FYFD

Tag: deformation

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    Non-Newtonian Raindrops

    Fluids like air and water are called Newtonian because their viscosity does not vary with the force that’s applied to them. But many common fluids — almost everything in your fridge or bathroom drawer, for example — are non-Newtonian, meaning that their viscosity changes depending on how they’re deformed.

    Non-Newtonian droplets can behave very differently than Newtonian ones, as this video demonstrates. Here, their fluid of choice is water with varying amounts of silica particles added. Depending on how many silica particles are in the water, the behavior of an impacting drop varies from liquid-like to completely solid and everything in between. Why such a great variation? It all has to do with how quickly the droplet tries to deform and whether the particles within it can move in that amount of time. Whenever they can’t, they jam together and behave like a solid. (Image, video, and research credit: S. Arora and M. Driscoll)

  • Stretching Ant Rafts

    Stretching Ant Rafts

    In their natural habitat, fire ants experience frequent floods and so developed the ability to form rafts. Entire colonies will float out a flood in a two-ant-thick raft anchored to whatever vegetation they can find. Because ants in the upper layer of the raft are constantly milling about, the rafts have some ability to “self-heal” as they’re stretched.

    Pulling slowly gives the ants time to "heal" their stretching raft.
    Pulling slowly gives the ants time to “heal” their stretching raft.

    In these experiments, researchers slowly (above) and quickly (below) stretched ant rafts to see how they responded. Given a slow enough stretch, the ants were able to adjust and keep the raft together until it doubled in length. In contrast, a faster stretching rate overwhelmed the raft by the time it was 30% longer. (Image credit: top – Wikimedia Commons, others – C. Chen et al.; research credit: C. Chen et al.; via APS Physics)

    Pulling quickly breaks an ant raft because the ants cannot react quickly enough to heal the raft.
    Pulling quickly breaks an ant raft because the ants cannot react fast enough to heal the raft.
  • Testing Vesicles

    Testing Vesicles

    In biology, vesicles contain a liquid surrounded by a lipid membrane. The characteristics of that membrane – like its stiffness – can change over time in ways that indicate other changes. For example, vesicles carrying HIV become stiffer as they grow more infectious. In the past, to observe these properties scientists used atomic force microscopes, which require removing the vesicles from the liquid in which they naturally reside. That’s problematic because it potentially changes how the vesicle responds. 

    Now researchers have developed a new method: a microfluidic system that subjects vesicles to electric fields in order to deform them and measures their properties without removing them from their carrier fluid. This provides a faster and more reliable method of testing a vesicle’s deformation, capable of testing hundreds of samples at a time. (Image credit: Wikimedia; research credit: A. Morshed et al.; submitted by Eric S.)

  • Oobleck Under Impact

    Oobleck Under Impact

    Fluids like air and water are Newtonian, which means that the way they deform does not depend on how the force on them gets applied. Many other fluids, however, are non-Newtonian. How they behave depends on how force is applied to them. The Internet’s favorite non-Newtonian fluid is probably oobleck, a mixture of cornstarch and water with some fairly extreme properties. When deformed quickly, like when struck with a bat, oobleck doesn’t flow; it shatters.

    What’s happening at the microscopic level is that the cornstarch particles in the oobleck are jamming together. They simply cannot move quickly and avoid one another. When they jam together, the friction between them goes way up and so does the apparent viscosity of the oobleck. Because it doesn’t have time to flow, all that energy goes into breaking off “solid” chunks instead. Once they hit the ground, the pieces of oobleck will puddle, just like any other liquid. (Image and video credit: Beyond the Press; via Nerdist)