FYFD

Tag: droplets

  • Listening to the Sizzle

    Listening to the Sizzle

    The sizzle of frying food is familiar to many a cook, and that sound actually conveys a surprising amount of information. In this study, researchers suspended water droplets in hot oil and observed their behavior, both with high-speed video and with microphones. They found that these vaporizing drops created three types of cavities in the oil: an exploding cavity that breaks the surface, an elongated cavity that remains submerged, and an oscillating cavity that breaks up well below the surface. All three cavities flung oil droplets upward, and all three were acoustically distinct from one another. That means, as the authors suggest, that it might be possible to measure the aerosol droplets generated during frying simply by listening! (Image credit: fries – W. Dharma, others – A. Kiyama et al.; research credit: A. Kiyama et al.; via Cosmos; submitted by Kam-Yung Soh)

  • Asperitas Formation

    Asperitas Formation

    In 2017, the World Meteorological Organization named a new cloud type: the wave-like asperitas cloud. How these rare and distinctive clouds form is still a matter of debate, but this new study suggests that they need conditions similar to those that produce mammatus clouds, plus some added shear.

    Using direct numerical simulations, the authors studied a moisture-filled cloud layer sitting above drier ambient air. Without shear, large droplets in this cloud layer slowly settle downward. As the droplets evaporate, they cool the area just below the cloud, changing the density and creating a Rayleigh-Taylor-like instability. This is one proposed mechanism for mammatus clouds, which have bulbous shapes that sink down from the cloud.

    When they added shear to the simulation, the authors found that instead of mammatus clouds, they observed asperitas ones. But the amount of shear had to be just right. Too little shear produced mammatus clouds; too much and the shear smeared out the sinking lobes before they could form asperitas waves. (Image credit: A. Beatson; research credit: S. Ravichandran and R. Govindarajan)

  • When Seeing a Flow Changes It

    When Seeing a Flow Changes It

    Adding dye to a flow is a common technique for visualization. After all, many flows in fluids like air and water are invisible to our bare eyes. But for some classes of flows — especially those driven by variations in surface tension — adding dye can have unforeseen effects. A recent study shows how true this is for bursting Marangoni droplets, where evaporation and alcohol concentration can pull a water-alcohol droplet apart.

    Composite series of photos showing the effect of increased dye concentration on Marangoni bursting.
    As more dye is added to the experiment, the daughter droplets grow larger and more ligaments form. In the first three images, a dashed black line has been added to show the location of the droplet rim.

    Without dye, it’s nearly impossible to see the phenomenon since the refractive indices of the two component liquids are so close. But the researchers found that, as they added more methyl blue dye, it did more than increase the contrast in the flow. It changed the flow, making the droplets larger and creating ligaments between them. They believe that the dye’s own surface tension creates local gradients that alter the flow. It’s a reminder that experimentalists have to be careful to consider how our efforts to measure and observe a flow can change it. (Image credit: top – The Lutetium Project, bottom – C. Seyfert and A. Marin with modification; research credit: C. Seyfert and A. Marin)

  • Merging Along Wires

    Merging Along Wires

    As oil slides down two slowly converging wires, the droplets will merge into a sheet that stretches between both wires. When this happens can vary somewhat but occurs somewhere around the liquid’s capillary length.

    In the poster above, the leftmost image (not the illustration) shows three possible merger points. To the right of the image, is a teal curve; this is a probability density function. Essentially, this curve shows where the merger is most likely to occur. The peak of the curve corresponds to the most probable point of merger.

    The following two composite images show the same system — same oil flow rate, same wire spacing — with gas blowing upward along the wires. As the gas’s flow rate increase, the oil drops get larger, making the oil films thinner. The result? The wires have to get closer to one another before the oil merges. That’s reflected in the yellow and orange probability density functions, which have peaks further along the wires than the no-gas-flow case. (Image credit: C. Wagstaff et al.)

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    Self-Stopping Leaks

    A leak can actually stop itself, as shown in this video. To demonstrate, the team used a tube pierced with a small hole. When filled, water initially shoots out the hole in a jet. The pressure driving the jet comes from the weight of the fluid sitting above the hole. As the water level drops, the pressure drops, causing the jet to sag and eventually form a rivulet that wets the side of the tube. As the water level and driving pressure continue to fall, the rivulet breaks up into discrete droplets, whose exact behavior depends on how hydrophobic the tube is. Eventually, a final droplet forms a cap over the hole and the leak stops. At this point, the flow’s driving pressure is smaller than the pressure formed by the curvature of the capping droplet. (Image and video credit: C. Tally et al.)

  • Dripping Impact

    Dripping Impact

    How does water drip, drip, dripping onto stones erode a crater? Water is so much more deformable that it seems impossible for it to wear harder materials away, even over thousands of impacts. To investigate this, a team of researchers developed a new measurement technique: high-speed stress microscopy. In the process, they found that water owes its incredible erosive power to three factors: 1) The drop’s impact creates surface shock waves along the material, which helps increase erosive power; 2) After the shock wave passes, a decompression wave in the material helps loosen surface matter; and 3) The spreading drop sends a non-uniform wave of stress across the material that simultaneously presses and scrubs at the surface. Together, these factors enable simple, repetitive droplet impacts to wear away at hard surfaces. (Image credit: cottonbro; research credit: T. Sun et al.; via Cosmos; submitted by Kam-Yung Soh)

  • Coalescence Symmetry

    Coalescence Symmetry

    When droplets coalesce, they perform a wiggly dance, gyrating as the capillary waves on their surface interfere. When the droplets have matching surface tensions, like the two water droplets in the animation on the lower left, the coalescence dance is symmetric. But for differing droplets, like the water and ethanol droplets merging on the lower right, coalescence is decidedly asymmetric.

    Two water droplets merge symmetrically.
    A water droplet and an ethanol droplet merge asymmetrically.

    The asymmetry arises from the droplets’ different surface tensions. The size and speed of the capillary waves that form on a droplet depend on surface tension, so droplets of different liquids have inherently different capillary waves. During merger, the interference of these capillary waves causes the asymmetry we see. (Image credit: top – enfantnocta, coalescence – M. Hack et al.; research credit: M. Hack et al.)

  • Raindrops on the Windshield

    Raindrops on the Windshield

    When I was a child, I was fascinated by the raindrops that shimmied along the windshield of our car. Some would slide up the glass. Some would run down. And some just seemed to wiggle in place, until the car’s speed changed. As common as this sight is, the physics of these droplets is quite complicated and not completely understood.

    Each droplet has a host of forces on it: gravity flattening it or pulling it down an incline; a drag force from the wind flowing over it; and friction between the drop and the surface trying to pin it in place. Recently, scientists have developed a new mathematical model that captures some of the behaviors behind these drops. The work describes the wind speed necessary to move a drop of a given size sitting on a flat surface. The authors also explored how that critical wind speed changes when a drop sits on a tilted surface aligned or against the wind. (Image credit: P. Gupta; research credit: A. Hooshanginejad and S. Lee; via Science News; submitted by Kam-Yung Soh)

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    “ColorLover”

    “ColorLover,” a short film by artist Rus Khasanov, is a delightful liquid rainbow. The video’s ingredients seem to be ink, paint, oil, and a bit of superhydrophobic coating primed to reveal a heart. I love that latter touch; it’s a cool way to use regular materials in a way that some might assume involved digital effects! (Video credit: R. Khasanov)

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    Within the Bubble’s Pop

    To our eyes, a soap bubble appears to pop instantly, but when observed in high-speed video, the process is far more complex. In this video, the Slow Mo Guys pop human-sized bubbles, giving us an opportunity to appreciate the rupture process at speeds up to 50,000 frames per second.

    Once the rupture starts, the hole spreads very symmetrically. But as the hole grows, the remaining soap film starts distorting. As Gav and Dan observe, the far side of the bubble actually wrinkles up before the rupture front arrives and tears the remaining fluid into droplets! (Image and video credit: The Slow Mo Guys)