FYFD

Tag: jets

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    Cavitation-Induced Microjets

    In cavitation, tiny bubbles of vapor form and collapse in a liquid, often sending shock waves ricocheting. In most occurrences beyond the lab, cavitation bubbles aren’t a solo act; many bubbles can form and interact. This video takes a look at some of the effects of those interactions. When close together, two cavitation bubbles can act to focus the flow during collapse, generating a microjet strong enough to penetrate into nearby surfaces. Researchers hope this technique may one day be used for needle-free injections. (Image, video, and submission credit: A. Mishra et al.)

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    Pressure At The Dam

    Hydrostatic pressure in a fluid is based on the fluid’s depth. You’ll rarely see a more dramatic example of that power than with a water release from a dam. Here we see the outlet of the Verbund Hydro Power dam in Austria. With 190 meters of water behind the dam, the outlet jet is massive. It moves 20,000 liters of water per second at a speed of 50 meters per second. Imagine what it would be like to stand next to that! (Image and video credit: Discovery UK; submitted by Olwyn B.)

  • Breaking Up Is(n’t) Hard to Do

    Breaking Up Is(n’t) Hard to Do

    Engineers often need to break a liquid jet up into droplets. To do so quickly, they surround the jet with a ring of fast-moving air in a set-up known as a coaxial jet. Shear between the gas and liquid creates instabilities that quickly distort the jet’s initial cylinder into sheets and ligaments. Those formations then undergo their own instabilities to break up into drops. The method is, as you can see in the high-speed images above, quite effective, though the breakup mechanism itself is tough to quantify. (Image credit: G. Ricard et al.)

  • Microjets and Needle-Free Injection

    Microjets and Needle-Free Injection

    Some people don’t mind needles, and others absolutely detest them. But to replace needles with needle-free injections, we have to understand how high-speed microjets pass through skin. Given skin’s opacity, that’s tough, so researchers are instead using droplets as a model. If we can understand the dynamics of a microjet passing through different kinds of droplets, getting jets of medicine into arms becomes easier.

    Researchers found that jets passed completely through a droplet if they impacted above a critical velocity. For Newtonian droplets, the jet creates a cavity and shoots straight through because the inertia of the impact outweighs the countering force of surface tension. But with viscoelastic drops, the jet goes through, slows down, and gets sucked back into the droplet. In this case, the combination of surface tension and viscoelasticity can, eventually, overpower the jet’s inertia. (Image, research, and submission credit: M. Quetzeri-Santiago et al.)

  • Bubbles Rising

    Bubbles Rising

    Here we see high-speed video of air bubbles rising through sesame oil. The flow rate of air is just right for one bubble to catch up to and merge with the previous bubble. As it the trailing bubble pinches off from the valve, it shoots a small jet through itself and into the prior bubble. For information on how to recreate this and related experiments, check out this article. (Image credit: C. Kalelkar and S. Paul, source; see also C. Kalelkar)

  • Decelerating Jets

    Decelerating Jets

    For more than a century, scientists have been fascinated by the jet that forms after a drop impacts a liquid. In this study, researchers tracked fluorescent particles in the fluid to understand the velocity and acceleration of flow inside the jet. They found that, within the first 10ms after the jet appears, it decelerates at up to 20 times the gravitational acceleration. That’s much too fast for gravity to cause, pointing instead to the critical importance of surface tension in dictating the behavior of these fast-moving jets. (Image and research credit: C. van Rijn et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Jets Beneath Leidenfrost Drops

    Jets Beneath Leidenfrost Drops

    When a droplet impacts, it’s not unusual for converging ripples to form an upward jet, like the one seen here. But under the right circumstances, jets can form downward, too. This study looks at the ultrafast jets that can form beneath an impacting Leidenfrost drop.

    These Leidenfrost drops are striking a surface much hotter than their boiling point, so a large vapor cavity forms quickly beneath them. Using x-ray imaging, the researchers were able to capture the dynamics of this cavity’s formation and collapse (Image 2). The field of view in the animation shows only a portion of the drop’s cavity, so Image 3 may help you orient relative to the drop at large.

    Initially, we see the center of the droplet hitting the surface, followed by the fast growth of a vapor cavity. Rippling capillary waves converge on top of the cavity, creating a pinch-off. From there, a bubble rises up while a fast jet shoots downward. (Image credit: water jet – A. Min, others – S. Lee et al.; research credit: S. Lee et al.)

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    Coalescing Drops

    This year’s Nikon Small World in Motion competition was won by fluid dynamics! The first place video shows droplets on a superhydrophobic surface coalescing. The droplets are a mixture of water and ethanol. Their initial merger creates a ripple of waves that’s followed by a ghostly vortex ring that jets into the interior. Previous research on coalescence during impact shows jets driven by surface tension but the jet here doesn’t appear to be confined to the surface. (Image and video credit: K. Rabbi and X. Yan; via Nature; submitted by Kam-Yung Soh)

    Droplets on a superhydrophobic surface coalescing.

  • Surface Jets in Coalescing Droplets

    Surface Jets in Coalescing Droplets

    What goes on when droplets merge is tough to observe, even with a high-speed camera. There are many factors at play: any momentum in the droplets, surface tension, gravity, and Marangoni forces, to name a few. A new study that simultaneously records multiple views of coalescence is shedding some light on these dynamics.

    The results are particularly interesting for droplets that are somewhat physically separated so that they only coalesce after one drop impacts near the other. In this situation, with droplets of equal surface tension, researchers observed a jet that forms after impact (Image 1) and runs along the top surface of the coalescing drops (Image 2). That location is a strong indication that the jet is created by surface tension and not other forces.

    To test that further, the researchers repeated the experiment but with droplets of unequal surface tension. They found that when the undyed droplet’s surface tension was higher (Image 3), Marangoni forces enhanced the surface jet, as one would expect for a surface-tension-driven phenomenon. But if the dyed droplet had the higher surface tension (Image 4), it was possible to completely suppress the jet’s formation. (Image, research, and submission credit: T. Sykes et al., arXiv)

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    Swinging Jets

    In the tiny realm of microfluidics, flows are, in general, completely laminar. That makes mixing a challenge. But it turns out that pumping water steadily into multiple inlets can spontaneously generate oscillations between the jets, allowing dramatic mixing even at low Reynolds numbers. Two inlets in a parallel channel (first image) oscillate steadily over a small range of conditions, but widening the channels (second image) allows the jets to switch back and forth over a larger range. And adding additional inlets (third image) can create even more complex fluid oscillators! (Image, video, and research credit: A. Bertsch et al.)