You've got something on your head there

May 15, 2009 5:03 PM

I've just poured myself a Dark Horse Brewery Tres Blueberry Stout. A slow, steady pour that left a modest head on top. And then a droplet or two skidded around over the foam like a water droplet on a hot frypan for maybe five seconds before disappearing. Darndest thing I've ever seen. Beer geniuses, how did this happen? How can I make it happen again?
posted by ardgedee to food & drink (6 comments total)

You can do that with plain old water, though it's not quite as tasty. I'm wagering something with surface tension. I find myself doing that in the bath sometimes, it's kinda pretty.
posted by fiercecupcake at 5:39 PM on May 15

I see this sometimes with Little Creatures Pale Ale. Temperature seems to be the biggest variable in getting it to repeat. But I've ceased trying to *make* it happen and just enjoy it when it does, like it's a good omen from the Gods of Beer; a sign that all is right with the world and all of nature's forces are in harmony.
posted by tim_in_oz at 8:41 PM on May 15

I've seen this happen dozens of times on plain water.

No idea what causes it, but it's not a beer-specific phenomenon.
posted by Ookseer at 11:35 PM on May 15

Happens with milky tea, too.
posted by pseudostrabismus at 11:51 PM on May 15

I've never seen this with beer, but it's easy to do with coffee or water. Use a short styrofoam cup, fill it almost to the top, and push it around the table with your fingers near the bottom of the cup. The vibrations will set up a standing wave pattern on the surface which causes individual droplets to form and detach. The surface tension is high enough for them to remain stable for several seconds, even after the cup comes to rest.
posted by Wet Spot at 5:14 AM on May 16

I think you've managed to produce the classic leidenfrost effect without the heat.

With a droplet of liquid on a hot surface, bubbles of vapor form underneath the droplet, and when they escape to the side, they kind of blow the droplet in the opposite direction, making it skitter all over the place.

The essential reason this happens with your stout has to do with the fact that for a given liquid, the smaller the bubble or droplet, the higher the pressure inside it (this is why fog often fails to freeze out even when the air temperature is well below 0 C., the pressure inside the droplet is high enough to significantly depress the freezing point of water).

In the case of your stout, assuming the droplet is larger than many bubbles in the foam, when the droplet comes in contact with the tiny bubbles, the surfaces merge, and because the pressure in the liquid layer of the bubble is higher than the pressure in the droplet, the liquid of the bubble is in effect injected into the droplet, causing the bubble to pop, and the gas of the bubble propels the droplet around the way escaping gas does in the classical situation.

I'd guess the trick to reproducing this is managing the bubble size in the head; if the bubbles are too small, the impulses the droplet receives from popping them may average out too much to make it skitter, if too large, the pressure differential may not be there, or it might be in the wrong direction.
posted by jamjam at 12:37 PM on May 16