Liquid Nitrogen on TV

This past weekend, Union played host to the New York State Association for College Admissions Counseling's Camp College program. This is a three-day summer program where students from disadvantaged backgrounds (the vast majority of this year's students were from New York City, with a handful of local students, and one group from Philly) spend a weekend on a college campus, sleeping in the dorms, eating in the dining halls, and attending simulated classes. The classes are taught by faculty memebers recruited from the host institutions-- I did a lecture for them a few years back, in the early days of this blog, and had a blast, so I was happy to do it again this year.

The lecture I gave for this (860kB PDF) is based on lectures I give in our "freshman seminar" class on the basics of laser cooling, but I play up the liquid nitrogen demos quite a bit more than in a real class. There's a certain magic show quality to the whole thing, which I swiped from Bill Phllips's public talks. It's a real kick to do this sort of thing-- the looks on the kids' faces when I smash the raquetball are more than worth the minor hassles of pulling the gear together.

The last time I did this, in 2002, it got written up in the paper. This time around, I ended up on tv, in a short segment on the local news. We recorded it onto Kate's computer, and she was good enough to cut it down to a reasonable size and host it on Steelypips. If you'd like to watch it for yourself, here's the file (5.1MB .avi) (see technical note at the end of this post, and please right-click and save it, rather than playing it directly from Steelypips)-- the highlight is the bit about 15 seconds in where a kid pulls a balloon out of the dewar and says "Ooo!"

Since I described most of these demos in detail back in 2002, I'll quote the text of that old post below the fold, with some additional comments indicating where I changed things this time out.

I did my Mr. Wizard act this morning for the high school kids I mentioned a few days ago. It's always good to do these things, or see them done, just as a way of calibrating how jaded scientists can become regarding some fairly amazing things.

My talk was about the basics of laser cooling (there's a nifty explanation at the Physics 2000 web site (it comes complete with video game applets that just have to be seen, and a cheesy question-and-answer format), if you're sick of waiting for me to do a plain-text version), and to provide a little context for what "cold" means in a laser cooling context, I broke out the liquid nitrogen (sorry about the hideous background on that page).

Nitrogen, as any geek can tell you, is a gas which makes up seventy-odd percent of air. If cooled to extremely low temperatures-- around 77 K (where one Kelvin is one degree Celsius above absolute zero-- 77 K is somewhere in the neighborhood of -320 Fahrenheit)-- it becomes a liquid which can be used for all sorts of things, from cooling experimental apparatus, to chilling ceramic superconductors, to freezing baseball legends. It's really common stuff in science circles, and while everybody in physics research has probably wasted at least one day playing with the stuff, it's easy to forget how neat the concept is if you haven't seen it before.

The classic liquid nitrogen demo is to dip flowers into the stuff, and chill them down. After a minute or so, you can tap the frozen flowers on the edge of a table, and they'll shatter like glass. It's a classic, but in some ways, it never gets old. The kids at today's talk loved that one, and several of them stuck around after I had finished to take turns freezing and then breaking things (the newspaper photographers who showed up (helping disadvantaged kids is good PR) loved that one-- they shot several rolls of film of kids breaking stuff. I'll have to check the papers tomorrow to see if that made it in) [Ed. 2006: It did, but the story isn't available on-line any more.]. Another good variation on it is to take something elastic-- a racquetball, say-- and chill it in liquid nitrogen for a while. Where you previously had a nice, springy, bouncy ball, the frozen ball will shatter on a hard wooden floor. It makes a really loud cracking noise, and always gets a big reaction.

(Product Placement Moment: I bought four Penn racquetballs (warning: Flash) to use for this demo. The can advertises a two-for-one guarantee-- two balls to replace any one that breaks before the logo wears off. Prior to the talk, I jokingly suggested that I should send them the shattered fragments, and keep myself stocked up for the future... The first time through (I gave the same lecture twice, to two different bunches of kids-- it was surprisingly tiring...), the ball broke easily. The second time, the damn thing just wouldn't break-- I ended up having to fling it into the floor really hard to get it to crack. A third one was similarly resilient. Rather than sending in the pieces and asking for replacements, I'm tempted to send them a testimonial... "Even in the most extreme conditions, Penn racquetballs hold up!") [Ed. 2006: The $3 racquetballs I got at Target this year broke easily (but impressively) in all three classes.]

Even simpler demos are enough to really impress people who haven't seen the stuff before, though. As a simple demonstration of how cold the liquid is, I dumped probably three liters of the stuff out on a table, a little at a time, to show that it boils instantly on contact with the table. It makes an impressive cloud of vapor, and hisses and spatters all over the place (which makes it a good way of recapturing the attention of an audience that's started to drift...), but to anyone who works with the stuff on a regular basis, that's old hat-- when you fill a dewar with liquid nitrogen, it spatters all over as a matter of course. You learn to stop flinching at the idea of the drops hitting your skin-- they don't do any damage, due to the "Leidenfrost Effect." (This has led some wits in the science community to suggest that liquid nitrogen is best handled naked...) Still, this was impressive enough to some of the students that a couple of them asked me after the lecture if I'd just let them pour some of the nitrogen out on the floor... [Ed. 2006: Again, dumping it out was a big draw after I finished.]

The final demo I did is sort of an exercise in showmanship that I lifted from Bill Phillips when he used it for a public talk here. The key to it is that if you blow up a balloon and dunk it in liquid nitrogen, it shrinks dramatically as the gas inside cools, liquifies, or freezes. A balloon about six inches across (just under the width of the dewar) will collapse down to become a little pancake if you stuff it into the liquid. This means you can take a dozen or more balloons, each about six inches around, and stuff them into a dewar that's eighteen inches high. It works really well if you do them a couple at a time, saying "If you want to cool down a sample of gas, you'd think that putting it in liquid nitrogen would be a good way to do it," then cover the dewar, and go on with the lecture. The first two, the students didn't think much of it. The second two, a few of them started to notice that there was something weird going on. By the eight or ninth, I could hear them asking "How's he doing that?" The looks on their faces when I started throwing the pancaked balloons out into the crowd were worth the hassle of dragging the nitrogen tank over into the auditorium where I gave the talk...

[Ed. 2006: After seeing Bill Phillips do this at DAMOP this year, I added a new element-- I only put about four balloons in during the talk, but I pre-loaded it with a bunch more, of different colors. When I went to start taking them out, I asked the students "How many yellow balloons did I put in?" They said "Four" and I pulled out four yellow balloons. Then I asked "How many red ones?" "None." The looks on their faces when I pulled out four or five red balloons were priceless.]

(The point of the demo is to show that conventional methods of cooling don't work for getting most gases down to extremely low temperatures, as the gas will liquify, or even solidify. If you want to work at low temperature, and still have a vapor, you need to be clever about it, hence the need for laser cooling...)

The one drawback to doing these sorts of stunts is that the audience tends to remember the demos more than the science being discussed. This crew was no exception-- one pain-in-the-ass kid kept asking nitrogen related questions through the whole talk ("What would happen if you poured it out on your hand?" "It would sting. Please shut up.") Both groups wanted to know where to get liquid nitrogen ("I get it from a big tank in the next building. You can't get it, period. Unless you become a scientist."). Still, both groups had at least one student who asked good questions at the end, one of them a question of the "I was hoping you wouldn't notice that" variety, zeroing in on some stuff I'd swept under the rug. That alone was worth the sacrifice of a sunny Saturday morning. [Ed. 2006: Fewer questions of any sort this time out. There were a couple of kids who wanted to know what laser-cooled atoms were good for, but that was about it.]

Oh, and about the statement above that this was all in the interest of providing context for what "cold" means to a laser cooling person? The temperature of the liquid nitrogen Ted Williams is frozen in is roughly one million times higher than run-of-the-mill temperatures in laser cooling experiments-- we routinely deal with samples of atoms at a temperature of 100 micro-Kelvin, or a hundred one-millionths of a degree (Celsius) above absolute zero. With a bit of work, you can push that down by another three orders of magnitude, or more.

The details of how you get those temperatures will have to wait for another post. [Ed. 2006: Two posts, actually, that were recently re-posted here: one, two.]

Technical Note on Video File (from Kate):

5.1 MB, 320x240, 1:24 long.

It was compressed using Virtual Dub and the XviD codec. If you can't
play it on an existing player, and don't want to mess about with
codecs, I recommend tryiing the href="http://www.videolan.org/vlc/">freeware VLC media player,
which is good at playing all kinds of encodings.

(And yes, I know that in the compression, weird color stripes and
blocks were introduced. Since there was some kind of flaw in the
original .avi file that kept crashing Virtual Dub, it took me all
night to get a reasonably-sized copy of the segment; since this
version is watchable, I decided to quit while I was ahead.)