Tag: Leidenfrost effect

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)
May 27, 2016
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)
March 9, 2016
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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February 10, 2016
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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January 29, 2016
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.)
December 9, 2015
Reader Question: When Mercury Meets Lava
Reader lucondri asks:
What happens when mercury touches lava?
That’s an interesting thought experiment, but hopefully no one tries it any time soon given mercury’s toxicity. So, what might happen? Mercury has a boiling point just under 630 Kelvin, and, although the temperature of molten lava varies, it’s between 970 and 1470 Kelvin when it first erupts. So mercury would definitely vaporize (i.e. boil) on contact with lava. (Again, this is very bad for anyone nearby.) If you’re curious what boiling liquid mercury looks like, wonder no further.
Molten lava is much, much hotter than the boiling point of mercury, though, so there’s a possibility that the mercury won’t boil away instantly. This is because of the Leidenfrost effect, where a thin layer of vapor forms between a liquid and an extremely hot surface. The vapor has such low friction that the liquid can essentially skate across a surface, and it doesn’t boil away instantly because the vapor insulates it from the extreme heat. After some digging, I found a paper that placed the Leidenfrost temperature of mercury between about 850 and 950 Kelvin, meaning that fresh lava is probably hot enough to generate mercury Leidenfrost drops.
So pouring a lot of mercury on lava will probably result in some boiling, but there’s also a good chance that it will form a bunch of skittering mercury droplets that will stick around awhile before they evaporate into toxic mercury gas. That said, it’s a lot easier and safer to watch awesome Leidenfrost drop videos with other liquids. (Collage credit: N.Sharp; images sources: Z. T. Jackson, and A.Biance)
March 20, 2015
When Lava Meets Ice
What happens when lava meets ice or water? Artists and geologists are working together to explore these interactions by melting crushed basalt and pouring it onto different substrates. Ice is their classic example; instead of melting instantly through the ice, the lava is so hot that it creates a layer of steam between it and the ice. This steam helps the lava flow due to lower friction while also insulating the ice from the lava. It’s an example of the Leidenfrost effect. The end result is a very bubbly lava flow thanks to the steam trying to escape through the viscous lava. (Video credit: Science Channel; submitted by @jchawner)
March 16, 2015
Bead-Infused Droplet
A Leidenfrost droplet impregnated with hydrophilic beads hovers on a thin film of its own vapor. The Leidenfrost effect occurs when a liquid touches a solid surface much, much hotter than its boiling point. Instead of boiling entirely away, part of the liquid vaporizes and the remaining liquid survives for extended periods while the vapor layer insulates it from the hot surface. Hydrophilic beads inserted into Leidenfrost water droplets initially sink and are completely enveloped by the liquid. But, as the drop evaporates, the beads self-organize, forming a monolayer that coats the surface of the drop. The outer surface of the beads drys out, trapping the beads and causing the evaporation rate to slow because less liquid is exposed. (Photo credit: L. Maquet et al.; research paper – pdf)
August 20, 2014
Droplets Surfing
The Leidenfrost effect can make water droplets skitter across a hot griddle or briefly protect a hand dunked in liquid nitrogen. When a liquid is exposed to a solid surface much, much hotter than its boiling point, the contact vaporizes part of the liquid, and, in the case of a droplet, forms a thin lubricating layer of vapor that the liquid drop can skate around on. Researchers have found that releasing these Leidenfrost droplets on textured surfaces creates self-propelling drops by directing the flow of vapor. In this video, one team demonstrates some of the neat tracks they’ve built for their drops. (Video credit: D. Soto et al.)
May 27, 2014
Flowing Uphill
Science Friday takes an inside look at self-propelled Leidenfrost droplets like those we’ve featured previously. The Leidenfrost effect takes place when a liquid comes in contact with a surface much, much hotter than its boiling point. Part of the liquid is vaporized, creating a thin gas layer that both insulates the remaining liquid and causes it to move with very little friction. Over a flat surface, this underlying vapor will spread in any direction. But by covering the surface with ratchets, it’s possible to direct the vapor in a particular direction, which propels the droplet in the opposite direction. Check out the video and our previous posts for more! (Video credit: Science Friday; via io9 and submitted by Urs)
March 18, 2014