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  • Charged Drops Don’t Splash

    Charged Drops Don’t Splash

    When a droplet falls on a surface, it spreads itself horizontally into a thin lamella. Sometimes — depending on factors like viscosity, impact speed, and air pressure — that drop splashes, breaking up along its edge into myriad smaller droplets. But a new study finds that a small electrical charge is enough to suppress a drop’s splash, as seen below.

    Video showing three different droplets, each with a different electrical charge, impacting an insulated surface. From left to right, the charges are: 0.0 nC, 0.08 nC, and 0.1 nC. The uncharged drop splashes, the low charge drop splashes less, and the final charged droplet spreads without splashing.

    The drop’s electrical charge builds up along the drop’s surface, providing an attraction that acts somewhat like surface tension. As a result, charged drops don’t lift off the surface as much and they spread less overall; both factors inhibit splashing.* The effect could increase our control of droplets in ink jet printing, allowing for higher resolution printing. (Image and research credit: F. Yu et al.; via APS News)

    *Note that this only works for non-conductive surfaces. If the surface is electrically conductive, the charge simply dissipates, allowing the splash to occur as normal.

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  • Inside an Alien Atmosphere

    Inside an Alien Atmosphere

    Studying the physics of planetary atmospheres is challenging, not least because we only have a handful of examples to work from in our own solar system. So it’s exciting that researchers have unveiled our first look at the 3D structure of an exoplanet‘s atmosphere.

    Using ground-based observations, researchers studied WASP-121b, also known as Tylos, an ultra-hot Jupiter that circles its star in only 30 Earth hours. One face of the planet always faces its star while the other faces into space. The team found that the exoplanet has a flow deep in the atmosphere that carries iron from the hot daytime side to the colder night side. Higher up, the atmosphere boasts a super-fast jet-stream that doubles in speed (from an estimated 13 kilometers per second to 26 kilometers per second) as it crosses from the morning terminator to the evening. As one researcher observed, the planet’s everyday winds make Earth’s worst hurricanes look tame. (Image credit: ESO/M. Kornmesser; research credit: J. Seidel et al.; via Gizmodo)

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  • A Stellar Look at NGC 602

    A Stellar Look at NGC 602

    The young star cluster NGC 602 sits some 200,000 light years away in the Small Magellanic Cloud. Seen here in near- and mid-infrared, the cluster is a glowing cradle of star forming conditions similar to the early universe. A large nebula, made up of multicolored dust and gas, surrounds the star cluster. Its dusty finger-like pillars could be an example of Rayleigh-Taylor instabilities or plumes shaped by energetic stellar jets. (Image credit: NASA/ESA/CSA/JWST; via Colossal)

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  • Imaging a New Era of Supersonic Travel

    Imaging a New Era of Supersonic Travel

    Supersonic commercial travel was briefly possible in the twentieth century when the Concorde flew. But the window-rattling sonic boom of that aircraft made governments restrict supersonic travel over land. Now a new generation of aviation companies are revisiting the concept of supersonic commercial travel with technologies that help dampen the irritating effects of a plane’s shock waves.

    One such company, Boom Supersonic, partnered with NASA to capture the above schlieren image of their experimental XB-1 aircraft in flight. The diagonal lines spreading from the nose, wings, and tail of the aircraft mark shock waves. It’s those shock waves’ interactions with people and buildings on the ground that causes problems. But the XB-1 is testing out scalable methods for producing weaker shock waves that dissipate before reaching people down below, thus reducing the biggest source of complaints about supersonic flight over land. (Image credit: Boom Supersonic/NASA; via Quartz)

    The XB-1 test aircraft in flight.
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  • “Kirigami Sun”

    “Kirigami Sun”

    Kirigami is a variation of origami in which paper can be cut as well as folded. Here, researchers look at flow through a cut kirigami sheet and how that flow changes with the cuts’ length. In the top central image, white lines mark the paper boundaries. As the cut gaps get larger, flow through them transitions from a continuous jet to swirling vortex shedding. Along the bottom, we see similar patterns emerge in the wake of uniformly-cut sheets, too. On the right, the flow comes through in jets; moving leftward, it transitions to an unsteady vortex shedding flow. (Image credit: D. Caraeni and Y. Modarres-Sadeghi)

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  • Instabilities in Competition

    Instabilities in Competition

    When two liquid jets collide, they form a thin liquid sheet with a thicker rim. That rim breaks into threads and then droplets, forming a well-known fishbone pattern as the Plateau-Rayleigh instability breaks up the flow. This poster shows a twist on that set-up: here, the two colliding jets vary slightly in their velocities. That variability adds a second instability to the system, visible as the wavy pattern on the central liquid sheet. The sheet’s rim still breaks apart in the usual fishbone pattern, but the growing waves in the center of the sheet eventually that structure apart as well. (Image credit: S. Dighe et al.)

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    Cavitation Near Soft Surfaces

    Collapsing cavitation bubbles are sometimes used to break up kidney stones, and they may find other uses in medicine as well. Here, researchers investigate the collapse of laser-triggered cavitation bubbles near tissue-mimicking hydrogel. The bubbles take on a very different form than they do near solid surfaces. Near hydrogel, the bubbles become mushroom-shaped. During their collapse, they release a rainy microjet that moves at nearly 2,000 meters per second! Even at 5 million frames per second, the jet is practically a blink-and-you-miss-it phenomenon. (Image and video credit: D. Preso et al.)

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  • Inside the Squirting Cucumber

    Inside the Squirting Cucumber

    Though only 5 cm long, the squirting cucumber can spray its seeds up to 10 meters away. The little fruit does so through a clever combination of preparation and ballistic maneuvers. Ahead of launch, the plant actually moves water from the fruit into the stem; this reorients the cucumber so that its long axis sits close to 45 degrees. It also makes the stem thicker and stiffer.

    This high-speed video shows the explosive release of the squirting cucumber's seeds.
    This high-speed video shows the explosive release of the squirting cucumber’s seeds.

    When the burst happens, fruit spews out a jet of mucus that propels the seeds at up to 20 m/s. The initial seeds move the fastest — thanks to the fruit’s high-pressure reservoir — and fly the furthest. As the pressure drops, the jet slows and the fruit’s rotation sends the seeds higher, causing them to land closer to the original plant. With multiple fruits in different orientations, a single plant can spread its seeds in a fairly even ring around itself. (Research and image credit: F. Box et al.; via Gizmodo)

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  • Predicting Droplet Sizes

    Predicting Droplet Sizes

    Squeeze a bottle of cleaning spray, and the nozzle transforms a liquid jet into a spray of droplets. These droplets come in many sizes, and predicting them is difficult because the droplets’ size distribution depends on the details of how their parent liquid broke up. Shown above is a simplified experimental version of this, beginning with a jet of air striking a spherical water droplet on the far left. In less than 3 milliseconds, the droplet has flattened into a pancake shape. In another 4 milliseconds, the pancake has ballooned into a shape called a bag, made up of a thin, curved water sheet surrounded by a thicker rim. A mere 10 milliseconds after the jet and drop first meet, the liquid is now a spray of smaller droplets.

    Researchers have found that the sizes of these final droplets depend on the balance between the airflow and the drop’s surface tension; these two factors determine how the drop breaks up, whether that’s rim first, bag first, or due to a collision between the bag and rim. (Image credit: I. Jackiw et al.; via APS Physics)

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    Tweaking Coalescence

    When a drop settles gently against a pool of the same liquid, it will coalesce. The process is not always a complete one, though; sometimes a smaller droplet breaks away and remains behind (to eventually do its own settling and coalescence). When this happens, it’s known as partial coalescence.

    Here, researchers investigate ways to tune partial coalescence, specifically to produce more than a single droplet. To do so, they add surfactants to the oil layer surrounding their water droplet. The surfactants make the rebounding column of water skinnier, which triggers the Rayleigh-Plateau instability that’s necessary to break the column into more than one droplet. (Image and video credit: T. Dong and P. Angeli)

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