Superhydrophobic surfaces have a complicated microscale structure that changes how water interacts with them, like the hairs on a lotus leaf or the scales of a butterfly’s wing. The photo above shows snapshots at each millisecond as a water drop hits a superhydrophobic surface covered in rows of 18 micron-tall posts. The drop hits with enough speed to drive some water into the space between posts, as shown by the dark area near the center of the splash. As the rest of the droplet spreads, four microjets form along the directions of the micropost array. Those jets remain apparent until the drop reaches its maximum radius and starts to recoil. The rectangular shape of the post array affects how the water pulls away from the surface, or depins, causing the round droplet to instead take on a square-like shape as it pulls back. (Image credit: M. Reyssat et al.)
Search results for: “jet”
Putting Out Fires
Fires in large, open spaces like aircraft hangers can be difficult to fight with conventional methods, so many industrial spaces use foam-based fire suppression systems. These animations show such a system being tested at NASA Armstrong Research Center. When jet fuel ignites, foam and water are pumped in from above, quickly generating a spreading foam that floats on the liquid fuel and separates it from the flames. Since the foam-covered liquid fuel cannot evaporate to generate flammable vapors, this puts out the fire.
The shape of the falling foam is pretty fascinating, too. Notice the increasing waviness along the foam jet as it falls. Like water from your faucet, the foam jet is starting to break up as disturbances in its shape grow larger and larger. For the most part, though, the flow rate is high enough that the jet reaches the floor before it completely breaks up. (Image credit: NASA Armstrong, source)
Cavity Collapse
One of the most iconic images in fluid dynamics is that of a drop impacting a liquid. When a drop hits a pool, it creates a crater, or cavity. That cavity expands and then collapses to form a jet that rebounds above the pool’s surface. If the jet is fast enough, it will eject one or more droplets before it falls back into the pool. Faster droplets, like the one that formed the cavity and jet shown above, actually create slower and fatter jets. In this regime, the complicated interplay of surface tension and gravity effects results in a jet velocity that is independent of impact speed and the liquid’s viscosity. Understanding this jet and splash dynamics is important for many industrial applications, including ink-jet printing. (Image credit: G. Michon et al.)
Titan’s Bubbly Islands
Titan, Saturn’s largest moon, is a fascinating world with remarkable similarities to our own. It is the only other world we know of with stable bodies of liquid at its surface. Unlike Earth, frigid Titan’s lakes and seas are filled with methane and ethane. Radar data from the Cassini mission has shown oddly changing shorelines on Titan, above, with islands that seem to magically appear and disappear over time.
Researchers at NASA’s Jet Propulsion Laboratory now think these islands may, in fact, be bubbles. As Titan’s lakes cool, they’re better able to absorb nitrogen gas, but when temperatures warm up, that gas comes out of solution and floats to the surface, much like the bubbles of carbon dioxide in a soda. If this hypothesis holds up, there are some interesting implications for Titan’s atmosphere. Here on Earth, bubbles popping in the ocean are a major source of aerosol particles. It’s possible migrating rafts of bubbles could behave similarly on Titan. (Photo credit: NASA/JPL-Caltech/ASI/Cornell; submitted by jpshoer)
I’m excited to announce I will be visiting JPL later this month (March 30th), where I have the honor of giving a Women’s History Month talk. If there are any JPLers who are FYFD fans, I hope to see you there. Be sure to RSVP to the ACW luncheon by the March 24th deadline.
Molten Copper
In this video, the Slow Mo Guys prove that pouring molten copper in slow motion is every bit as satisfying as one would imagine. Because they pour the metal from fairly high up, they get a nice break-up from a jet into a series of droplets; that’s due to the Plateau-Rayleigh instability, in which surface tension drives the fluid to break up into drops. Upon impact, the copper splashes and splatters very nicely, forming the crown-like splash many are familiar with from famous photos like Doc Edgerton’s milk drop. The key difference between the molten copper and any other liquid’s splash comes from cooling; watch closely and you’ll see some of the copper solidifying along the edges and surface of the fluid as it cools. In this respect, watching the molten copper is more like watching lava flow than seeing water splash. (Video and image credit: The Slow Mo Guys)
The Archer Fish’s Arrow
Archer fish have a remarkable superpower. When hunting, they target insects above the water and knock them down with a precision strike from a jet of water they spit out. As previous research has shown, the archer fish packs an impressive punch by carefully modulating the water jet so that its tail travels faster and catches up to the front of the jet just as it strikes its target. Even more impressively, the archer fish can make this perfect strike on targets at different distances, which requires the fish to make significant adjustments to each jet. As this video from Deep Look discusses, the archer fish’s impressive hunting hints that it may have greater intelligence than we thought possible, given a comparison of its brain to ours. (Video credit: Deep Look)
Crowns On Impact
Dropping a partially-filled test tube of water against a table makes the meniscus at the air-water interface invert into a jet of liquid. In some cases, the impact is strong enough to generate splashing crowns of water around the base of the jet. These crowns come in two forms – one with many splashes layered upon one another and the other with only a few splashes and a faster jet.
The many-layered splash crowns come from the pressure wave that reflects back and forth from the bottom of the tube to the surface and back. This pressure wave moves at the speed of sound and vibrates the water surface, creating the many splashes. The same reflected pressure wave occurs in the second type of splash crown, but it gets disrupted by cavitation bubbles that form in the water (visible in the lower left image). Instead the splash crowns form from the shock waves generated when the cavitation bubbles collapse. (Image credits: A. Kiyama et al.)
Paint Spilling Physics
There is a remarkable amount of physics contained in art. In this video, scientists from The Splash Lab explore some of the physics involved in pouring paint atop a rectangular post. The spreading paint transforms its shape repeatedly, and, at the corners of the post, it preserves a tiny history of all the colors poured. Paint sliding down the sides shifts from a thin sheet to a thicker jet that deposits color in waves. For tall posts, the distance the paint falls is long enough for instabilities to set in, producing a paint puddle that’s riddled with curves and waves between each color of paint. It’s a lovely reminder of the complexity inherent even within a simple action. (Video credit: R. Hurd et al.)
Spore Squirting
The fungus Pilobolus spreads its spores with a squirt cannon. Each spore sits on the end of a round fluid-filled pod. Like many plants, the fungus uses a process called osmosis to pump water into the pod. Through osmosis, the fungus increases the concentration of certain molecules inside the pod, which draws water into the pod and increases its pressure. Eventually, the pod ruptures, sending the spore aloft on a jet of fluid that accelerates it at 20,000+g! (Image credit: BBC Earth Unplugged, source; research credit: L. Yafetto et al.)
Fish, Feathers, and Phlegm
Inside Science has a new documentary all about fluid dynamics! It features interviews with five researchers about current work ranging from the physics of surfing to the spreading of diseases. Penguins, sharks, archer fish, 3D printing, and influenza all make an appearance (seriously, fluid dynamics has everything, guys). If you’d like to learn more about some of these topics, I’ve touched on several of them before, including icing, penguin physics, shark skin, archer fish, and disease transmission via droplets. (Video credit: Inside Science/AIP)
