A few weeks back, a Union alumnus who works at Troy Prep contacted the college to arrange a visit for a bunch of second-graders, and asked if faculty would be willing to arrange talks and demos for the kids. I said something like "Sure, we could probably make liquid nitrogen ice cream for them," and then basically forgot about it until last week, when I said "Oh, crap, I have to make liquid nitrogen ice cream for 60 seven-year-olds on Monday!"
Fortunately, our students in the Department of Physics and Astronomy are awesome, and I was able to round up a handful of helpers from the summer research crowd, who handled most of the presentation themselves. I did the frantic running-around to get the materials together (including a second trip to the store, because I hadn't bought enough milk products for two batches, the math of portion sizes not being a thing we study in physics grad school). The students found lab coats and some other props, and handled all the pouring, mixing, serving, and of course the all-important breaking of stuff:
They did a great job, based on basically zero preparation-- I spent about fifteen minutes explaining the idea behind the demos (but not actually doing any), and we mixed a test batch of ice cream during lunch, and that was it. All the credit for the success of the show goes to them.
My own contribution to the show was basically two bits of shameless pandering to the second-grade audience. For the first, I introduced the show, which was right after lunch, by asking "Do you guys like ice cream?"
"Yes!!!" they all yelled, because they're second-graders.
I looked at the teachers, and said "Wow, you've found the only second-graders in the world who don't like ice cream." Then back to the students, and a repeat of "Do you like ice cream?"
Which of course got the desired "YEEEEESSSSSSS!!!!" that rattled the windows.
"OK, good. We all like ice cream. But we have one problem: We don't have any ice cream." Which got the desired goggle-eyes of disappointment from a good chunk of the class. "We do have the stuff to make ice cream, though, so these students are going to show you how to make ice cream really fast using science."
I told Kate about that, and she said "You're shameless." Which is true, but they're second-graders. You have to play things really broadly for second-graders...
The other bit of pandering I contributed was when they started asking about why soft objects frozen in liquid nitrogen break. The students running the show tried a couple of answers without much luck, then I offered "How many of you have seen the movie Frozen?" Which of course got everyone bouncing in their seats raising their hands. "What happened to the bad guy's sword at the end? It broke into a million pieces, right? Because when things get really cold, they get brittle, and can shatter."
This, of course, isn't actually an answer to the question in any scientific sense. It did, however, relate the question to something they all cared deeply about, and that seemed to satisfy them. And, again, it's kind of shameless, but you need to know your audience.
Anyway, a fun day for all. Many thanks to Andrew, Beka, Harrisonn, Matthew, and Shauna, who were thoroughly awesome on short notice, and made the college look really good. And, as one of the teachers said, we now have 60-odd kids from Troy who are going to want to join the Union SPS chapter in 2024...
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It was pretty interesting to see the reaction of the guy when the ball shattered like glass against the wall. He was more surprised than the students! No acting needed, but ...
First lesson to undergrads: always test your demos!
I hope you and he realized that the fragments could have injured someone if the ball had hit the wall closer to where they were sitting.
That's actually from the second round of demos (they split the kids into two groups), so he had done it before. I think that throw went higher than the first, though, and he was afraid he'd broken something in the room.
We deliberately started the students in the second row of seats so they were clear of flying pieces. The ball in question was a dollar-store dog toy, and sufficiently flimsy that the pieces weren't much danger, even frozen.
I'm pretty sure he threw it up high and away from the students specifically so they were out of harms way.
I'm pretty sure the student knew what would happen when he threw it against the wall.......come on mannnnnnnnnnn
LED on a long pair of wires plus its ballast and a battery. Run it at room temp, then dip in liquid nitrogen. There is a nice spectral shift. Grain of wheat incandescent and 90V neon bulbs s are different.
The higher it is, the farther they travel. Basic kinematics. They get close enough to the people sitting by that wall to make me nervous about having people there the next time around. Some people use a shield between the demo and the audience.
This is no joke. It shatters like glass because the cold fragments are sharp as glass until they warm up. All it takes is the bad luck to have one hit someone one in the eye, even with low mass.