FYFD

Tag: gels

  • Why Slicing Tomatoes Works

    Why Slicing Tomatoes Works

    Picture it: a nice, ripe tomato. Your not-so-recently sharpened kitchen knife. You press the blade down into the soft flesh and… it explodes. Soft solids – like a tomato – don’t react well to cutting, but they slice just fine. Examining why that’s the case is at the heart of this model.

    Tomatoes are essentially a gel encased in a thin skin. Gels are a kind of hybrid material — not quite liquid and not quite solid. They consist of a network of particles or polymers bonded together and immersed in a liquid. To cut that network apart, the downward force of the blade has to strain the gel past its limits, which squeezes out the surrounding liquid.

    The researchers found that this liquid layer is key to how force from the knife’s motion gets transmitted. In particular, they found that the horizontal motion of a slice is necessary to initiate a cut, and that the gel parts most easily when the downward knife velocity is no more than 24% of the horizontal cutting speed. Press down any faster and the strain propagation fluctuates, creating that unfortunate tomato explosion. (Image credit: G. Fring; research credit: S. Mora and Y. Pomeau; via Ars Technica; submitted by Kam-Yung Soh)

  • Branching Gels

    Branching Gels

    If you sandwich a viscous fluid between two plates, then pull the plates apart, you’ll often get a complex branching pattern that forms as air pushes its way into the fluid. But the exact results depend strongly on what kind of viscous fluid you used. A new study looks specifically at what happens when that fluid is a yield-stress gel.

    Yield-stress fluids behave like a solid until a critical amount of force causes them to flow. Think about your toothpaste. When you take the cap off, the toothpaste stays put until you squeeze the tube enough to make it flow. The gels used in this experiment behave similarly.

    The researchers found that their gels required a critical energy input in order to branch and flow. If the energy applied in pulling the plates apart was too low, no branching occurred (Image 1). But beyond that critical energy, separating the plates created intricate branching patterns consistent with those seen in simpler, Newtonian fluids. (Image, research, and submission credit: T. Divoux et al.; via APS)

  • Drying Out

    Drying Out

    Look closely at old paintings, and you’ll notice arrays of tiny, straight cracks that form as the paint dried. This sort of pattern formation during drying is not unusual. Here we see the patterns formed when a thin layer of hydrogel sandwiched between two glass plates dries. As the water evaporates, stress builds at the interface between the air and gel, causing bubbles to form. The bubble size and shape depend on the size on the gap between the plates and the characteristics of the gel. The resulting patterns can be entirely disordered, or they can form worm-like designs that curl throughout the domain. (Image and research credit: R. Pic et al.)

  • Salty Comets

    Salty Comets

    Many of the products we use every day in our homes behave like solids until the right force is applied. These yield-stress fluids are like hand sanitizer – strong enough to suspend millimeter-sized particles when still but capable of flowing easily when pumped. In hand sanitizer, this is because the fluid is made up of swollen microgel particles that are jammed together. To rearrange, they need a certain amount of force applied. The weight of the sugars, capsules, and particulates added to the product aren’t heavy enough to move the jammed microgels, so they stay suspended.

    But researchers found that if they add a salt crystal of the same size and weight (bottom image), it sinks steadily through the gel. The salt’s velocity is constant; it doesn’t change with size as we might expect. That’s because it’s not falling by forcing the microgel particles to move. Instead, its salinity forces the microgel to release its absorbed liquid; basically, it’s collapsing the jammed particles. It falls steadily because it takes a given amount of time to collapse each gel particle.  (Image credits: microgel – N. Sharp; salt comet – A. Nowbahar et al.; research credit: A. Nowbahar et al.)

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    Liquid Pearls

    Researchers create liquid pearls–a liquid droplet surrounded by a gel-like exterior–by dropping the fluid through a special bath. The initial droplet contains a mixture of the liquid core and an alginate solution. When the drop falls through a bath containing calcium ions, the alginate turns into a hydrogel shell around the liquid core. In order to prevent mixing during the droplet impact, researchers use a surfactant that helps the thin alginate layer persist while gelling takes place. The resulting liquid pearl is permeable to chemicals; researchers hope this may allow them to be used to contain microorganisms or cells in a three-dimensional environment during testing. (Video credit: New Scientist, N. Bremond et al.; see also Gallery of Fluid Motion)

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    Gelatin

    Gelatins are actually colloidal gels, or a liquid dispersed inside a solid, cross-linked network. The crosslinks give the gelatin structure, but much of its dynamic behavior remains reminiscent of fluid motion.