignition. In contrast, cooler ambient winds can hinder a fire by keeping nearby grass and twigs too cool to ignite. (Image credit: fire – M. Finney/US Forest Service; simulation – R. Linn; research credit: R. Linn et al.; for more, see Physics Today)

  • Volcanic Plume

    Volcanic Plume

    Astronauts aboard the International Space Station captured this dramatic image of Raikoke Volcano’s eruption in late June. This uninhabited Pacific Island is part of the Kuril Islands off mainland Russia. The hot plume of ash and volcanic gas rose until its density matched that of the surrounding air, at which point it could only expand horizontally. This is why the plume appears to have such a flat top. It’s similar to the cumulonimbus clouds we associate with severe thunderstorms. Scientists speculate that the white ring around the plume’s base might be water vapor condensed from ambient air pulled in to the plume’s base or a side-effect of magma flowing into the surrounding sea. (Image credit: NASA; via NASA Earth Observatory)

  • Granular Instabilities

    Granular Instabilities

    Granular mixtures show surprising similarities to fluids, even though their underlying physics differ. The latest example of this is a Rayleigh-Taylor-like instability that occurs when heavy particles sit atop lighter ones. By combining vertical vibration and an upward gas flow, researchers found that the lighter particles form fingers and bubbles that seep up between the heavier grains (upper left). Visually, it looks remarkably similar to a lava lamp or other Rayleigh-Taylor-driven instability (upper right).

    But the physics behind the two are distinctly different. In the fluid, buoyancy drives the instability while surface tension acts as a stabilizing force. There’s no surface tension in a granular material, though. Instead, the drag force from gas flowing upward provides the vertical impetus while friction between the grains – essentially an effective viscosity – replaces surface tension as a stabilizing influence.

    The similarities don’t stop there, though. When the researchers tested a “bubble” of heavy grains suspended in lighter ones (lower left), they found that, instead of sinking, the granular bubble split in two and drifted downward on a diagonal. Eventually, those daughter bubbles also split. Again, visually, this looks a lot like what happens to a drop of ink or food coloring falling through water (lower right), but the physics aren’t the same at all. 

    In the fluid, the breakup happens when a falling vortex ring splits. In the granular example, gas moving upward tends to channel around the heavy grains because they’re harder to move through. Eventually, this builds up a solidified region under the bubble. When the heavy grains can’t move directly down, they split and sink through the surrounding suspended particles until they build up another jammed area and have to split again. (Image credits: granular RTI – C. McLaren et al.; RTI simulation – M. Stock; bag instability – D. Zillis; research credit: C. McLaren et al.; submitted by Kam-Yung Soh)

  • The Bouncing Drop

    The Bouncing Drop

    For a droplet to bounce, we expect it to hit a wall or a sharp interface of some kind. But in a new study, researchers demonstrate a droplet that bounces with neither. Shown above is an oil droplet sinking through a stratified mixture of ethanol (toward the top) and water (toward the bottom). Because the oil is heavier than ethanol, it initially sinks, dragging some of the ethanol with it as it falls. Over time, some of that ethanol rises again, forming what’s known as a buoyant jet.

    Simultaneously, the gradient of ethanol to water between the top and bottom of the drop creates an imbalance in surface tension. The ethanol near the top of the drop has a lower surface tension than the water at the bottom. This creates a downward Marangoni flow along the drop interface.

    The bounce itself happens quickly after a long, slow sinking period. As the drop’s sinking slows, the buoyant jet weakens until it disappears completely. At the same time, the downward Marangoni flow pulls fresh ethanol-rich fluid toward the top of the drop. That increases the surface tension difference and strengthens the Marangoni flow, creating a positive feedback loop. In less than a second, the Marangoni flow increases by two orders of magnitude, pulling so hard that the drop shoots upward.

    That resets the cycle by weakening the Marangoni flow and strengthening the buoyant jet. The droplet can continue bouncing for about 30 minutes until the concentration gradient is so well-mixed that the cycle can’t continue. (Image and research credit: Y. Li et al.; via APS Physics; submitted by Kam-Yung Soh)

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    The Art of Paper Marbling

    Known as ebru in Turkey and suminagashi in Japan, the art of paper marbling has flourished in cultures around the world since medieval times. The details of methods vary, but in general, the technique uses a base of oily water to float various dyes and pigments. Artists then use brushes, wires, and other tools to manipulate the dyes into the desired pattern. Paper is spread over the top to soak up the color pattern before being hung to dry. Every print made in this manner is a unique result of buoyancy, surface tension variation, and viscous manipulation. Check out the video above to watch a timelapse video showing the technique in action. (Video and image credit: Royal Hali)

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    The Shaky Life of a Droplet

    An evaporating drop of ouzo goes through several stages due to the interactions of oil, alcohol and water. If you turn the situation around by placing a drop of (blue-dyed) water in a mixture of alcohol and anise oil (top image), you get some similarly odd behavior. The drop of water shimmies and grows as alcohol dissolves into it, carrying the occasional oil droplet with it. Eventually, the droplet grows large enough and buoyant enough that part of it detaches and floats to the surface (middle image). If you increase the alcohol ratio in the surrounding fluid, you speed up this process, causing droplets to stream up to the surface (bottom image). (Image and video credit: O. Enriquez et al., source)

  • Lava Bomb

    Lava Bomb

    What you see above is a homemade lava bomb. To systematically study what happens when groundwater meets lava, scientists melted basalt and created their own meter-scale explosion-on-demand. Inside the container, they can inject water and observe the resulting dynamics.

    Beneath the lava, the water forms what scientists call a domain. Thanks to the Leidenfrost effect, it can be protected from direct contact with the lava by a thin vapor layer that boils off it. If the water domain is large enough, buoyancy will pull it upward through the lava. Whether the water maintains a spherical shape or begins to distort and break up into smaller domains depends on the speed of its rise.

    At some point, though, either naturally or through an external trigger (like the sledgehammer you see above), the water and lava can contact, resulting in explosive vaporization of the water and an explosion. What’s visible at the surface depends on the depth at which the explosion takes place. Scientists are eager to characterize these variations, which will help them better predict the explosive danger of eruptions like Kilauea and Eyjafjallajökull. (Image and research credit: I. Sonder et al.; video credit: NYTimes; submitted by Kam-Yung Soh)

  • What Drives Droplets

    What Drives Droplets

    There’s been a lot of interest recently in what goes on inside droplets made up of more than one fluid as they evaporate. This can be entertaining with liquids like whiskey or ouzo, but it has practical applications in ink-jet printing and manufacturing as well. And a new experiment suggests that we’ve been fundamentally wrong about what drives the flow inside these drops.

    As these drops evaporate, a donut-shaped recirculating vortex forms inside them, as seem in the cutaway views above. Conventional wisdom says that vortex is driven by surface tension. Evaporation of components like alcohol is more efficient at the edges of the drop, and as the alcohol evaporates, it creates a higher surface tension at the drop’s edge than at its peak. Marangoni forces then pull fluid down toward the edges, creating the vortex. That explanation is  consistent with observations of a sessile drop sitting on top of a surface (left side of images).

    But those observations are also consistent with another explanation: evaporating ethanol makes the local density higher, so alcohol-rich parts of the drop rise toward the peak while alcohol-poor regions sink. This difference in density would also create a flow pattern consistent with observations. So which is the real driver, surface tension or gravity?

    To find out, researchers flipped the drop upside-down (right side of images). When hanging, the preferred flow direction due to surface tension doesn’t change; flow should still go from the deepest point on the drop toward the edge. But gravity is swapped; alcohol-rich areas should be found near the edge and attachment points of the drop because buoyancy drives them there. And that is exactly what’s observed. The flow direction inside the hanging droplet is consistent with the direction prescribed by buoyancy-driven flow, thereby upending conventional wisdom. It turns out that gravity, not surface tension, is the major driver of internal flow in these multi-component droplets! (Image and research credit: A. Edwards et al.; via APS Physics; submitted by Kam-Yung Soh)