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  • Rainfall Beyond Earth

    Rainfall Beyond Earth

    Rain is not unique to our planet: Titan has methane rain and exoplanet WASP 78b is home to iron rain (ouch). A new study examines rainfall across planets from the perspective of individual rain drops. The authors examine raindrop shape, terminal velocity, and evaporation rate as a function of droplet size for a wide range of known and speculated atmospheres.

    They found that raindrops are surprisingly universal. Although planets with higher gravity tend to produce smaller raindrops, they found a remarkably narrow range for maximum drop size. That’s a pretty wild result, all things considered! The idea that iron, ammonia, methane, and countless other fluids falling through vastly different atmospheres all share very common characteristics is fascinating. (Image credit: NASA/JPL-Caltech/SwRI/MSSS/Brian Swift; research credit: K. Loftus and R. Wordsworth; via Science News; submitted by Kam-Yung Soh)

  • Airborne Aerosol Transmission of COVID-19

    Airborne Aerosol Transmission of COVID-19

    Early in the COVID-19 pandemic health officials resisted the idea that the novel coronavirus was transmissible through tiny aerosol droplets rather than larger, non-buoyant droplets. One case that made headlines and helped shift opinion was that of an outbreak among patrons of a Guangzhou restaurant traced to a single, pre-symptomatic patient zero. The pattern of who became sick at the carrier’s table and those nearby made little sense unless the restaurant’s air flow played a role in spreading the virus.

    https://www.youtube.com/watch?v=WaZiCqQmO4g

    This paper studies the incident in detail, using an in-house computational fluid dynamics (CFD) code to simulate both airflow in the restaurant and the paths aerosol droplets would follow in that environment. It takes into account flow from the air conditioner and the warm air rising from customers. The study’s predictions of which areas would have the highest concentrations of virus-laden aerosols matches well with the actual pattern of the outbreak. The authors hope that tools like theirs can help prevent future outbreaks by indicating the most dangerous paths for transmission and measures that can block those. (Image credit: Center for Disease Control; video, research, and submission credit: H. Liu et al.)

  • Oil-Coated Bubbles

    Oil-Coated Bubbles

    Bubbles in industrial applications are often more complicated than a simple pocket of air surrounded by water. Here researchers investigate the formation of an air bubble coated in oil before it rises through water. The photo above shows a series of snapshots as the bubble forms. Initially, a droplet of oil sits pinned on the surface. As air gets injected, the oil stretches around the growing bubble. Eventually, buoyancy pulls the bubble off the injector, creating a rising air bubble coated in oil. The team found that oil-coated bubbles could grow much larger than those in water alone. (Image and research credit: B. Ji et al.)

  • Swapping Emulsions

    Swapping Emulsions

    Chemically speaking, oil and water don’t mix. But with a little fluid mechanical effort, it’s possible to make them an emulsion — a mixture of oil droplets in water or water droplets in oil. Researchers in the Netherlands discovered that the viscosity of these emulsions depends critically on which of those mixtures you have.

    To create their emulsions, the team used a tank consisting of two concentric cylinders. When the inner cylinder spins, it creates a well-understood flow field between the inner and outer cylinder. By varying the ratio of oil to water in the tank, they could explore a wide range of emulsions. They found that the emulsion’s viscosity changed dramatically when the emulsion shifted from oil droplets in water to water droplets in oil, something known as a catastrophic phase inversion. During this switch the viscosity dropped from 3 times higher than pure water to 2 times lower! (Image credit: A_Different_Perspective; research credit: D. Bakhuis et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Whiskey Webs

    Whiskey Webs

    Unlike scotch whisky, when American bourbon whiskeys are diluted, they form unique web-like evaporation patterns. These differences arise in part from the way the liquors are aged: scotch is aged in re-used barrels, whereas bourbons require aging in a new, charred American white oak barrel*.

    During aging, the whiskey picks up water-insoluble chemicals from the barrel. When water is added to the bourbon, it helps transport those insoluble components to the surface of a droplet, where they form a monolayer of fatty acid chains (Image 2; in green). As evaporation continues and the droplet gets smaller, the molecules at the shrinking surface collapse inward, forming the rigid web structure we see left behind. The patterns that form act as a kind of fingerprint for the bourbon. Check out some of the brand-to-brand variations over at the researchers’ Whiskey Webs site. (Image and research credit: S. Williams et al.; via Physics Today)

    * In case you were wondering, this is actually a legal requirement in order to be considered bourbon. Bourbons must also be made from a grain mixture that is >50% corn.

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    Visualizing Radiation

    Radiation is invisible, but it’s not too difficult to build an apparatus that lets you see it. This video shows the ghostly aftermath of passing radiation in a cloud chamber, one of the first set-ups used to study radiation. The chamber contains a radioactive source and chilled isopropyl alcohol. The alcohol forms a supersaturated vapor — essentially a cloud in waiting — inside the chamber.

    When a radioactive particle gets emitted from the source, it streaks through the chamber, colliding with atoms and ionizing them. Those ions then serve as nucleation sites where alcohol condenses into droplets. It’s these condensation trails that we see bloom and decay in the particle’s wake. (Image and video credit: L. Gledhill)

  • Freezing Splats

    Freezing Splats

    In fluid physics, there’s often a tug of war between different effects. For droplets falling onto a surface colder than their freezing point, the hydrodynamics of impact, sudden heat transfer, and solidification processes all compete to determine how quickly and in what form droplets freeze.

    The images above form a series based on changing the height from which the droplet falls. Each image is divided into two synchronized parts. On the left, we see a visible light, top-down view of the freezing droplet; on the right, we see an infrared view of freezing. As the height of impact increases, the shape of the frozen drop becomes more elaborate, moving from a flat splat with a small conical tip all the way to one with a concentric double-ring in its center. (Image and research credit: M. Hu et al.)

  • Fallstreak Holes

    Fallstreak Holes

    Occasionally clouds appear to have a hole in them; these are known as fallstreak holes or hole-punch clouds. To form, the water droplets in the cloud must be supercooled; in other words, they must be colder than their freezing point but still in liquid form. When disturbed — say, by the temperature drop caused by flowing over an airplane wing — the supercooled water droplets will suddenly freeze. This typically kicks off a chain reaction in which many droplets freeze and the heavy ice crystals fall out of the sky, leaving behind a void in the cloud. Because airplanes are particularly good at creating these fallstreak holes, they’re often seen near busy airports. (Image credit: J. Stevens/NASA; via NASA Earth Observatory)

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    “Radiolarians”

    “Radiolarians” is a short film by artist Roman De Giuli using ink, alcohol, and oil. Much of the fluid motion involves break-up into droplets. The effects appear to rely heavily on Marangoni bursting, the physics of which you can learn about in this previous post. (Image and video credit: R. De Giuli)

  • The Sounds of Leidenfrost Stars

    The Sounds of Leidenfrost Stars

    On a hot surface, droplets can float on a layer of their own vapor and vibrate in star-like shapes. These so-called Leidenfrost stars also make noise, with distinct beats that match the oscillations of the vapor layer beneath them. Researchers found that the frequency of the sound shifts with droplet size, increasing as the drop size decreases. Physically, the droplets act much like a wind instrument! (Image and research credit: T. Singla and M. Rivera; via APS Physics)

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