FYFD

Tag: vaporization

  • Rolling Along

    Rolling Along

    Leidenfrost drops – droplets deposited onto a surface much hotter than their boiling point – are known for their mobility. With the right surface, they can be propelled, trapped, and even guided through a maze, typically by directing the vapor layer that cushions them. But new work shows that these drops have internal dynamics that also contribute to their propulsion.

    By adding tracer particles to each droplet, researchers can visualize flows inside the droplet. Large drops tend to have a flatter shape and contain two or more rotating vortices. Such drops won’t propel themselves without another force in play. But smaller droplets are more spherical and contain only a single rotating flow. Once these drops detach, they roll away! Despite the similarity to wheels, these liquid drops aren’t moving the same way. Remember that the drop is not actually in contact with the surface. To see what sets the drop’s direction, researchers examined the shape of the bottom of the drop. They found that it sits at a slant on its vapor cushion. That pushes evaporating gases out one side, propelling the drop the other way. (Image and video credit: A. Bouillant et al., source)

  • Oil Splatters

    Oil Splatters

    Most cooks have experienced the unpleasantness of getting splattered with hot oil while cooking. Here’s a closer look at what’s actually going on. The pan is covered by a thin layer of hot olive oil. Whenever a water drop gets added – from, say, those freshly washed greens you’re trying to saute – it sinks through the oil due to its greater density. Surrounded by hot oil and/or pan, the water heats up and vaporizes with a sudden expansion. This throws the overlying oil upward, creating long jets of hot oil that break into flying droplets. These are what actually hit you. This is a small-scale demonstration, but it gets at the heart of why you don’t throw water on an oil fire. (Image credit: C. Kalelkar and S. Paul, source)

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    The Elastic Leidenfrost Effect

    Drop some hydrogel beads in a hot frying pan and they’ll bounce, hiss, and screech. Normally, if you drop a ball, it bounces to ever smaller heights until it comes to rest. In contrast, on a hot surface the hydrogel can bounce to a steady height for minutes at a time, raising a question: where does it get the energy for its incessant bounce? 

    Upon close examination of the impact, researchers found the hydrogel beads are actually slapping the surface over and over on each bounce. The frequency of the slapping exactly matches that of the audible screech, so what you’re actually hearing is this bounce-slap. Now what causes the slapping?

    Contact with the hot surface vaporizes some of the water inside the hydrogel. If it were a droplet, this vapor would form a thin, almost frictionless layer the droplet could glide on; that’s the classic Leidenfrost effect. Here the shell of the bead prevents that until the pressure really builds up. When the pressure gets high enough, the vapor finally escapes, opening up a gap. As the gap reaches its largest point, the bead rebounds elastically, bringing it back in contact with the surface and starting the process again. Each of these cycles acts like a tiny engine, harvesting energy that drives the larger bounce. This elastic Leidenfrost effect may be particularly helpful in soft robotics, providing robots with a new mechanism for movement. (Image and video credit: S. Waitukaitis et al.,arXiv)

  • Surfing on Vapor

    Surfing on Vapor

    Place a drop of liquid on a surface much, much hotter than the liquid’s boiling point, and the portion of the drop that impacts will vaporize immediately. This leaves the droplet hovering on a thin layer of vapor. With a fluid like water, the vapor state is a much more efficient insulator than the liquid state. Thus, the vapor layer actually protects the liquid droplet, enabling it to boil off at a much slower rate than if the drop were touching the heated surface. This is known as the Leidenfrost effect, and it can be used to create self-propelled droplets.  (Image credit: R. Thévenin and D. Soto)

  • When Lasers Strike

    When Lasers Strike

    Lasers are a great way to deliver a lot of energy very quickly. In this animation, you see a jet of water get struck by a pulse from a powerful X-ray laser. The energy from that laser pulse gets absorbed by the water in a matter of picoseconds – that’s trillionths of a second. All that energy in so little time makes the water vaporize explosively. It’s this vapor explosion that breaks the jet in two. As the vapor expands outward, it forces water from the jet into a thin film that forms a cone. The conical film bends back on itself until it strikes the jet and coalesces. For more, check out this video of a similar experiment that looked at laser impacts on droplets. (Image credit: C. Stan et al., from Supplementary Movie 5; via Gizmodo)

  • Skating on Vapor

    Skating on Vapor

    Turn the stove up high enough and you may have noticed that drops of water stop boiling away and instead skate across the surface. This is the Leidenfrost effect, which occurs when a surface is so much hotter than a liquid’s boiling point that any liquid that contacts instantly vaporizes. That thin vapor layer insulates the rest of the drop and makes it skate around on very little friction. Previously, researchers found that putting these drops on patterned surfaces causes them to self-propel. Here you see Leidenfrost drops on a V-shaped “herringbone” surface. The grooves in the surface catch and direct the vapor out the Vs. If it seems counter-intuitive that the drops move in the same direction as their vapor, you’re not alone! It turns out that Leidenfrost drops aren’t propelled by vapor moving away from them – like, say, a rocket is. Instead the drops are being dragged along by friction between them and the escaping vapor. By controlling the direction of the vapor, researchers were able to create race tracks (top) and even traps (bottom) for the drops. (Image credit: D. Soto et al., from Supplemental Movies 2 and 3)

  • Molten Salt in Water

    Molten Salt in Water

    In his latest video, The Backyard Scientist explores what happens when molten salt (sodium chloride) gets poured into water. As you can see, the results are quite dramatic! He demonstrates pretty convincingly that the effect is physical – not chemical. The extreme difference in temperature between the liquid water (< 100 degrees Celsius) and the molten salt (> 800 degrees Celsius) causes the water to instantly vaporize due to the Leidenfrost effect. This vapor layer protects the liquid water from the molten salt – until it doesn’t. When some driving force causes a drop of water to touch the salt without that protective vapor layer, the extreme temperature difference superheats the water, causing it to expand violently, which drives more water into salt and feeds the explosion.

    But why don’t the other molten salts he tests explode? Sodium carbonate, the third salt he tests, has a melting point of 851 degrees Celsius, 50 degrees hotter than sodium chloride. Yet for that test, the Leidenfrost effect prevents any contact between the two liquids. The key in this case, I hypothesize, is not simply the temperature difference between the water and salt, but the difference in fluid properties between sodium chloride and sodium carbonate. The breakdown of the vapor layer and subsequent contact between the water and the molten salt depends in part on instabilities in the fluids. A cavity where instabilities can grow more easily is one where the Leidenfrost effect is less likely to protect and separate the two fluids. And, in fact, it turns out that the surface tension of molten sodium chloride is significantly lower than that of molten sodium carbonate! A lower surface tension value means that the molten sodium chloride breaks into droplets more easily and its vapor cavity will respond more strongly to fluid instabilities, making it more likely to come in contact with liquid water and, thus, cause explosions. (Image/video credit: The Backyard Scientist; submitted by Simon H)

  • The Leidenfrost Dunk

    The Leidenfrost Dunk

    The Leidenfrost effect occurs when a liquid is exposed to a surface so hot that it instantly vaporizes part of the liquid. It’s typically seen with a drop of water on a very hot pan; the drop will slide around, nearly frictionless, upon a cushion of its own vapor. You can see the effect when plunging a hot object into a bath of liquid, too. This is what happens when you quickly dunk a hand in liquid nitrogen (not recommended, incidentally) or when you drop a red hot steel ball into water like above. In this case, the object is so hot that it gets encased in a layer of water vapor. If you could maintain the temperature difference necessary to keep the vapor layer intact, you could move underwater at high speeds with low drag, similar to the effects of supercavitation. (Image credit: Paul Pyro, source)

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  • Pouring Molten Aluminum on Dry Ice

    Pouring Molten Aluminum on Dry Ice

    What happens when you pour molten aluminum on dry ice? As the Backyard Scientist shows, you get what looks like slippery, sliding, boiling metal. In fact, what you see may remind you of the Leidenfrost effect, where a liquid can slide around over an extremely hot surface on a thin film of its own vapor. Despite the opposite temperature extremes–this is a very cold surface rather than a very hot one–a very similar thing is happening here. The molten aluminum is so much hotter than the dry ice that it causes the dry ice to sublimate, releasing gaseous carbon dioxide that the aluminum slides around on. For the same reason, the aluminum appears to boil in the bottom animation. What we’re really seeing is carbon dioxide gas rising and escaping the aluminum so violently that it carries some of the metal with it. Be sure to check out the full video for more awesome physics!  (Image credit: The Backyard Scientist, source; via Gizmodo)

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    The Tightrope Dancers

    Boiling is a process most of us don’t pay much attention to. But it can be remarkably entertaining and beautiful. This award-winning video shows boiling on and around a heated wire immersed in oil. Depending on the diameter of the wire and the power used to heat it, the researchers observe several different regimes of behavior. In one, vapor bubbles form on the wire and interact with one another: bouncing, merging, and dancing back and forth. When the bubbles become large enough, their buoyancy lifts them upward. In another regime, the wire is hot enough for film boiling. Like the Leidenfrost effect, film boiling occurs when a surface is so hot that it instantly vaporizes any liquid near it. The vapor layer then acts like coating, insulating the remaining liquid from the hot surface. The bubbles formed on the wire in this regime are mesmerizing, rising in periodic patterns or shifting back and forth gobbling up lesser bubbles. (Video credit: A. Duchesne et al.)