FYFD

Category: Research

  • Creating Clouds

    Creating Clouds

    What you see here is the formation of clouds and rain – but it’s not quite what you’re used to seeing outside. This is an experiment using a mixture of sulfur hexafluoride and helium to create clouds in a laboratory. Everything is contained in a cell between two transparent plates. Liquid sulfur hexafluoride takes up about half of the cell, and when the lower plate is heated, that liquid begins evaporating and rising in the bright regions. When it reaches the cooled top plate, the liquid condenses into droplets inside the dimples on the plate, eventually growing large enough to fall back as rain. The dark wisps you see are areas where cold sulfur hexafluoride is sinking, much like in the water clouds we are used to. Setups like this one allow scientists to study the effects of turbulence on cloud physics and the formation of droplets. (Image credit: E. Bodenschatz et al., source)

    Boston-area folks! I’ll be taking part in the Improbable Research show Saturday evening at 8 pm at the Sheraton Boston. Come hear about the Boston Molasses Flood and other bizarre research!

  • Leidenfrost Atop a Fluid

    Leidenfrost Atop a Fluid

    Leidenfrost droplets typically hover on a thin layer of vapor above a surface that is much hotter than the boiling point of the liquid. Such drops move almost frictionlessly across these surfaces and can even propel themselves. The question of how hot is hot enough to produce the Leidenfrost effect is still being debated, but recent research suggests that the answer may depend strongly on surface roughness.

    To test the role of surface roughness, one group tested drops of ethanol atop a heated pool of silicone oil, as pictured above. Ethanol’s boiling point is 78 degrees Celsius, and the researchers found they could hold the ethanol drop in a Leidenfrost state by heating the pool to 79 degrees Celsius – only 1 degree above ethanol’s boiling point! Thanks to surface tension, a liquid surface is essentially molecularly smooth. The fact that solid surfaces require much higher temperatures before the Leidenfrost effect is observed indicates that even the slightest roughness can have a large impact on the Leidenfrost temperature. (Image credit: F. Cavagnon; research credit: L. Maquet et al., pdf)

    Heads-up for Boston-area folks! I’ll be taking part this Saturday evening in the Improbable Research show at the AAAS conference. The show is free and open to the public but fills up quickly, so be sure to come early for a seat.

  • Self-Wrapping Drops

    Self-Wrapping Drops

    A liquid drop can fold itself up in a thin sheet. The animation above shows a drop of water with an ultra-thin (79nm) circular sheet of polystyrene atop it. As a needle removes water from the underside of the droplet, the shrinking droplet causes wrinkles and folds to form in the sheet. What’s going on here is a competition between the energy required to change the droplet’s shape and the energy needed to bend the sheet. Eventually, the droplet’s volume is small enough that the bending of the sheet overrules surface tension in dictating the droplet’s shape. The result is a tiny empanada-shaped droplet completely encapsulated by the sheet. (Image credit: J. Paulsen et al., source; research paper)

  • Microgravity Can Change Vision

    Microgravity Can Change Vision

    In recent years, astronauts have reported their vision changing as a result of long-duration spaceflight. Pre- and post-flight studies of astronauts’ eyes showed flattening along the backside of the eyeball, and scientists hypothesized that the redistribution of body fluids that occurs in microgravity could be reshaping astronauts’ eyes by increasing the intracranial pressure in their skulls.

    A new study tested this hypothesis with the first-ever measurements of intracranial pressure during microgravity flights and during extended microgravity simulation (a.k.a. bedrest with one’s head pointed downward). The authors found that humans here on Earth experience substantial changes in intracranial pressure depending on our posture – while upright we experience much lower intracranial pressure than we do when we’re lying flat. In both microgravity flights and simulation, patients had intracranial pressures that were higher than earthbound upright values but lower than what is experienced when lying flat on Earth.

    Since we humans on Earth spend about 2/3rds of our time upright and 1/3rd prone, our bodies are accustomed to regular variations in intracranial pressure. In space, astronauts don’t receive that regular unloading of intracranial pressure we have when we’re upright. So now researchers suggest that it is the lack of daily variation in intracranial pressure that is the culprit behind astronauts’ vision changes – not the absolute value of the pressure itself. (Image credit: NASA; N. Alperin et al.; research credit: J. Lawley et al.)

  • Swimming with Corkscrews

    Swimming with Corkscrews

    E. coli, like many bacteria, swim using corkscrew-like appendages called flagella. Because the bacteria are extremely tiny – their flagella may be less than ten microns long – their swimming is overwhelmingly dependent on viscosity. (Inertial effects are 100 to 10,000 times smaller than viscous effects for swimming E. coli.) Rotating their helical flagella generates viscous drag along the surface of the corkscrew. Because the flagella is asymmetric when you add all of those drag components together, the net force is thrust that moves the bacterium forward. Watch carefully in the animation above and you’ll see that E. coli have multiple flagella and will swing one out to the side during maneuvers. (Image credit: L. Turner et al., source; reproduced in a review by E. Lauga, pdf)

  • Featured Video Play Icon

    Unboiling an Egg

    Cooking is something we think of as a one-way process. You add heat to food, it changes forms, and there’s undoing that. But that process is less one-directional than we thought, at least in some cases. Take boiling an egg. When you add heat to egg whites, it breaks down bonds between the folded proteins and lets those proteins build more bonds with other sections of proteins, eventually solidifying into a seemingly unbreakable mess. You can’t break those bonds by adding or removing thermal energy, but you can shake the proteins apart and refold them into their original shapes.

    Researchers accomplish this by putting the boiled egg whites in a solution of water and urea and spinning them. When they spin the fluid mixture, the fluid near the wall spins faster than the fluid in the center of the vial, which creates shear stress. That shear stress helps untangle the proteins and reform them into their original shape–thereby unboiling the egg white. Now you definitely don’t want to eat the results – urea is, of course, a component of urine – but it does demonstrate that fluid dynamics can be used to reverse chemical processes we thought were irreversible. And that surprising discovery nabbed the researchers an Ig Nobel Prize in 2015. (Video credit: TedEd/E. Nelson; research credit: T. Yuan et al.)

  • Soft Robots

    Soft Robots

    A research group at MIT has created a new class of fast-acting, soft robots from hydrogels. The robots are activated by pumping water in or out of hollow, interlocking chambers; depending on the configuration, this can curl or stretch parts of the robot. The hydrogel bots can move quickly enough to catch and release a live fish without harming it. (Which is a feat of speed I can’t even manage.) Because hydrogels are polymer gels consisting primarily of water, the robots could be especially helpful in biomedical applications, where their components may be less likely to be rejected by the body. For more, see MIT News or the original paper. (Image credit: H. Yuk/MIT News, source; research credit: H. Yuk et al.)

  • Freezing Impact

    Freezing Impact

    When a water droplet hits a frozen surface, what happens depends significantly on the temperature of the substrate. At relatively high temperatures (-20 degrees C), the droplet freezes without any cracking (upper left). As the surface gets colder, drops begin to crack. At first the cracks are relatively large and unstructured (upper right), but at lower temperatures, they grow in a network of smaller cracks with more distinctive structure (lower left). Cold temperatures can also affect the contact line where water, air, and substrate meet. This can cause droplets to splash even as they’re freezing (lower right). (Image credit: V. Thievenaz et al.; see also E. Ghabache et al.)

  • Accidental Painting

    Accidental Painting

    Some paintings of Mexican artist David Alfaro Siqueiros feature patchy, spotted areas of contrasting color formed by what Siqueiros described as “accidental painting”. Many modern artists use this technique as well. By pouring thin layers of two different colors atop one other, Siqueiros was able to generate seemingly spontaneous patterns like those shown above. In fact, what Siqueiros was using was a density-driven fluid instability! These patterns will only appear when a denser paint is poured atop a lighter one. They’re the result of a Rayleigh-Taylor instability – the same behavior that makes beautiful swirls of cream in coffee and the finger-like protrusions seen in supernovae.

    Although a density difference is necessary to generate accidental painting, other factors like the paint layer’s thickness and viscosity affect the final pattern. For those who are mathematically-inclined, this paper has a linear stability analysis that shows how density difference, viscosity, and other factors affect the cell sizes in the pattern. (Image and research credits: S. Zetina et al.; GIF source)

  • Dissolving

    Dissolving

    It looks like the fiery edge of a star’s corona, but this photo actually shows a dissolving droplet. The droplet, shown as the lower dark region in this shadowgraph image, is a mixture of pentanol and decanol sitting in a bath of water. Pentanol is a type of alcohol that is fully miscible with decanol and is water soluble, so that it will dissolve into the surrounding water over time. Decanol, on the other hand, is immiscible with water, so that part of the droplet won’t mix with the surrounding water.

    The bright swirls along the droplet’s edge show areas with more pentanol. As the alcohol dissolves into the water, it forms a buoyant plume at the top of the droplet that rises due to pentanol’s lower density. That rising plume draws fresh water in from the sides, shown by the upper white arrows. Inside the droplet, flow moves in the opposite direction, from the top toward the outer edges. This is a result of uneven surface tension within the droplet. Scientists are interested in understanding the dynamics of these multiple component drops for applications like printing, where it’s desirable for pigments in an ink drop to be distributed evenly as the drop dries.  (Image credit: E. Dietrich et al.)