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Search results for: “droplet”

  • Leidenfrost On Ice

    Leidenfrost On Ice

    We’ve seen many forms of Leidenfrost effect — that wild, near-frictionless glide that liquid droplets make on a very hot surface — over the years, but here’s a new one: the three-phase Leidenfrost effect. Researchers found that they could generate a Leidenfrost effect using an ice disk placed on an extremely hot surface. During the effect, the ice and its melting layer of water glide on vapor, hence the name.

    The team found that getting a three-phase Leidenfrost effect requires a much, much higher temperature than the regular Leidenfrost effect. Water will get its glide on at 150 degrees Celsius. Getting ice to glide on the same surface required a stunning 550 degrees Celsius! Why the big difference? The challenge is that water layer, which, by definition, has a 100-degree difference between its boiling side and its frozen boundary. It takes so much heat to maintain that layer that there’s little energy left over for evaporation; that’s why it takes so much more energy to get the three-phase Leidenfrost effect. (Image and research credit: M. Edalatpour et al.; via Ars Technica; submitted by Kam-Yung Soh)

  • Superhydrophobic Drag

    Superhydrophobic Drag

    Using air or bubbles to reduce drag on boats is a popular idea, whether using supercavitation, the Leidenfrost effect, or superhydrophobic coatings. But most of the experiments done thus far use spheres rather than realisitic boat shapes. In this study, the researchers used two model boats — one with a hydrofoil and the other in a conventional motorboat shape — and applied superhydrophobic coatings to different parts of the model to see how superhydrophobicity affected the overall drag.

    Perhaps surprisingly, they found that superhydrophobic coatings can actually increase the drag! The effect was particularly stark for the hydrofoil boat (Image 2), where the surface jets (lower half) caused by the superhydrophobic coating slowed the boat by 30% compared to its unmodified speed (upper half).

    For the speedboat, a superhydrophobic hull made no overall difference in its drag, though it changed how water splashed in its wake. And coating the boat’s propeller was particularly detrimental, resulting in a speed up to three times slower. Overall, the study suggests that superhydrophobic coatings may be useful in some circumstances, but they have to be applied carefully, as they can have negative impacts, too. (Image credits: top – S. Anghan, others and research credit: I. Vakarelski et al.)

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    When Bubbles Don’t Die

    In a pure liquid, most bubbles pop almost immediately. But with a simple ingredient — a little heat — bubbles can live almost indefinitely. The mechanism is revealed in this video when the researchers use an infrared camera to watch a bubble on a heated pool. The top of the bubble is cooler than the rest of the liquid, forming colder, denser droplets that slide down. But the cooler liquid also has a higher surface tension, which draws warm liquid up the bubble, replenishing it. The result is a stable bubble that simply carries on. (Image and video credit: S. Nath et al.)

  • Splashing on Spheres

    Splashing on Spheres

    The splash of a droplet is a surprisingly complex phenomenon, depending not only on the droplet’s characteristics but also the surrounding air pressure, the roughness and temperature of the impact surface, and the surface’s curvature. In this study, researchers investigated the effects of surface curvature on splashing, finding that it’s harder for a drop to splash on spheres of smaller radius than ones with a larger radius of curvature.

    In Image 1, the falling droplet coats the 2-mm sphere with no sign of splashing. But as the radius gets larger (Images 2 and 3), splashing becomes more and more pronounced. They found that the splash suppression is due to a modification of the lift force on the leading edge of the lamella, the thin liquid layer created as the drop impacts and spread. (Image, research, and submission credit: T. Sykes et al.; also available here)

  • Stopping The Drop

    Stopping The Drop

    When a droplet falls on a mesh surface, some of the liquid can burst through the holes (top row). But subsequent drops have a harder time penetrating the prewetted mesh. After a few drops have impacted (rows 2-3), the wetted mesh can completely suppress penetration (rows 4-5). The authors found that the taller drops sitting atop the mesh were better at stopping penetration from an incoming drop. (Image and research credit: L. Xu et al.)

  • Swept Along

    Swept Along

    When a car drives over a leaf-strewn autumn road, it pulls leaves up with its passage. This tendency to drag fluid along when an object passes is called entrainment, and it may be a key to transporting loads like medicine in microfluidic applications.

    As shown above, a self-propelled microswimmer — in this case, an oil droplet — pulls the surrounding fluid and tracer particles with it (Image 1). Researchers modeled this single-swimmer entrainment (Image 2) to quantify just how much fluid the droplet pulls with it. Then they studied what happens when many swimmers pass through an area (Image 3). They found that the droplet swarm entrained ten times the volume of fluid compared to the fluid entrained by the same number of isolated droplets. The fluid volume pulled along was also far larger than any payload the droplets themselves could carry. So future microswimmer swarms may simply sweep their cargo along in their wake. (Image and research credit: C. Jin et al.; via APS Physics)

  • 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)

  • 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)

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    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)

  • 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)

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