FYFD

Search results for: “density”

  • Aqueous Chandeliers

    Aqueous Chandeliers

    Colorful dyes falling through water form chandelier-like, branching shapes. These formations are the result of a slight density difference between the heavier dyes and the surrounding water. As the dye falls, Rayleigh-Taylor instabilities cause the mushroom-like blobs and their branches. With creativity and photographic skill, Mark Mawson turns these ephemeral shapes into bold liquid sculptures, frozen in time. See more of his work in these previous posts, on his website, and on Instagram. (Image credit: M. Mawson)

  • Asperitas Formation

    Asperitas Formation

    In 2017, the World Meteorological Organization named a new cloud type: the wave-like asperitas cloud. How these rare and distinctive clouds form is still a matter of debate, but this new study suggests that they need conditions similar to those that produce mammatus clouds, plus some added shear.

    Using direct numerical simulations, the authors studied a moisture-filled cloud layer sitting above drier ambient air. Without shear, large droplets in this cloud layer slowly settle downward. As the droplets evaporate, they cool the area just below the cloud, changing the density and creating a Rayleigh-Taylor-like instability. This is one proposed mechanism for mammatus clouds, which have bulbous shapes that sink down from the cloud.

    When they added shear to the simulation, the authors found that instead of mammatus clouds, they observed asperitas ones. But the amount of shear had to be just right. Too little shear produced mammatus clouds; too much and the shear smeared out the sinking lobes before they could form asperitas waves. (Image credit: A. Beatson; research credit: S. Ravichandran and R. Govindarajan)

  • Merging Along Wires

    Merging Along Wires

    As oil slides down two slowly converging wires, the droplets will merge into a sheet that stretches between both wires. When this happens can vary somewhat but occurs somewhere around the liquid’s capillary length.

    In the poster above, the leftmost image (not the illustration) shows three possible merger points. To the right of the image, is a teal curve; this is a probability density function. Essentially, this curve shows where the merger is most likely to occur. The peak of the curve corresponds to the most probable point of merger.

    The following two composite images show the same system — same oil flow rate, same wire spacing — with gas blowing upward along the wires. As the gas’s flow rate increase, the oil drops get larger, making the oil films thinner. The result? The wires have to get closer to one another before the oil merges. That’s reflected in the yellow and orange probability density functions, which have peaks further along the wires than the no-gas-flow case. (Image credit: C. Wagstaff et al.)

  • Featured Video Play Icon

    Double Diffusive Flow

    Diffusion is the tendency for differences in a fluid — in density, temperature, or concentration — to even out over time. Think about a drop of food coloring in a glass of water. Even without stirring, that dye will eventually disperse throughout the glass through diffusion. But when there is more than one factor controlling diffusion — like temperature and salinity — things get more complicated. In the ocean, for example, this double-diffusion causes salt fingers like those shown in the first image.

    But what happens when the two diffusing fluid layers are flowing? That’s the question at the heart of this video, which explores the intricate mixing that takes place between doubly-diffusing liquids in a channel. (Video and image credit: A. Mizev et al.)

  • Mushy Layers

    Mushy Layers

    In many geophysical and metallurgical processes, there is a stage with a porous layer of liquid-infused solid known as a mushy layer. Such layers form in sea ice, in cooling metals, and even in the depths of our mantle. Within the mushy layer, temperature, density, and concentration can vary dramatically from one location to another.

    The image above shows a mushy layer made from a mixture of water and ammonium chloride. Above the mushy layer, green plumes drift upward, carrying lighter fluid. Look closely within the mushy layer and you’ll see narrow channels feeding up to the surface. These are known as chimneys. In sea ice, chimneys like these carry salty brine out of the ice and into the seawater, increasing its salinity. See this Physics Today article for more details on the dynamics of mushy layers. (Image credit: J. Kyselica; via Physics Today)

  • The Shapes of Melting Ice

    The Shapes of Melting Ice

    Water is an odd substance because it is densest at 4 degrees Celsius, well above its melting point at 0 degrees Celsius. This density anomaly means that melting ice takes on very different shapes, depending on the temperature of the water surrounding it. At low temperatures (under 4 degrees Celsius), the cold water melting off the ice is denser than the surroundings, so it sinks. The sinking fluid melts lower portions of the ice faster, leading to an inverted pinnacle (Image 1).

    In contrast, at higher temperatures (above 7 degrees Celsius), the meltwater is lighter than the surroundings and therefore rises, creating an upward-pointing pinnacle (Image 3). At intermediate temperatures, some areas of the ice see rising meltwater and some see sinking. This complicated flow pattern sets up vortices that result in a scalloped edge along the ice (Image 2). (Image and research credit: S. Weady et al.; via APS Physics)

  • Featured Video Play Icon

    Acrylic Paint Fractals

    Here’s a simple fluids experiment you can try at home using acrylic paints, ink, isopropyl alcohol and a few other ingredients. When dropped onto diluted acrylic paint, a mixture of black ink and alcohol spreads in a fractal fingering pattern. The radial (outward) flow is driven by the alcohol’s evaporation, which increases the local surface tension and draws fluid outward. The shape and density of the fingers depends, at least in part, on the viscosity of the underlying paint layer; more viscous paint layers grow smaller and denser fractal patterns. (Image and video credit: S. Chan et al.)

  • Featured Video Play Icon

    Filming the Brinicle

    It may have been 10 years since the BBC filmed the first timelapse of a growing brinicle, but the footage is just as amazing now as it was then! This video gives you the behind-the-scenes story of what it took to capture this natural wonder under the Antarctic ice. It’s incredible to see the shots of sinking brine streaming off the brinicles, too. The difference in density (and thus refractive index) of the brine and the ocean water is substantial enough that your eye can actually pick them out as separate fluids. I once went snorkeling in an area with similarly varied salinity and it was completely bizarre watching everything suddenly go wavy and blurry as I swam. (Image and video credit: BBC)

  • Featured Video Play Icon

    The Bubbly Escape

    Sometimes experiments don’t work as planned and, instead of answers, they lead to more questions. In this video, we see an experiment looking at an air bubble trapped beneath a cone. It’s the same situation you get by holding a mug upside-down in a sink full of water but with inclined walls. As the cone moves downward, it squeezes the trapped air bubble. A film of air gets pushed along the walls of the cone, eventually forming finger-like bubbles that wrap around the edge of the cone and get entrained into the vortex ring outside the cone.

    Clearly, there is some kind of instability that drives the air bubble to form these fingers rather than spreading uniformly. But the big question is which one? Is this a density-driven Rayleigh-Taylor instability caused by air getting pushed into water? Or is it a Saffman-Taylor instability causes by the less viscous air forcing its way into the more viscous water? What do you think? (Image and submission credit: U. Jain)

    A bubble trapped beneath a cone gets distorted and squeezed as the cone accelerates downward.
  • Noctilucent Clouds

    Noctilucent Clouds

    Noctilucent clouds are the “highest, driest, coldest, and rarest clouds on Earth.” Formed in the mesosphere at altitudes over 80 kilometers, these clouds typically form at polar latitudes where they can catch sunlight hours after sunset, hence their night-shining name. The clouds take shape when water vapor in cold mesospheric air layers freezes onto dust left behind by meteors.

    Fun fact: because of their high altitude and particle size and density, noctilucent clouds were considered a hazard for space shuttle reentry, and planners explicitly avoided trajectories that would take the spacecraft near potential clouds. (Image credit: top – N. Fewings, other – J. Stevens/NASA Earth Observatory)

    Satellite image of noctilucent clouds above the North Pole.

More posts