FYFD

Category: Research

  • Viscosity and Quantum Mechanics

    Viscosity and Quantum Mechanics

    Viscosity describes a fluid’s resistance to changing its shape. Like surface tension, it’s a fundamental property of a fluid that comes from the interactions between molecules. But viscosity is a slippery beast, and especially so for liquids. There is no generic way to calculate a liquid’s thermodynamic properties from quantum dynamical first principles. But that hasn’t stopped theoretical physicists from making progress on deducing the connections between quantum mechanics and liquids.

    Although viscosity changes with temperature, all liquids have a minimum viscosity, and those minima are all fairly close to the same value as water’s (excluding any superfluids, which are their own brand of quantum weirdness). Why would liquids share a similar minimum viscosity? Because it turns out the minimum viscosity is quantum! Physicists found that the minimum viscosity is set by an equation depending on Planck’s constant and the mass of an electron — both fundamental constants.

    Physicists sometimes like to conjecture about the habitability of the universe if fundamental quantities like Planck’s constant had a different value. This work shows that changing that value would alter water’s viscosity, completely changing the viability of microscopic life! (Image credit: A. Rozetsky; research credit: K. Trachenko and V. Brazhkin; via Physics Today)

  • Laser-Induced Jet Break-Up

    Laser-Induced Jet Break-Up

    A falling stream of water will naturally break up into droplets via the Plateau-Rayleigh instability. Those droplets are random, unless something like vibration of the nozzle sets their size. In this study, though, researchers found that shining a laser beam on the stream can trigger an orderly break-up with droplets that are consistent in size and spacing.

    The optofluidic phenomenon depends on a few different effects. The changing curvature of the liquid stream reflects the laser light, some of which undergoes total internal reflection and travels up the jet as if it were a fiber optic cable. Look closely in the right side of the second image, and you’ll see a periodic flicker of green light at the mouth of the nozzle. Those flashes of green reveal that the liquid jet is guiding the light upstream in bursts, each of which exerts an optical pressure that triggers the Plateau-Rayleigh instability.

    When the laser first turns on, there’s a transition period before the orderly break-up begins, and, likewise, turning the laser off triggers a transition from orderly to random (top image). (Image and research credit: H. Liu et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Changing with the Flow

    Changing with the Flow

    Chemically-reacting flows are some of the toughest problems to unravel. In this new study, researchers found that the very act of flowing through narrow channels can change the speed of chemical reactions. In particular, they found that protein molecules carried through a capillary tube (comparable in size to human capillaries) changed their local shape as a result of the shear forces they experienced. Those changes actually sped up the proteins’ chemical reactions compared to the reaction speed for the chemicals in bulk.

    That finding suggests two important takeaways: 1) chemicals may be absorbed in the human bloodstream differently in capillaries than in other parts of the cardiovascular system, and 2) mimicking these tiny capillaries in microfluidic devices could be useful in speeding up certain biochemical reactions. (Image credit: top – KazuN, visual abstract – T. Hakala et al.; research credit: T. Hakala et al.; via Science; submitted by Kam-Yung Soh)

    Graphical abstract showing that shear forces in small channels can cause local changes to protein structure that affect the rate of chemical reactions.
  • All Wound Up

    All Wound Up

    A thin fiber sitting atop a bubble can spontaneously coil around the bubble thanks to elastocapillarity. (This seemingly bizarre behavior is also why wet strands of hair clump together.) Here’s the situation: The dark circle you see is all bubble; only a portion of the bubble — known as a spherical cap — sticks above the surface of the liquid. When a fiber sits across the top of the bubble, two things can happen: 1) the fiber simply sits there until the bubble bursts, or 2) the fiber starts to bend and wind around the bubble’s cap.

    Bending the fiber takes energy. In this case, that bending energy comes from the system as a whole reducing its free energy. The fiber actually sinks into the bubble film in what the researchers call a “bridged” configuration, where the fiber sits inside the liquid film while also touching the air inside and outside the bubble. In this position, the interfacial energy of the fiber-bubble system is lower, leaving enough excess energy savings for the fiber to coil. (Image and research credit: A. Fortais et al.)

  • Inside a Coronavirus Aerosol

    Inside a Coronavirus Aerosol

    This is a glimpse inside a tiny aerosol droplet with a single SARS-CoV-2 coronavirus inside it. The numerical simulation required a team of 50 scientists, 1.3 billion atoms, and the second most powerful supercomputer in the world. By simulating every atom, the researchers hope to observe what happens to a coronavirus in these micron-sized, long-lasting droplets. Does the virus survive? How do variants fare?

    Their simulation shows that the positive charge of the coronavirus’s spike proteins helps attract mucins that shield the virus and protect it from the droplet interface where evaporation could destroy it. Variants like Delta and Omicron have even more positive charge to their spike proteins, giving themselves a better cloak of mucins and potentially making them all the more infectious. Definitely check out the full New York Times write-up for more spectacular visualizations from the work. (Image and research credit: R. Amaro et al.; via NYTimes; submitted by Kam-Yung Soh)

  • Featured Video Play Icon

    Opera Singer Air Flow

    What does the air flow from a trained opera singer look like? That’s the question behind this study, which combines music and fluid dynamics. Using an infrared camera tracking carbon dioxide (CO2) exhalations from a singer during a performance allowed researchers to identify several important flow features. When breathing, air flows out the singer’s nose in a tight, downward jet with an initial velocity around 1 m/s.

    While singing, air leaves the mouth at a much lower velocity, especially during vowels where the mouth is open. With less momentum behind these exhalations, they can drift upward on the buoyant warmth of the singer’s breath. During consonants — especially plosives like t, k, p, b, d, and g — a rapid burst of air leaves the mouth, traveling at nearly 10 m/s. From the perspective of COVID-19 safety, it’s these plosive jets that are likely to spread contaminated droplets. (Image and video credit: MET Orchestra; research credit: P. Bourrianne et al.; via Improbable Research; submitted by Kam-Yung Soh)

  • Featured Video Play Icon

    Tougher Hydrogels

    Hydrogels are soft, stretchy solids made from polymer chains immersed in water. Engineers hope these materials will be good candidates for medical implants, but to reach that goal, hydrogels need to be durable enough to withstand repeated stretching and contortion without tearing. One team has built a better hydrogel by encouraging entanglement within the gel’s polymer network.

    The polymers inside a hydrogel form their network with two main components: physical entanglements between polymer chains and chemical cross-links. If you imagine the polymers as a tangle of yarn, the cross-links would be spots where pieces of yarn are knotted together and the entanglements are spots where strands wrap and cross without knotting. If you pull on the network, cross-links (knots) will allow very little stretching, whereas the looser entanglements can stretch and deform without tearing. In a hydrogel with lots of entangled polymers but very few cross-links, the material is strong and stretchy without becoming brittle or easily torn. (Video credit: Science; research credit: J. Kim et al.)

  • Where Wind Meets Water

    Where Wind Meets Water

    That the wind causes ocean waves is obvious to anyone who has spent time near the water, but the details of that process remain fuzzy. Many of the explanations — like the Kelvin-Helmholtz instability — only explain part of the process, usually the beginning when the waves are very small. As the waves get larger, they affect the wind in turn, complicating matters.

    As messy as the theory gets, our ability to measure the wind and water in situ is limited, too. Just look at this wild research platform oceanographers designed to study wind and waves. It’s part of a 355-ft vessel that’s towed out to sea horizontally and then flipped so that 300 feet of it remain underwater to stabilize the remainder for measurements. Even with equipment like this, measuring the turbulent air and water near the ocean-sky interface is incredibly difficult.

    This review article gives a nice overview of different historical efforts to explain how wind makes waves and provides a snapshot of the latest research in the area. (Image credit: R. Bilcliff; see also N. Pizzo et al.)

  • Solving the Teapot Effect

    Solving the Teapot Effect

    The teapot effect — that tendency for liquid to dribble down the outside of the spout when pouring — is a frustration to many tea drinkers. Unraveling the fluid dynamics of this phenomenon has taken various researchers decades, but a team now believe they’ve captured the problem fully. Their full mathematical description is quite dense, but it boils down to a subtle interplay of capillary, viscous, and inertial forces.

    Essentially, they found that droplets will always form just under the lip of the spout, thereby keeping that area wetted. The flow rate of the pour (along with the geometry and surface characteristics of the spout) determines how large those droplets can grow. At low flow rates, the droplets can grow large enough to redirect the entire stream around the spout’s edge, creating a hugely frustrating mess. You can see this flow rate effect in the high-speed video below. (Image credit: S. Ferrari; video and research credit: B. Scheichl et al.; via Ars Technica; submitted by Kam-Yung Soh)

  • Wet Masks Block Droplets Better

    Wet Masks Block Droplets Better

    As wearing face masks for long periods has become more typical, you may have wondered whether a soggy mask offers less protection. All masks — cloth, surgical, and N-95s — get moist from their wearer’s breath. A recent study indicates this isn’t a cause for alarm, though.

    Researchers looked at how relatively high-speed droplets (like those from a cough or sneeze) impact dry and wet masks. These high-speed droplets can break into smaller droplets upon impact with a mask layer. The more layers a mask has, the fewer droplets make it through. But even for single-layer masks*, a moistened mask layer lets fewer droplets through. So you don’t have to worry if it’s a little humid in there. Your mask is still working! (Image credit: top – V. Davidova, other – S. Bagchi et al.; research credit: S. Bagchi et al.; via APS Physics)

    * To be clear, you should be wearing masks that are more than a single layer thick. Personally, I’m still only going into indoor public spaces in an N-95 at this point.

    Droplet penetration through a mask. Top row: dry, single layer mask. Middle row: wet, single layer mask. Bottom row: wet, triple layer mask.
    Droplet penetration through a mask. Top row: dry, single layer mask. Middle row: wet, single layer mask. Bottom row: wet, triple layer mask. When wet, masks permit fewer droplets through.