FYFD

Tag: granular raft

  • Waves Break Up Floating Rafts

    Waves Break Up Floating Rafts

    Small particles can float on a liquid, held together as a raft through capillary action. But those rafts — like the tea skin below — break up when waves jostle them. In this study, researchers looked at how standing waves broke up a raft of graphite powder. Although the raft’s break-up resembles fields of sea ice breaking apart, the researchers found that different mechanisms were responsible. In their experiment, waves pushed and pulled horizontally at the raft, causing it to fracture. But that push-and-pull is negligible in sea ice, where sheets instead break from the up-and-down motion of waves vertically bending the ice. Nevertheless, the new insights are valuable for various biofilms and some ice floes. (Image and research credit: L. Saddier et al.; via APS Physics)

    The skin atop a cup of tea breaks up into polygons after stirring with a spoon.
    The skin atop a cup of tea breaks up into polygons after stirring with a spoon. Although the effect resembles sea ice breakup, the specific wave mechanism differs.
  • “Coat or Collapse?”

    “Coat or Collapse?”

    Imagine a layer of particles sitting at the interface between oil and water. Known as a granular raft, these particles can interact in interesting ways with other objects. Here, researchers experiment with allowing different shapes to fall through the raft. At slow speeds, the raft deforms to coat the object, even if it has a complex shape (top images). At higher insertion speeds, however, the granular raft can break up around the object. The lower sequence of images show a cylinder interacting with the raft. Moving from left to right, each image shows a larger cylinder diameter and an increasingly complex break-up of the raft. (Image credit: C. Gabbard et al.)

  • Swedish Egg Coffee

    Swedish Egg Coffee

    In the mid-1800s, Scandinavian immigrants settling in the Midwest had no filters, no percolators, and no drip coffee makers to aid their quest for a cup of coffee. Instead, they used eggs to boil a smooth, grit-free cup. Mixing the coffee grounds with egg — sometimes with the shell and all — creates a protein-packed raft that floats when the coffee’s done boiling. Adding cold water sinks the raft of ground coffee, giving a clean final pour with no filter necessary. I’m not a coffee drinker, but for those of you who are, I’m curious: would you drink an egg coffee? (Image credit: K. Tomlinson; via Atlas Obscura; submitted by Richard B.)

  • Encapsulating Drops

    Encapsulating Drops

    Sometimes a droplet needs a little protection while it’s traveling to its destination. When that’s the case, we often try to encapsulate it in a layer of material that won’t be affected by whatever environment the drop is traveling through. In this study, researchers aimed to give their drops not one but two layers of protection — in as simple a way as possible.

    The team began with three layers of liquid. The lowest layer was water, the middle layer was an oil, and the top layer was a mixture of water and isopropyl alcohol. Next, they added glass particles that were denser than the alcohol, but less dense than the oil. This caused the particles to form a clump — a granular raft — along the interface between the alcohol and the oil (not shown). When the layer of particles became heavy enough, it began to sink into the oil, carrying some of the alcohol with them. This conglomeration formed the initial droplet of alcohol mixture encased in an armor of glass beads.

    As this armored droplet sank, it approached the second interface: the oil-water interface. At this juncture, the team observed three different outcomes. When the glass particles were small or light, the armored drop would come to a rest at the oil-water interface. As the drop deformed, water would pierce the armor, causing the whole drop to rupture (Image 1).

    In the second case, heavier particles caused the armored drop to sink through the oil-water interface, but a low oil viscosity meant that the oil film drained from the bottom of the drop before the drop was fully encapsulated. Once again, this let the water through and ruptured the droplet (Image 2).

    In the final case, armored drops with just the right bead density and oil viscosity would sink through the oil-water interface until the oil pinched off behind the drop. This pinch-off allowed the oil to redistribute around the drop, encapsulating it in layers of both oil and particles, thereby protecting it as it continued its journey (Image 3). (Image credits: top – Girl with red hat, experiment – A. Hooshanginejad et al.; research credit: A. Hooshanginejad et al.)

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    Rifts in Rafts

    A raft of particles floating on water has some natural cohesion from particle attraction and capillary action. But when the raft is pulled apart, what happens? Does it break cleanly in one spot? Does it stretch and deform? That’s what this video explores. It turns out that the speed you pull the raft at determines how it holds together. Every particle cluster has a preferred relaxation rate, and by choosing the pulling speed, you determine which relaxation rate — and therefore cluster size — can survive most effectively. (Image and video credit: K. Tô and S. Nagel)

  • Mimicking Asteroids

    Mimicking Asteroids

    In nature, objects like asteroids, black holes, and atomic nuclei can get distorted when spinning rapidly. Researchers are exploring these objects using a new model platform: particle rafts levitated by sound. The individual particles are less than a millimeter wide and tend to clump together due to the scattering of sound waves off neighboring particles. This effect provides a cohesive force — similar to surface tension or the effects of gravity — that draws the particles together. With the right frequency, the sound waves can also make the granular rafts spin, setting up a tug-of-war between cohesion and centrifugal force.

    Using sound waves for levitation, particles slowly rise and clump together. Particles are approximately 190 micrometers each, and the video is drastically slowed down from real-time.

    As the rafts spin, they distort, pull apart, and come back together. Interestingly, the cohesive force a raft experiences increases with the raft’s size. That makes the attractive force unlike surface tension (which is the same whether you have a bucket of water or a lake) and more like gravity (which is stronger with more material.) Because of this size dependence, the team hopes their granular rafts could be a new way to study the formation of rubble-pile asteroids and similarly granular systems.

    As the raft’s rotation increases, it’s pulled apart by centrifugal forces, but the pieces later reconnect. Video is slowed down by a factor of 60.

    (Video, image, and research credit: M. Lim et al.; via APS Physics)

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    Morphing Particle Rafts

    A layer of tiny glass beads sitting atop a pool of castor oil becomes a morphing surface in this video. Applying an electric field creates enough electrostatic force to draw the interface upward against the power of both gravity and surface tension. Moving the electric field — either by shifting the electrode or simply moving a finger over the surface — is enough to pull columns of fluid along! I could imagine this making some very cool human-machine interfaces one day. (Image and video credit: K. Sun et al.)

  • Breaking Up Granular Rafts

    Breaking Up Granular Rafts

    Particles at a fluid interface will often gather into a collection known as a granular raft. The geometry of the interface where it meets individual particles, combined with the surface tension, creates the capillary forces that attract these particles to one another. Colloquially, this is called the Cheerio’s effect; it’s the same physics that draws those cereal chunks together in your bowl.

    Once together, these granular rafts can be surprisingly difficult to break up. That’s the focus of a new study on erosion in granular rafts. As seen in the top image, the raft has to be moving quite quickly before individual beads get pulled away. The experimental set-up here is pretty neat, and it’s not apparent from the video, so I’ll take a moment to explain it. The particles you see are gathered at an interface between water and oil. To generate the movement we see, researchers take the metal cylinder seen at the left of the image and pull it downward. That curves the oil-water interface, effectively creating a hill for the raft to accelerate down.

    To focus in on the forces necessary to separate individual particles, the researchers also looked at a pair of particles (bottom image). With this set-up, they could more easily track the geometry of the contact line where the oil, water, and bead meet. What they found is that the attractive forces generated between the beads are two orders of magnitude larger than predicted by classical theory. To correctly capture the effect, they needed a far more precise description of the contact line geometry around a sphere than is typically used. (Image and research credit: A. Lagarde and S. Protière)