FYFD

Tag: buoyancy

  • Carbonation in Microgravity

    Carbonation in Microgravity

    Bubbly beverages are popular among humans, but there’s surprising complexity underlying their seemingly simply carbonation, as explored in a new Physics Today article. Most drinks get their bubbles from carbon dioxide, which at higher than atmospheric pressures, can stay dissolved inside water and other liquids. When that pressure gets released, any carbon-dioxide-filled gas cavity in the liquid adopts the allowable saturation concentration for the ambient pressure, which sets up a concentration gradient of carbon dioxide  between the liquid and the bubble. That causes carbon dioxide gas to diffuse into the bubbles, making them grow. 

    Here on Earth, those growing bubbles are buoyant, and they form rising plumes of bubbles. They continue gathering carbon dioxide as they rise, making them grow ever larger (lower left). In microgravity, on the other hand, the bubbles congregate where they form and continue growing through diffusion (lower right). This is one reason carbonated beverages are unpopular in space – instead of rising to the surface and escaping, all the carbon dioxide in a drink gets consumed, leaving astronauts with no way to expel it aside from burping!

    For lots more fascinating facts about bubbly drinks – including how they relate to geology! – check out the full Physics Today article. (Image credits: beer – rawpixel; bubbles – P. Vega-Martínez et al.; see also: R. Zenit and J. Rodríguez-Rodríguez)

  • Bubbling

    Bubbling

    Many chemical reactions produce gases as a stream of bubbles out of a solution. Here we see the electrolysis of an aqueous sodium hydroxide solution (NaOH), which produces hydrogen gas on the cathode (left) and oxygen gas on the anode (right). In timelapse, the gas bubbles nucleate on the electrode, slowly growing larger. Once the the bubbles are large enough to detach, though, they rise so quickly they look like they disappear! The large buoyant forces on them drive that brief journey to the surface. By contrast, the smaller bubbles rise slowly, held back by their lesser buoyancy and the viscous drag they experience. (Video and image credit: Beauty of Science)

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    From Firenado to Water Spout

    Just a few years ago, fire tornadoes were almost fabled because they were so rarely captured on video. Now, with worsening wildfire seasons and cell phone cameras everywhere, there are new videos all the time. This video captures a fire tornado that sets off a water spout as it reaches the river (~1:15 in).

    Neither the fire tornado or the water spout is truly tornadic; instead they are more like dust devils. They are driven by the rising heat of the fire. As cooler, ambient air flows inward to replace the rising air, it brings with it any vorticity it had. And, like an ice skater, the incoming air spins faster as it moves inward. This sets up both the fire tornado and the water spout’s vortices.

    Although this is the first example I’ve seen video of, fire tornadoes have been known to create water spouts before. Lava flowing into the ocean can create whole trains of them. (Video credit: C. & A. Mackie; via Jean H.)

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    Why Fish Don’t Freeze

    Have you ever wondered why it is that fish in a pond or lake don’t freeze during the winter? The secret is due to a peculiarity of water that’s vital for life here on Earth. In general, cold things are denser than warmer ones. This is why, for the most part, cold fluids tend to sink and warmer ones rise here on Earth. So as fall moves into winter and water near the surface of a pond cools, it sinks. But only to a point.

    Water is at its densest at 4 degrees Celsius. Any colder and the water will actually expand and become less dense. This is why you can’t fill ice cube trays to the very top before putting them in the freezer. In the pond it means that buoyant convection shuts down at 4 degrees Celsius. When the water at the top keeps cooling down to the freezing point, it doesn’t sink. Instead, the fish and other pond life get to spend the winter at a chill – but not freezing – 4 degrees. (Video credit: A. Fillo)

  • Tea Physics

    Tea Physics

    Tea is a popular beverage around the world, and nearly everyone has their own method for making the perfect cup. Perhaps unsurprisingly, scientists have studied tea physics as well. One such study used both experiments and numerical simulations to study tea infusion from teabags. The authors looked at round, two-dimensional teabags in two configurations – one in which the bag was left still during infusion and one in which the bag was dunked up and down in the water.

    In the static case, as the hot water leeches solutes out of the tea leaves, it forms a buoyant convection current. In this case, the convection is driven by solute concentration, not temperature. The convection creates a re-circulation in the cup that helps slowly distribute the tea solutes.

    The dunking method, unsurprisingly, distributes tea solutes much faster. In addition to stirring the cup’s contents, dunking helps drive flow through the tea leaves, releasing solutes faster. Although the authors study the two methods in detail, they decline to pass judgement on what method is “the best”. (Photo credit: T. Foster, source; research credit: G. Lian and C. Astill; submitted by Marc A.)

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    “Water Ballet”

    Artist Kamiel Rongen uses common substances like paint, oil, eggs, and even air freshener to create what he calls “water ballet.” His videos are full of ethereal and surreal landscapes full of color and motion. Buoyancy (or the lack thereof) plays a major role in his work – fluids often spurt upward like alien creatures emerging from a chrysalis. I’ve been debating with myself whether the fluids are actually rising or if they’re falling in front of an upside-down camera, and I’m not completely certain either way! I think that’s a testament both to Rongen’s artistry and to the awesome physics involved. Check out the full video below and you can see many more examples of Rongen’s work on his website. (Image and video credit: K. Rongen; h/t to James H.)

  • Impressionist Foams

    Impressionist Foams

    Imagine taking two panes of glass and setting them in a frame with a small gap between them. Then partially fill the gap with a mixture of dye, glycerol, water, and soap. After turning the frame over several times, the half of the frame will be filled with foamy bubbles. When you flip it again, the dyed glycerol-water will sink and penetrate the bubble layer, creating complex and beautiful patterns as it mixes. Some of the bubbles may get squeezed together until they coalesce into larger bubbles that shoot upward thanks to their increased buoyancy. Other smaller bubbles will wend their way upward as neighboring fluid shifts. If you examine the tracks left by individual bubbles, you can find patterns reminiscent of Impressionist paintings, as seen at the end of this Gallery of Fluid Motion video. (Image credit: A. Al Brahim et al., source)

  • Riding Across Water

    Riding Across Water

    Humans may not be fast enough to run across water, but we’ve found other ways to conquer the waves. It’s even possible (though definitely not recommended) to ride across stretches of water on a dirt bike. To do so, you have to keep the bike (hydro)planing, and to understand what that means, let’s take a moment to talk about boats.

    At low speeds, boats stay afloat based on buoyancy, a force that depends on how much water they displace. But when moving at high speeds, modern speedboats lift mostly out of the water and skim the surface instead. At this point, it’s hydrodynamic lift that keeps the boat above the surface and we say that the boat is planing. Calculating that hydrodynamic lift is fairly complicated and depends on many factors – for those who are interested, check out some of David Savitsky’s papers – but, generally speaking, going faster gives you more lift.

    This brings us back to the dirt bike. There’s nothing particularly hydrodynamic about a dirt bike. It’s not shaped to provide hydrodynamic lift, but it does come with a high power-to-weight ratio. It’s this ability to create pure speed, and a rider’s keen sense for holding the bike at the right angle, that enables pros to cross open water. Needless to say, this is the kind of stunt that could end really badly, so don’t try it yourself. (Image credits: C. Alessandrelli, source; EnduroTripster, source; via Digg; submitted by 1307phaezr)

  • Layered Latte Physics

    Layered Latte Physics

    Latte lovers may be familiar with the layered latte, a beverage with distinctive horizontal layers mixing espresso and milk, but you may not have taken the time to wonder how these layers form. Like many layering phenomena in our oceans, the layered latte is the result of double-diffusive convection. This means that there are two variables that both affect density in the fluid mixture and that they act at different rates.

    In the latte, those factors are 1) the different densities of the milk and espresso and 2) density changes caused as the latte cools to room temperature. A layered latte forms when the lighter espresso is poured into denser milk. If it’s poured quickly enough, the momentum of the pour forces some of the espresso down into the milk, despite the buoyant force that tries to keep the espresso on top. So that initial pour sets up a density gradient that runs from pure espresso at the top to pure milk on the bottom, with varying mixtures of the two in between.

    The distinct layers won’t form until the latte begins cooling off. Along the walls of the container, heat is lost more quickly, causing fluid to cool and start sinking. But a specific bit of fluid can only sink until the fluid surrounding it is the same density. That can carry a cooler bit of latte to the bottom of a layer, but not into the denser layer below. At this point, our bit of latte moves inward, starts to warm up, and circulates up through the center of its layer. As when it sank, the fluid can only move up until it encounters a layer with equal or lesser density, at which point it must move horizontally instead. This thermal convection, combined with the density gradient formed by the initial pour, sets up the distinctive layers of the latte. The layers are quite stable – neither gentle stirring nor taking a sip will disrupt them for long – provided the drink remains warmer than the surrounding air. (Image credits: kopeattugu/Instagram, N. Xue et al.; research credit: N. Xue et al.; via NYTimes; submitted by Kam-Yung Soh)

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    The Cheerios Effect

    You’ve probably noticed that cereal clumps together in your breakfast bowl, but you may not have given much thought as to why. This tendency for objects at an interface to attract is known as the Cheerios effect, although it happens in more than just cereal, as Joe Hanson from It’s Okay to Be Smart explains. The effect is a combination of buoyancy, gravity, and surface tension acting in concert.

    When air, a liquid, and a solid meet, they form a meniscus, the curvature of which depends on characteristics of their interaction. Light, buoyant cereal and the walls of your bowl both have upward-curving menisci. Denser objects, like the tacks shown below, stay at the surface only because surface tension holds them up. Their meniscus curves downward.

    Objects with a similar meniscus curvature will attract. For cereal approaching a wall, the light Cheerio is buoyant enough that there’s an upward force on it, but it’s constrained to stay at the interface. It cannot rise, but that buoyancy is enough to let it climb the meniscus at the wall. The two tacks attract one another for similar reasons, except this time their weight helps them fall into one another. Check out the full video to see more examples of this effect in nature! (Video and image credit: It’s Okay to Be Smart; research credit: D. Vella and L. Mahadevan, pdf)

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