FYFD

Category: Research

  • Unsticking in Jumps

    Unsticking in Jumps

    Soft materials tend to be sticky, and once they’re adhered to a surface, they’re often harder to remove than they were to attach — think of Scotch tape stuck to a desk. This difficulty separating sticky things — known as adhesion hysteresis — has been attributed to various causes, like energy lost to viscoelasticity or age-related chemical bonding. But a new study shows that both those explanations are unnecessary.

    Instead, the difficult removal comes from the way two surfaces separate in fits and starts. No two surfaces are perfectly smooth, and soft surfaces are able to conform to all the nooks and crannies of their partner surface. That molding results in a lot of surface contact, all of which must break for the materials to detach. That peeling doesn’t take place smoothly. Instead, the two surfaces part a little at a time in discrete jumps, as shown in the image above. The colors in the illustration show how much energy is dissipated in each jump, with darker colors indicating higher energy. The team found that this stick-slip mechanism is enough to account for the struggles we have un-sticking objects. They’re now looking at how water affects these narrow meeting places between sticky surfaces. (Image and research credit: A. Sanner et al.; via Physics World)

  • Seeking Rogue Wave Origins

    Seeking Rogue Wave Origins

    Rogue waves — rare waves much larger than any surrounding waves — have long been a part of sailors’ tales, but their existence has only been confirmed relatively recently. The exact mechanisms behind them are still a matter of debate. Laboratory experiments with mechanically-produced waves have created miniature rogue waves, but we still lack real-world observations of their formation.

    To that end, researchers sailed the Southern Ocean, known for its rough waves, during austral winter and observed the state of the wind and waves nearby using stereo cameras. They found that young wind-driven waves tend to be steeper, and they move slower than the wind, as they’re still drawing energy from it. Older waves, in contrast, were shorter, less steep, and less likely have white caps from breaking. Overall, they found that strong winds could more easily drive young waves into the nonlinear growth that leads to rogue waves. (Image credit: S. Baisch; research credit: A. Toffoli et al.; via APS Physics)

  • Why Inkjet Paper Curls

    Why Inkjet Paper Curls

    Printed pages from inkjet printers tends to curl up over time. Researchers found that this long-term curl correlates with the migration of glycerol — one of the solvents used in inkjet ink — through the paper’s fiber layers toward the unprinted side. The glycerol migration makes the cellulose fibers in the paper swell up, causing the curl. Changing the solvent used in inkjet inks could stop the curl but would likely lead to printing issues, since the glycerol helps the tiny droplets wind up in the right place on the page. Another solution? Print on both sides of the page! (Image credit: Lunghammer – TU Graz; research credit: A. Maass and U. Hirn; via Physics World)

  • How Moths Confuse Bats

    How Moths Confuse Bats

    When your predators use echolocation to locate you, it pays to have an ultrasonic deterrence. So, many species of ermine moths have structures on their wings known as tymbals. These areas have a band of ridges, and, when the moth’s wing lifts or falls, the ridges buckle one-by-one. A nearby bald patch on the wing acts as an amplifier, making these ultrasonic snaps louder. Altogether, the mechanism deters prowling bats anytime the moth flaps its wings — without any additional effort on the moth’s part. Since the moths have no ears, they presumably don’t even know that they’re making the sound! (Image credit: Wikimedia/entomart; research credit: H. Mendoza Nava et al.; via APS Physics)

  • Dendritic Painting Physics

    Dendritic Painting Physics

    In the art of Akiko Nakayama, colors branch and split in a tree-like pattern. In studying the process, researchers found the physics intersected art, soft matter mechanics, and statistical physics. In dendritic painting, the process starts with an underlying layer of acrylic paint, diluted with water. Atop this wet layer, you place a drop of acrylic ink mixed with isopropyl alcohol.

    The combination of both layers is key. The alcohol-acrylic drop on a Newtonian substrate will show spreading, driven by Marangoni forces, but no branching. It’s the slightly shear-thinning nature of the diluted acrylic paint substrate that allows dendrites to form. As the overlying drop expands, it shears the underlayer, changing its viscosity and allowing the branches to form. You can see video of the process here. (Image credit: A. Nakayama; research credit: S. Chan and E. Fried; via Physics World)

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    Stomp It Out

    Drop a ball that’s partially filled with water and it may or may not bounce. Why the difference? It all comes down to where the water is before impact. The more distributed the water is along the walls, the less likely a container will bounce. Researchers found they could control the bounce by spinning the bottles before they dropped. Centrifugal force flings the water all over the walls of the spinning bottle, and, when impact happens, the water concentrates into a central jet. For the spinning bottles, that jet is wide, messy, and swirling; it breaks up quickly, expending energy that could otherwise go into a bounce. In effect, the spinning bottle’s jet forms quickly enough to “stomp” the rebound. (Video and image credit: A. Martinez et al.; research credit: K. Andrade et al.)

  • Lasers and Soap Films

    Lasers and Soap Films

    Soap films are a great system for visualizing fluid flows. Researchers use them to look at flags, fish schooling and drafting, and even wind turbines. In this work, researchers explore the soap film’s reaction to lasers. When surfactant concentrations in the soap film are low, laser pulses create shock waves (above) in the film that resemble those seen in aerodynamics. The laser raises the temperature at its point of impact, lowering the local surface tension. That temperature difference triggers a Marangoni flow that draws the heated fluid outward. The low surfactant concentration gives the soap film relatively high elasticity, and that allows the shock waves to form.

    In contrast, a soap film with a high concentration of surfactants has relatively little elasticity. In these films (below), the laser creates a mark that stays visible on the flowing soap film. This “engraving” technique could be used to visualize flow in the soap film without using tracer particles. (Image and research credit: Y. Zhao and H. Xu)

    When surfactant concentrations are high, a laser pulse "engraves" spots onto a flowing soap film. Shown in terms of interference (left) and Schlieren (right) imaging.
    When surfactant concentrations are high, a laser pulse “engraves” spots onto a flowing soap film. Shown in terms of interference (left) and Schlieren (right) imaging.
  • Surviving Rainfall

    Surviving Rainfall

    Water striders spend their lives at the air-water boundary, skittering along this interfacial world. But what happens when falling rain destroys their flat existence? That’s the question that motivated today’s research study, which looks water striders subjected to artificial rain.

    Although the water drops themselves are far heavier than the insects, the water doesn’t strike hard enough to injure the insects. Neither a direct impact nor the forces from a neighboring impact, the researchers found, were enough to pose a problem for the water strider’s exoskeleton. Instead, they’re more likely to get flung or submerged, as follows:

    The initial impact of a raindrop creates a large crater. Depending on the position of the insect relative to the point of impact, this may fling the insect away or pull it down into the cavity.
    The initial impact of a raindrop creates a large crater. Depending on the position of the insect relative to the point of impact, this may fling the insect away or pull it down into the cavity.

    When the drop hits, it creates a big crater in the water’s surface. Insects to the outside of the splash get flung outward, while those closer to the point of impact ride the crater wall downward. As the crater collapses, it forms a thick jet that pushes nearby water striders up with it.

    As the initial cavity collapses, it creates a large jet that can push the strider into the air.
    As the initial cavity collapses, it creates a large jet that can push the strider into the air.

    As that initial jet collapses, it forms a second crater, which — being smaller and narrower — collapses much faster than the first one. That action, researchers found, often submerges a water strider caught in the crater.

    The first jet's collapse creates a second crater, and it's this one that tends to trap and submerge the water striders underwater.
    The first jet’s collapse creates a second crater, and it’s this one that tends to trap and submerge the water strider underwater.

    Fortunately for the insect, their water-repellent nature means they’re covered in a thin bubble of air that lets them survive several minutes underwater. That’s time enough for the water strider to rescue itself. (Image credit: top – H. Wang, animations – D. Watson et al.; research credit: D. Watson et al.; via APS Physics)

  • Tornadoes in a Bucket

    Tornadoes in a Bucket

    In nature, some powerful tornadoes form additional tornadoes within their shear layer. These subvortices revolve around the main tornado, causing massive destruction in their wake. In the laboratory, researchers create a similar multi-tornado system with a spinning disk at the bottom of a shallow, cylindrical layer of water. Depending on how fast the disk spins, different numbers of subvortices form around the main vortex.

    In this poster, researchers show the transition from a 3-subvortex system to a 2-subvortex one. Starting at the 12 o’clock position and moving clockwise, we see 3 subvortices arranged in a triangle. A sudden change in the disk’s rotation speed destabilizes the system, causing the subvortices to break down and shift into a new 2-subvortex configuration. As this happens, material that was isolated in each subvortex (darker blue regions) is suddenly able to mix. That suggests that a real-world multiple vortex tornado might suddenly shed debris if it lost enough angular momentum. Back in the lab, though, the shift to a stable 2-subvortex system once again isolates material in individual subvortices and prevents it from mixing with the rest of the flow. (Image and research credit: G. Di Labbio et al. 1, 2)

  • Vortex Below

    Vortex Below

    When a drop of ethanol lands on a pool of water, surface tension forces draw it into a fast-spreading film. Evenly-spaced plumes form at the edges of the film, then the film stops spreading and instead retracts. All of this takes place in about 0.6 seconds. But, as the image above shows, there’s more that goes on beneath the surface. A vortex ring forms and spreads under the film, driven by the shear layer under the edge of the plumes. Here, the vortex ring is visible in the swirling particles near the water surface. (Image and research credit: A. Pant and B. Puthenveettil)