The Leidenfrost Maze
October 2, 2013 8:37 AM   Subscribe

When a liquid is dropped onto a smooth plate that is heated to a specific temperature well above its boiling point, boiled vapor will get trapped underneath the remainder of the droplet insulating it from the hot plate, allowing it to dance around the plate like oil on a wet surface in what is known as the Leidenfrost effect. Intriguingly, surfaces that are grooved into the shape of a saw blade will cause droplets suspended by the Leidenfrost effect to predictably skitter in the direction of the groove, allowing University of Bath undergraduate students Carmen Cheng and Matthew Guy to build a fascinating maze.

Here's a cool video of a red hot nickel ball in water. (Previously) (Previouslier)

Incidentally the best temperature for frying anything you want to cook but not burn or stick, taking advantage of this effect to both steam the meat or eggs or whatever and produce the Maillard reaction without charring, can be found by flicking water onto the hot pan in a similar way. For extra precision go for the very small temperature window where the bubble that forms stays together.

It is also the means by which rats are able to skate on butter as they scat in The Muppets Take Manhattan
posted by Blasdelb (32 comments total) 35 users marked this as a favorite

 
I did this experiment, comparing mice and water droplets, and it turns out that water droplets are both smarter and less flammable than mice.
posted by Mister_A at 8:46 AM on October 2, 2013 [5 favorites]


Badass. I think I was actually more impressed by the short shot of water flowing uphill of its own accord (on an appropriately shaped and heated surface) than I was by the (on its own, very impressive) maze.
posted by 256 at 8:46 AM on October 2, 2013


Holy jeez.

Is this the same thing that makes liquid nitrogen drops skitter around on the floor? It looks very similar to me.
posted by showbiz_liz at 8:46 AM on October 2, 2013 [2 favorites]


I think it must be, Liz.

Also - the droplets in close-up are going the opposite way i'd have expected based on the shape of the sawteeth. Why is that?
posted by Mister_A at 8:48 AM on October 2, 2013 [1 favorite]


the droplets in close-up are going the opposite way i'd have expected based on the shape of the sawteeth. Why is that?

they move perpendicular to the largest surface
posted by sexyrobot at 8:50 AM on October 2, 2013 [2 favorites]


That was great. Also appreciated the music--so nice not to have dance or rock blaring in the background.
posted by ropeladder at 8:53 AM on October 2, 2013


"Is this the same thing that makes liquid nitrogen drops skitter around on the floor? It looks very similar to me."

It is! Its kind of a mindfuck to realize it but Liquid Nitrogen dropped onto a floor is indeed just a liquid dropped onto a surface well above its boiling point, all of the principles are identical just at a lower temperature. It is intuitive to think of the temperature that we live at as 'normal,' or a useful reference, but really to only meaningful reference point is 0°K (−273.15°C or −459.67°F), everything else is relative to that.
posted by Blasdelb at 8:58 AM on October 2, 2013 [2 favorites]


Huh, my intuition would have attributed this to another cool effect of hydrogen bonding, but if it works for nitrogen too...
posted by OHenryPacey at 8:59 AM on October 2, 2013


I just sat here with my mouth open for three minutes. Awesome find.
posted by en forme de poire at 9:01 AM on October 2, 2013


Very very cool!
posted by OmieWise at 9:07 AM on October 2, 2013


In my mind, every single one of those water drops was making its way back to the car over an asphalt parking lot after a day at the beach: "Ooh, aah, eek, aah..."
posted by yoink at 9:12 AM on October 2, 2013 [10 favorites]


At the end of the video they cite a 2006 paper in Physical Review Letters (the most prestigious place to get your physics published) describing the underlying mechanism. Pressure is highest at the peak of the sawtooth (hot tooth poking drop); the steam going rearward from the peak escapes sideways along the cliff of the sawtooth and doesn't do much, while the steam going forward has to force its way on the long face of the tooth and pushes the drop with it. PDF of relevant paper with pretty diagrams.
posted by drdanger at 9:13 AM on October 2, 2013 [4 favorites]


thanks drdanger, I think I grasp the concept now.
posted by Mister_A at 9:25 AM on October 2, 2013


We were fortunate enough to have an Aga in our kitchen growing up, and it was endlessly fascinating to all us children to throw various liquids upon it in various ways, and watch the bubbly skittling reactions of the grown ups.
posted by forgetful snow at 9:41 AM on October 2, 2013 [1 favorite]


I hope you also had a plentiful supply of unguents and bandages, forgetful snow!
posted by Mister_A at 9:51 AM on October 2, 2013


The slow-motion microscopic-camera snippets were amazing.

The u-turn close-up was also amazing.

The whole thing was pretty amazing. And fun. Thanks for sharing this.
posted by seyirci at 10:42 AM on October 2, 2013


Very, err, cool, but needs food coloring.

Also -- the droplets climb up the sawteeth rather than "down" them. (I assumed they would climb down, for the same reason drivers don't back up over the parking lot exit guards.)
posted by notyou at 10:42 AM on October 2, 2013


Well, apparently you can see the Leidenfrost effect by dipping your fingers into molten lead! Mythbusters even did it.
posted by klausman at 11:18 AM on October 2, 2013 [2 favorites]


[Oops...I missed the previously. Still cool, though.]
posted by klausman at 11:25 AM on October 2, 2013


Ha! Very interesting. thanks!
posted by BlackLeotardFront at 11:58 AM on October 2, 2013


I used to demo the hand in liquid nitrogen thing for science shows, but no one was willing to risk the molten lead one, even the guy who would "drink" LN2. It's an apparently reverse Leidenfrost effect, hot liquid on a cool surface, with the liquid moisture on your hand providing the ablative protection.

Ice skating also works sort of the same way, but rather than a liquid-gas phase change, it's a solid ice->liquid water one, with the heat to liquefy the water coming from the pressure of the skate blade. You can even cut wire through ice this effect.

In summary, phase changes are really neat.
posted by bonehead at 12:27 PM on October 2, 2013 [2 favorites]


For some reason I wasn't expecting the string to take on a paraboloid shape as it cut through the ice in that video. Is it because the string isn't an ideal cable and so the tension at the edges of the ice block is higher initially? Maybe too that the ice is warmer and therefore melts a little more easily there?
posted by invitapriore at 1:11 PM on October 2, 2013


Well, apparently you can see the Leidenfrost effect by dipping your fingers into molten lead! Mythbusters even did it.

Yeah, that was totally stupid. You're supposed to dip your hands in a solution of liquid ammonia. Back when I was a chemistry student, I recall reading that Leidenfrost used to demonstrate the effect by dipping his hands in liquid ammonia and then having an assistant pour the lead into his cupped hands, then holding it for a moment before letting it go. According to legend, he wanted all students of physics and chemistry to perform this demonstration as a prerequisite to receiving their degree. It would be a demonstration of their trust in physics.

For some reason I wasn't expecting the string to take on a paraboloid shape as it cut through the ice in that video.

It's a catenary curve.
posted by charlie don't surf at 1:42 PM on October 2, 2013 [1 favorite]


Yeah, that was totally stupid. You're supposed to dip your hands in a solution of liquid ammonia.

Yeah that part where using water worked just fine was dumb as hell
posted by invitapriore at 1:48 PM on October 2, 2013


pour the lead into his cupped hands

This is the thing we were warned, even with our complete lack of H&S training, absolutely under no circumstances not to do with the LN2. The thinking was that first, you didn't want sustained contact with either hot or cold liquids, so no cupping of hands, and secondly, if the liquid did break through the gas barrier, it would cause pain (before it killed the nerve), causing one to close their fist by instinct. So dunking happened with hands straight, fingers spread, exactly as Adam and Jamie did it. Not disbelieving, that just that sounds completely crazy.
posted by bonehead at 1:55 PM on October 2, 2013


Yeah that part where using water worked just fine was dumb as hell

You could dip your hand in molten lead that quickly without even dunking in the water first. That isn't even the Leidenfrost effect, it's just too quick to transfer heat and cool down the lead enough for it to change to a solid. The metal that stuck to their experimental sausage was cooled slag from the top of the liquid. If they had cleared off the slag, like they did for their fingers, it would have been fine.

I sometimes demonstrate this non-Liedenfrost effect by showing how to measure the temperature of water up to boiling, with an accuracy of about 2 degrees, by dipping your bare finger into the water. I call it The Human Thermometer. The temperature is indexed to how fast the heat transfers to your fingertip before the finger warms and the heat receptor nerve endings fire.
posted by charlie don't surf at 1:57 PM on October 2, 2013 [2 favorites]


That red hot nickel ball video is the gateway to a wide world of cool related YouTube videos. Search for RHNB and look for results by user carsandwater.
posted by JHarris at 2:01 PM on October 2, 2013


You could dip your hand in molten lead that quickly without even dunking in the water first.

I'd want proof of the assertion that you can do this without getting even minorly burned, even if the lead doesn't clump on your hand. Lead has a much higher thermal effusivity than your boiling water and will be hotter besides.
posted by invitapriore at 2:23 PM on October 2, 2013


According to legend, he wanted all students of physics and chemistry to perform this demonstration as a prerequisite to receiving their degree. It would be a demonstration of their trust in physics.

That's stupid. Physics isn't trustable -- its just natural processes. What it would be a demonstration of is the student's, and the faculty's for allowing him to make such a rule, trust that it would do what he says it does -- basically, a power trip.
posted by JHarris at 2:36 PM on October 2, 2013 [1 favorite]


That is one cool video. Amazing to see them climbing the sawtooth bits.
posted by marienbad at 4:56 PM on October 2, 2013


effusivity

That isn't even a real word. Perhaps you meant thermal conductivity? It doesn't matter. The thermal conductivity of human flesh is the same, whether you dip it in 621F molten lead, or 212F boiling water. The thermal energy of molten lead is higher merely because its temperature is higher. So you'd dip your finger in and out faster than you could tolerate in boiling water. Energy is only transferred when the finger is in contact with something of higher energy. Just as an example, I can dip my finger in boiling water briefly. But once I spilled some on my hand and it stayed in continuous contact, resulting in a 3rd degree burn.

Anyway, if you want proof, you have probably seen people test the temperature of an iron by licking their finger and pressing it into the hot metal for a moment. Try it without the finger being wet. You still won't get burned.
posted by charlie don't surf at 5:07 PM on October 2, 2013


WTF? Effusivity. And it does matter, since the contact temperature between two bodies is a function of both of their effusivities.
posted by invitapriore at 5:50 PM on October 2, 2013 [6 favorites]


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