FYFD

Category: Research

  • Fish Fins Work Together

    Fish Fins Work Together

    Researchers studying how fish swim have long focused on their tail fins and the flows created there. But a fish’s other fins have important effects, too, as seen in this recent study. Researchers built a CFD simulation based on observations of a swimming rainbow trout, focusing on the flow from its back and tail fins. They found that the vortex created by the back fin stabilizes and strengthens the one generated by the tail. It also played a role in reducing drag on the fish by maintaining the pressure difference across the body. When they tried changing the size and geometry of the fins, the fish’s efficiency suffered, indicating that evolution has already optimized the trout’s fins for swimming efficiency. (Image credits: top – J. Sailer, simulation – J. Guo et al.; research credit: J. Guo et al.; via APS Physics)

    Visualization of flow around a digitized rainbow trout.
    Visualization of flow around a digitized rainbow trout.
  • Gravity Changes Droplet Shapes

    Gravity Changes Droplet Shapes

    With small droplets, gravity usually has little effect compared to surface tension. An evaporating water droplet holds its spherical shape as it evaporates. But the story is different when you add proteins to the droplet, as seen in this recent study.

    The protein-filled sessile drop starts out largely spherical, but as the drop evaporates, the concentration of proteins reaches a critical point and an elastic skin forms over the drop. From this point onward, the drop flattens.
    The protein-filled sessile drop starts out largely spherical, but as the drop evaporates, the concentration of proteins reaches a critical point and an elastic skin forms over the drop. From this point onward, the drop flattens.

    As a protein-doped droplet sitting on a surface evaporates, it starts out spherical, like its protein-free cousin. But, as the water evaporates, it leaves proteins behind, gradually increasing their concentration. Eventually, they form an elastic skin covering the drop. As water continues to evaporate, the droplet flattens.

    For a hanging droplet, the shape again starts out spherical. But as the drop's water evaporates and the proteins concentrate, it also forms an elastic skin. As the drop evaporates further, the skin wrinkles.
    For a hanging droplet, the shape again starts out spherical. But as the drop’s water evaporates and the proteins concentrate, it also forms an elastic skin. As the drop evaporates further, the skin wrinkles.

    In contrast, a hanging droplet with proteins takes on a wrinkled appearance once its elastic skin forms. The key difference, according to the model constructed by the authors, is the direction that gravity points. Despite these droplets’ small size, gravity makes a difference! (Image, video, and research credit: D. Riccobelli et al.; via APS Physics)

  • Testing Turbulence’s Limits

    Testing Turbulence’s Limits

    Understanding chaotic, turbulent flows has long challenged scientists and engineers due to their sheer complexity. In turbulent flows, energy cascades from the largest scales — like the kilometer-size cross-section of a cloud — to the very smallest scales, less than a millimeter in size, where viscosity transforms the flow’s motion to heat. For nearly a century, our theoretical understanding of turbulence has posited that there are certain universal behaviors in the statistics of a turbulent flow — essentially that, due to this energy cascade, some aspects of every turbulent flow are the same from clouds to ocean currents to your coffee cup.

    Accordingly, experimentalists have tried for decades to measure this expected universality. Often, there are some signs of agreement, and any deviation was attributed to the finite difference between the large and small scales of the flow. (The theory assumes the difference in these scales’ size is effectively infinite.) But now researchers have achieved the largest range of scales yet — comparable to those found in the atmosphere — and the gaps between theory and experiment remain. The new study does show signs of universality but in a different way than existing theory predicts. As the authors point out, we’ll need new theories to explain these findings. (Image credit: D. Páscoa; research credit: C. Küchler et al.; via APS Physics)

  • Sliding on Sand

    Sliding on Sand

    Getting around on sandy slopes is no easy feat. On steep inclines, even small disturbances will cause an avalanche. The predatory antlion takes advantage of this fact by building a conical pit that makes ants that walk in slide down into its waiting jaws. But a new study shows that it’s more than just pressure that determines when an object slides down the slope.

    To simulate hapless ants sliding into an antlion’s pit, researchers used plexiglass disks with four smaller disks that act as legs on the granular slope. By varying the distance between these points of contact, researchers found that stance also affects when a slide starts. The closer together the contacts are, the more likely the disk would slide. In contrast, spreading the points of contact increased stability, meaning that adopting a wider stance could keep an animal, human, or robot from sliding as easily. (Image credit: NEOM; research credit: M. Piñeirua et al.; via APS Physics)

  • Bubble Growth, Inspired By Art

    Bubble Growth, Inspired By Art

    Eighteenth- and nineteenth-century French painters like Chardin and Manet had a certain fascination with bubble-blowing physics. Both left behind artwork depicting children blowing soap bubbles through straws. Now researchers are exploring this bubble-making method in a recent study.

    To blow a bubble from a straw or other narrow constriction, there are three basic stages. In the first, the soapy interface bulges and takes on a spherical shape. That’s followed by a period of rapid growth in less than 100 milliseconds. And, finally, the bubble will pinch off and detach from the straw. So far, most studies have focused on that third phase. Instead, this team focused on those early stages.

    In that first stage, the bubble’s growth depends on air getting forced out of an attached reservoir. For children, that’s their lungs reducing in volume as they blow air into the straw. In their experiments, the team found that the initial volume of the air reservoir is an important (and previously overlooked) factor in controlling bubble growth. (Image credit: J. Chardin; research credit: M. Grosjean and E. Lorenceau; via Ars Technica)

  • Puddle Depth Matters for Stalagmites

    Puddle Depth Matters for Stalagmites

    In a cave, mineral-rich water drips from the ceiling, spreading ions used to build stalagmites. A recent study considers how the depth of a pool affects the droplet’s splash and how material from the droplet spreads. The authors found several scenarios that vary widely depending on pool depth.

    A droplet falling into a shallow pool creates a splash that quickly breaks up into droplets. This flings the red droplet material in many directions.
    A droplet falling into a shallow pool creates a splash that quickly breaks up into droplets. This flings the red droplet material in many directions.

    A drop falling into a shallow pool had a splash that quickly broke up into droplets (above). By dyeing the pool green and the droplet red, they could track where the droplet’s material wound up. The spray of small droplets carried fluid far, but the main point of impact had a strong concentration of the drop’s fluid.

    With a deeper pool, the drop's impact creates a thick crown splash that collapses in on itself. The drop's fluid is quickly mixed into the pool.
    With a deeper pool, the drop’s impact creates a thick crown splash that collapses in on itself. The drop’s fluid is quickly mixed into the pool.

    In contrast, a deeper pool sent up a thick-walled splash crown that collapsed in on itself. This droplet’s material saw lots of mixing with the pool, but only near the point of impact. From their work, the authors concluded that models of stalagmite growth should incorporate pool depth in order to capture how minerals actually concentrate and move. (Image credit: cave – H. Roberson, others – J. Parmentier et al.; research credit: J. Parmentier et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Overheating Slows Large Animals

    Overheating Slows Large Animals

    As climate change and human development continue to encroach on animals’ territories, mass migrations will become more and more common. But animals aren’t all equally able to travel long distances at speed. In general, larger animals are faster than smaller ones. But a new study shows that there’s another important factor in an animal’s top speed: heat dissipation.

    By studying the characteristics of over 500 animals that walk, fly, and swim, the team found that animals were limited in their speed by how well they could dissipate heat. This makes sense, even from a human perspective; we may be able to run long distances, but once we’re too hot, we have to slow down. The same principle holds for animals, and the bigger the animal, the longer it takes to dissipate heat. As a result, the team found that the fastest animals over long distances all have intermediate body mass. At their size, they can balance the mechanical ability to produce speed with the thermodynamic requirement to dissipate heat. (Image credit: N. and Z. Scott; research credit: A. Dyer et al.; via APS Physics)

  • Getting Water Out of Your Ear

    Getting Water Out of Your Ear

    Swimming often results in water getting stuck in our ear canals. The narrow space, combined with the waxy surface, is excellent at trapping small amounts of water. If left in place, that excess fluid distorts hearing, can cause pain, and may eventually lead to an ear infection. So most people’s common response is to tilt their head sideways and shake it or jump to knock the water out. This recent study looks at just how much acceleration is needed to dislodge that water.

    An acceleration of 7.8g isn't enough to remove the water from this artificial ear canal.
    An acceleration of 7.8g isn’t enough to remove the water from this artificial ear canal.

    The team built an artificial ear based on the shape of a human’s ear canal and observed how much acceleration was needed to knock the water out. The answer? Quite a bit. As seen above, nearly 8g of acceleration was enough to distort the interface of the water in the ear canal, but it didn’t move the water out.

    At higher accelerations — above 20 times the acceleration due to gravity – the air-water interface distorts enough to get the water to flow. But accelerations that large are enough to potentially damage brain tissues.

    At over 24g, the acceleration is enough to dislodge the water from this artificial ear canal. But accelerations this high can cause brain damage.
    At over 24g, the acceleration is enough to dislodge the water from this artificial ear canal. But accelerations this high can cause brain damage.

    The problem is worse for children and babies, whose tiny ear canals necessitate even larger accelerations. For them, shaking hard enough to remove water could cause real damage. Instead, a couple drops of vinegar or alcohol in the ear will lower the surface tension and make the fluid easier to remove. (Image credit: top – J. Flavia, others – S. Kim et al.; research credit: S. Kim et al.; submitted by Sunny J.)

  • Shouting Into the Wind is Easier Than You Think

    Shouting Into the Wind is Easier Than You Think

    “Shouting into the wind” usually means a failure to communicate, but it turns out that shouting into the wind doesn’t work the way people usually think. In fact, it’s easy for people upstream to hear your shouting, thanks to an acoustical effect called convective amplification. You’ve likely experienced it firsthand as an ambulance approaches. With its sirens blaring, the ambulance sounds louder as it comes toward you and quieter after it’s past. (This is separate from the Doppler effect, which changes the pitch of the approaching and receding vehicle.)

    So why does shouting into the wind seem so hard? It’s because your ears are downstream of your mouth. Like the ambulance that’s already gone by, your voice comes from ahead of your ears and therefore sounds quieter to you than it does to your audience upstream. (Image credit: I. Huhtakallio; research credit: V. Pulkki et al.; via Science News; submitted by Kam-Yung Soh)

  • Fixing Reverse Osmosis

    Fixing Reverse Osmosis

    Desalination and water treatment plants both rely on reverse osmosis to generate clean water for human use. The standard theory behind reverse osmosis for the last half century suggested that the membranes separated water and other chemicals by forcing water molecules, driven by chemical gradients, to travel one-by-one through a dense membrane forest. But over the years, researchers saw signs that this theory didn’t hold up; for one, the membranes water travels through have pores in them that are larger than individual water molecules.

    A new study examines the underlying assumptions of the prevailing model and finds instead that water moves through reverse osmosis membranes by pore flow. Instead of individual molecules pushed by concentration, flow takes place through pores and is driven by a pressure gradient. The difference is important because it enables engineers to design more efficient membranes according to real-world physics. By understanding the underlying mechanism, designers can tweak the pore size, density, and other features of reverse osmosis membranes to better filter unwanted chemicals and to remove salt from water with less energy input. (Image credit: Florida Water Daily; research credit: L. Wang et al.; via Wired; submitted by Kam-Yung Soh)