FYFD

Tag: jamming

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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)

  • Jamming Soft Grains

    Jamming Soft Grains

    Hard granular materials — sand, gravel, glass beads, and so on — can flow, but, in narrow regions or under large forces, they can also jam up, essentially turning into a solid. Soft particles can also flow and jam, but do so under different conditions than hard particles. One group of researchers used a custom-built rheometer to measure jamming in soft particles like the hydrogel beads pictured here. They found that they could extend existing models for jamming in hard particles, but they had to rescale the mathematics to account for the way soft particles change their shape under pressure. (Image credit: Girl with red hat; research credit: F. Tapia et al.; via APS Physics)

  • Jamming Inside

    Jamming Inside

    Worm-like Spirostomum ambiguum are millimeter-sized single-cell organisms that live in brackish waters. In milliseconds, these cells can retract to half their original length, generating g-forces greater than a Formula One driver experiences when cornering. How, researchers wondered, do these cells avoid shredding their internal structure with forces that strong?

    Spirostomum ambiguum, they found, contain fluid-filled sacs called vacuoles that are entangled with the folds of a membrane-like structure called the endoplasmic reticulum. The researchers constructed a simulated cell, based on the properties of the living ones, and tested it under retraction. Without the endoplasmic reticulum, the insides of their model acted like a liquid, with vacuoles moving past one another readily. That’s not good for staying alive since swapping positions can disrupt bodily functions.

    An artificially-colored micrograph highlights the different structures inside Spirostomum ambiguum. The red strings are a membrane-like endoplasmic reticulum entangled between yellow, fluid-filled vacuoles.
    An artificially-colored micrograph highlights the different structures inside Spirostomum ambiguum. The red strings are a membrane-like endoplasmic reticulum entangled between yellow, fluid-filled vacuoles.

    With the vacuoles connected by a model endoplasmic reticulum, the cell’s insides acted more like a solid during retraction. The vacuoles deformed but fewer of them traded places, instead jamming together to prevent rearrangement. Mimicking this structure at a larger scale, the team suggests, could enable new types of shock absorbers. (Image and research credit: R. Chang and M. Prakash; via APS Physics)

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    To Clog or Not to Clog?

    The clear plastic disks use to study clogging appear rather plain — at least until you look at them through polarizers. Then the disks light up with a web of lines that reveal the unseen forces between the particles. In this video, researchers use this trick to explore how spontaneous clogs occur. If particles jam together into an arch, that bridge can be strong enough to hold the weight of all the particles above it, bringing the flow to a halt. Some arches aren’t strong enough to hold for long; they can break in moments. Other more stable arches persist. By watching the flow through polarizers and carefully tracking the ebb and flow of the forces between particles, researchers can predict which clogs will have staying power. (Video credit: B. McMillan et al.)

  • Slow to Relax

    Slow to Relax

    Oobleck is a decidedly weird substance. Made from a dense suspension of cornstarch in water, oobleck is known for its mix of liquid-like and solid-like properties, depending on the force that’s applied. In a recent study, researchers took a look at what happens when you really push oobleck to the extreme. When the force applied to oobleck is small or slowly added, the water between cornstarch particles helps keep the particles apart and free of contact. It’s when the force is large that those particles start jamming up against each other and having friction between them, and then the oobleck suddenly acts like a solid. But what happens once that force is removed?

    When the force is gone, we expect the particles to repel and for water to squeeze back into the spaces between them, breaking up the friction and allowing the oobleck to relax back to a liquid-like form. But the team found that sometimes the oobleck doesn’t relax as easily as expected; instead, it seems to retain some memory of its solid-like state, due to persisting friction between particles. (Image credit: T. Cox; research credit: J. Cho et al.)

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    Modeling Oobleck

    Oobleck – that peculiarly behaved mixture of cornstarch and water – continues to be a favorite of children and researchers both. Oobleck flows like a liquid when deformed slowly, but try to move it quickly and it will seize up like a solid. This sudden change depends on the tiny particles of cornstarch suspended in the liquid. When they’re given time, electrostatic forces between the particles help them repel one another and keep the liquid flowing. But under sudden impacts, the particles get jammed together and the friction between neighboring grains makes the viscosity of the fluid increase by orders of magnitude. 

    Researchers are now able to model these particle interactions numerically, which will help them predict how oobleck and similar substances will behave in applications like body armor or pothole repair. (Video credit: MIT; via MIT News; research credit: A. Baumgarten and K. Kamrin)

  • Shearing Grains

    Shearing Grains

    Granular materials, like beads and sand, demonstrate both solid and fluid-like behaviors, which makes them difficult to study. Traditionally, one method for studying how fluids respond to deformation places the fluid in a ring-shaped cell with a rotating outer wall. That creates a uniform shear, as indicated by the red arrows above. For granular materials, though, this classic set-up usually breaks the grains up into two separate regions, one that behaves solidly and the other that behaves fluidly.

    To get past that issue and study grains under truly uniform shear, researchers built a new version of the classic apparatus. In this new ring-shaped cell, the outer wall moves but so do independent concentric rings beneath the grains. This allows researchers to see how grains move under uniform shear (left) and what kinds of forces develop between jammed grains in the system (right). (Image and research credit: Y. Zhao et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Storing Memory in Bubbles

    Storing Memory in Bubbles

    Soft systems like this bubble raft can retain memory of how they reached their current configuration. Because the bubbles are different sizes, they cannot pack into a crystalline structure, and because they’re too close together to move easily, they cannot reconfigure into their most efficient packing. This leaves the system out of equilibrium, which is key to its memory. 

    By shearing the bubbles between a spinning inner ring (left in image) and a stationary outer one (not shown) several times, researchers found they they could coax the bubbles into a configuration that was unresponsive to further shearing at that amplitude. 

    Once the bubbles were configured, the scientists could sweep through many shear amplitudes and look for the one with the smallest response. This was always the “remembered” shear amplitude. Effectively, the system can record and read out values similar to the way a computer bit does. Bubbles are no replacement for silicon, though. In this case, scientists are more interested in what memory in these systems can teach us about other, similar mechanical systems and how they respond to forces. (Image and research credit: S. Mukherji et al.; via Physics Today; submitted by Kam-Yung Soh)

  • 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.)

  • Giving Chocolate that Smooth Finish

    Giving Chocolate that Smooth Finish

    Anyone who’s tried to make chocolate confections at home can tell you that achieving that perfect smooth consistency isn’t easy. It was only after Rodolphe Lindt invented the process of conching in 1879 that anyone enjoyed smooth chocolate. Conching is what allows granular solids like sugar, milk and cocoa powders to mix with liquid cocoa butter into a smooth, homogeneous liquid. Although the process has been known for more than a century, it’s only recently that researchers have unraveled the underlying physics that enables it.

    One of the key parameters to conching is the a mixture’s jamming volume fraction; in other words, the point where the fraction of solid particles in the mixture is too high for it to flow freely. In the first stage of conching, the solid particulates and a small amount of liquid are stirred and slowly heated. The mechanical action of stirring breaks up aggregates and raises the jamming volume fraction. By the end of the dry conche, the mixture could flow, in theory, except that it fractures at a lower stress than what’s necessary to flow.

    At this point, chocolatiers add the remainder of the liquid ingredients. That infusion of moisture decreases the friction between solid particles and further raises the jamming volume fraction. With the system now far below that jamming point, the mixture transforms into a freely-flowing, smooth fluid. By understanding the intricacies of the process, scientists hope to reduce the energy necessary in chocolate production and similar industrial processes.  (Image credit: A. Stein; research credit: E. Blanco et al.; via Physics World; submitted by Kam-Yung Soh)