Self Propelled Liquid Droplets

April 6, 2006 12:57 AM

Self Propelled Liquid Droplets : When a liquid drop is placed (.mov files) on a surface held at a temperature much higher than the liquid’s boiling point it hovers on its own vapor cushion, without wetting the surface.
posted by dhruva (24 comments total)

that's how you walk on hot coals right?
posted by matteo at 1:57 AM on April 6, 2006

very cool... thanks for this.
posted by cadastral at 2:01 AM on April 6, 2006

that's how you walk on hot coals right?

Related Mefi post.
posted by dhruva at 2:05 AM on April 6, 2006

The effect can be achieved with a wok or iron skillet. it's especially fun with a wok.
posted by 2sheets at 2:25 AM on April 6, 2006

Easy to do in your own home by just heating up a stove plate and dropping some water on it.
posted by insomnus at 2:25 AM on April 6, 2006

I think people are missing the point of the post. Yes, you can do this on a skillet. But, if you see that last link you'll see that the real discovery here is that these droplets can be made to move uphill!
posted by vacapinta at 2:41 AM on April 6, 2006

I was under the impression the hovercrafts slid down hills and that's why you only see them on relatively horizontal surfaces such as water. N'est pas?
posted by furtive at 4:03 AM on April 6, 2006

i have eels in my hovercraft.
posted by quonsar at 4:26 AM on April 6, 2006

I don't care about your hovercraft, quonsar, I'm too busy trying to get these scratches off my tobbaconist's.
posted by bouncebounce at 4:40 AM on April 6, 2006

I never realized quonsar's pants were in effect a ground effect vehicle.


These droplets roll uphill - when said inclined plane is sawtoothed or ratcheted and is significantly warmer then the droplet of liquid placed upon it.

So, plain water on a sawtoothed, sufficiently hot metal plate will "defy gravity" by rolling uphill against the grain of the sawtooth due to the way the steam vapors escape differently from the droplet, depending on where on the sawtooth the droplet is.

Same goes for very cold liquid nitrogen on a much warmer yet room-temperature sawtoothed plate.

This isn't just a droplet skittering around a hot griddle. It's a counterintuitive discovery that could actually lead to some useful things.

Say, a pumpless water cooled engine, or methods of cooling chips in computers without resorting to moving parts or exotic materials, or utilizing waste heat to pump or manipulate liquids.

Note that in the videos they also use a channel with sawtoothed walls, and large water droplets zip through it like they were shot out of a gun.

A channel is practically a tube - just one wall short, so it'd be logical to assume that tubes might also work as well.
posted by loquacious at 4:46 AM on April 6, 2006

wonderful link, thanks.
posted by hypersloth at 5:03 AM on April 6, 2006

that's cool in the first slowmotion video the water looks like a slinky
posted by The Boy at 5:48 AM on April 6, 2006

It's not REALLY self-propelled, as you're applying energy from the plate, but it is still very nifty.

Perhaps, PERHAPS, even niftier than putting liquid nitrogen in your mouth (which is safe due to the same effect - but don't swallow it!).
posted by solotoro at 6:24 AM on April 6, 2006

I'm so hot -- that why shower never work for me.
posted by NewBornHippy at 6:42 AM on April 6, 2006

Perhaps, PERHAPS, even niftier than putting liquid nitrogen in your mouth (which is safe due to the same effect - but don't swallow it!).

Yeah, it's safe as long as your mouth is dry. If your mouth is too wet then the water will flash freeze and crack a tooth.
posted by Farengast at 7:27 AM on April 6, 2006

Very cool mechanism. But I don't see how using this phenomenon adds anything to a micropump. Aren't we still relying on steam to get to the condenser?
posted by horsewithnoname at 7:52 AM on April 6, 2006

The Leidenfrost effect is neat, but I doubt that it contributes much to successful fire-walking. You don't see any sane people walking on very hot cast iron, where the LE should work as well as anywhere else. Firewalking occurs on coals of various sorts.

The explanation I've heard is that the coals are at a high temperature, but actually retain relatively little heat because they're so porous. (Think: sticking your hand into the hot air of a 400 degree oven; touching a potato baking at 400 degrees; touching an oven rack at 400 degrees.) It's also a good idea to walk briskly and forget about dawdling on the coals.

The experience is supposed to be like walking on hot beach sand, although it can subjectively feel cooler if you do those New Age chants and crap before and you really psych yourself into thinking they'll have an effect.

The link? Water roling uphill is awesome.
posted by rosemere at 7:59 AM on April 6, 2006

The droplets aren't really rolling uphill. They are making small random movements in a rachet-like topology that prevents reversion, much like the Bush administration going up the creek.
posted by weapons-grade pandemonium at 8:15 AM on April 6, 2006

Oooh... the glitter is really pretty. I always kinda wondered how water skated along hot surfaces (as in cooking).
posted by OverlappingElvis at 9:27 AM on April 6, 2006

If liquid gets out of laptops as easily it can accidentally get in, i'm not sure i want mine filled with near-vaporizing temperature liquids.
posted by wumpus at 10:50 AM on April 6, 2006

i have eels in my hovercraft.

Eels on a hovercraft, man.

Eels on a motherfuckin' hovercraft.
posted by Alex404 at 10:51 AM on April 6, 2006

I hesitate to substitute my own judgement for a professor's of physics and all his students', but if you look at the slow motion movie they provide, I think you can see that their
explanation for the uphill motion of the drop is wrong.

Instead of being pulled along by viscous forces acting at the base as they suggest, it's clear to me that, as the droplet passes over the deepest part of the ratchet, a sudden large jet of vapor escapes behind the droplet, which generates a quite large wave in the body of the droplet. This wave, in turn, moves up over the top of the
droplet to the right, shifting the center of gravity of the droplet in that direction, and, by virtue of the wave's momentum as well, carrying the rest of the droplet along with it.
posted by jamjam at 3:17 PM on April 6, 2006

That's a strange definition of "safe" you have there, Farengast. I imagine a lot could go wrong, I'd rather not tempt the Physics Gremlins like that.
posted by JHarris at 3:44 PM on April 6, 2006

Eels on a motherfuckin' hovercraft.
posted by Alex404 at 12:51 PM CST on April 6 [!]

No, no, no.

*ahem*

I want these motherfucking eels off this motherfucking hovercraft!
posted by Ynoxas at 7:55 PM on April 6, 2006