FYFD

Tag: instability

  • Acrylic and Oil

    Acrylic and Oil

    Photographer Alberto Seveso is well-known for ink in water art, some of which FYFD has featured previously (1, 2, 3). More recently, he’s been experimenting with alternative methods, dropping fluids like acrylic paint into sunflower oil. The effect is quite different but no less beautiful. Because the paint and oil are immiscible, the boundaries between the two fluids are much more clearly defined and highlighted in an iridescent sheen. Instead of appearing like billowing waves of silk, the paint forms abstract and alien shapes driven by gravity, inertia, and density differences. For many more great examples, check out Seveso’s website. (Photo credit: A. Seveso)

  • Breaking Down Vortices

    Breaking Down Vortices

    Vortex rings are ubiquitous in nature, showing up in droplet impacts, in propulsion, and even in volcanic eruptions. Understanding the interaction and breakdown of multiple vortices with one another is therefore key. The image above shows a circular disk that’s being oscillated up and down (in and out of the page). As the disk moves and changes direction, it generates vortices that interact with one another. Here some of those interactions are visualized with fluorescent dye. The overlapping vortices form complex and beautiful shapes on their way to breakdown. (Image credit: J. Deng et al., poster, paper)

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    Bursting Droplets

    Mixing multiple fluids can often lead to surprising and mesmerizing effects, whether it’s droplets that dance or tears along the walls of a wine glass. A recent paper highlights another such mixture-driven instability – the bursting of a water-alcohol droplet deposited on an oil bath. The Lutetium Project tackles the physics behind this colorful burst in the short video above. The behavior is driven by the quick evaporation rate of alcohol in the droplet and the way this changing chemical concentration affects surface tension in the droplet. Alcohol evaporates more quickly from the edges of the drop, creating a region of higher surface tension around the edge. This pulls fluid to the rim of the drop, where it breaks up into droplets that get pulled outward as the inner drop shrinks.

    The oil bath plays an important role in the instability, too. Without it, friction between the drop and a wall is too high for the droplet to “burst”. A thick layer of oil acts as a lubricant, allowing the escaping satellite drops to speed away. (Video and image credit: The Lutetium Project; research credit: L. Keiser et al.; submitted by G. Durey)

  • Accidental Painting

    Accidental Painting

    Some paintings of Mexican artist David Alfaro Siqueiros feature patchy, spotted areas of contrasting color formed by what Siqueiros described as “accidental painting”. Many modern artists use this technique as well. By pouring thin layers of two different colors atop one other, Siqueiros was able to generate seemingly spontaneous patterns like those shown above. In fact, what Siqueiros was using was a density-driven fluid instability! These patterns will only appear when a denser paint is poured atop a lighter one. They’re the result of a Rayleigh-Taylor instability – the same behavior that makes beautiful swirls of cream in coffee and the finger-like protrusions seen in supernovae.

    Although a density difference is necessary to generate accidental painting, other factors like the paint layer’s thickness and viscosity affect the final pattern. For those who are mathematically-inclined, this paper has a linear stability analysis that shows how density difference, viscosity, and other factors affect the cell sizes in the pattern. (Image and research credits: S. Zetina et al.; GIF source)

  • Saturnian Clouds

    Saturnian Clouds

    It may look like an oil slick, but the photo above actually shows the clouds of Saturn. The false-color composite image reveals the gas giant in infrared, at wavelengths longer than those visible to the human eye. NASA uses this infrared photography to identify different chemical compositions in Saturn’s atmosphere based on how they reflect sunlight. You can see an example of how they construct these images here. This detail shot appears to show cloud bands of different compositions mixing. You can see hints of shear instabilities forming along the edges  where the light and dark bands meet. (Image credit: NASA; via Gizmodo)

  • Fluid Fingers

    Fluid Fingers

    Fluid phenomena can show up in unexpected places. The collage above shows patterns formed when an aluminum block is lifted during wet sanding, a polishing technique. The dendritic fingers are formed from oil and the slurry of sanded particles being polished away. They are an example of the Saffman-Taylor instability, which forms when less viscous fluids (oil) protrude into a more viscous one (the slurry). Each image contains a different concentration of oil, resulting in very different fingering patterns. (Image credit: D. Lopez)

  • Dripping, Frozen

    Dripping, Frozen

    The simple drip of a faucet is more complicated when frozen in time. Any elongated strand of water tends to break up into droplets due to surface tension and the Plateau-Rayleigh instability. Whenever the radius of the water column shrinks, surface tension tends to drive water away from the narrow region and toward a wider point. This exaggerates the profile, making narrow regions skinnier and wider regions fatter. Eventually, the neck connecting the droplets becomes so thin that it pinches off completely, leaving a string of falling droplets.  (Image credit: N. Sharp)

  • Inside a Supernova

    Inside a Supernova

    During a supernova, shock waves moving outward push denser material into less dense plasma and gas. This causes what is known as a Richtmyer–Meshkov instability, where the interface between the two fluids first becomes wavy and then develops finger-like intrusions. Those too break down, as seen in the simulation above, causing large-scale mixing between the different fluids.

    Here on Earth this instability shows up in the process of inertial confinement fusion. In that case, the outer shell material is denser than the fuel core and the instability is triggered during the implosion process. As the fusion material is suddenly compressed, waviness and mixing occurs along the interface between the shell and the fuel. That’s undesirable because it reduces the efficiency of the fusion reaction.  (Image credit: E. Evangelista et al.)

  • Shark Tooth Instability

    Shark Tooth Instability

    Imagine that you partially fill a horizontal cylinder with a viscous fluid, like corn syrup or honey. If that cylinder is still, the fluid will simply pool along the bottom. On the opposite extreme, if you spin it very fast, that cylinder will become coated in an even layer of fluid that rotates along with the cylinder thanks to centrifugal force. Between those two extremes in rotational velocity, some interesting fluid behaviors occur. Start spinning the cylinder and some of the pooled fluid will be pulled up the sides, eventually forming a thicker film with a straight front along the bottom of the cylinder. Spin faster and that straight front starts to break down, forming sharper cusp-like waves known as shark teeth. (Image credit: S. Morris et al., source; research credit: S. Thoroddsen and L. Mahadevan)

  • Shear Across the Water

    Shear Across the Water

    This photo series shows the development of a Kelvin-Helmholtz instability. It’s formed when two layers of fluid move past one another at different speeds. In this case, the two fluids meet off the back of a flat plate (seen at the left of the top image) when fast-moving flow from the top of the plate encounters slower fluid beneath. Friction and shear between the fluid layers causes billows to rise up and form waves very similar to those on the ocean (wind across the water works the same way!). Those waves turn over into vortex-like spirals and keep mixing until they break down into turbulence. This pattern crops up pretty frequently, especially in clouds. (Image credit: G. Lawrence)