Well, I have been about the worst blogger in the world recently. Not by writing bad posts, or boring posts, or outrageous or insulting posts, but rather by writing no posts at all. It’s been another hectic month at Fermilab, at conferences and workshops, even a few days of vacation squeezed in there.
One thing definitely worth sharing here on QD is the grand finale of the Science Chicago LabFest! events which happened recently downtown at Chicago’s Millennium Park. I mentioned the LabFest events in an earlier post, but they are outdoor science fairs which have been organized by Science Chicago and have occurred in parks and at schools all over the Chicago area over the past year. A group from Fermilab has been at each event through the summer with our van full of displays and interactive demonstrations. The Millennium Park event was the final LabFest of the summer and the fourth one that I have attended.
Beyond the remarkable venue, however, this LabFest was special because I got to do, with another physicist from Fermilab, a 20 minute stage presentation using liquid nitrogen. Mike Cooke and I met with the master of the cryo show, Mr Freeze (a.k.a. Jerry Zimmerman from Fermilab), before the show and he was kind enough to let us borrow his stage equipment.
Jerry’s normal show runs about an hour, so it was a challenge to pick and choose the bits we would fit into 20 minutes but maintain some continuity in the material. We began by giving some idea of the basic properties of liquid nitrogen. What is it? Basically liquid air since nitrogen makes up almost 80% of the atmosphere. How cold is it? -321 degrees Fahrenheit. Yikes, not even a Chicago winter compares! What does it look like? Pour some into a plastic ziplock bag and note that it looks like water, but is boiling at room temperature, so it can’t be water.
Next we explored the expansion that occurs as the liquid nitrogen (LN) rapidly boils into a gas. Sealing the ziplock bag quickly leads to it ballooning until it ruptures. Next a typical kitchen garbage bag (~13 gallons) is loaded up with 1 cup of LN and sealed with the same destructive results. Finally, a lawn garbage bag (~45 gallons = 720 cups) is used and, upon fully inflating, is shown to barely contain 1 cup of liquid nitrogen converted into gas. The expansion ratio is 700:1. At this point, as a foreshadowing of things to come, Mike poured a small amount into a 20 oz. plastic bottle and sealed the cap, holding it up to the excited kids in the front rows. They seemed to really get the point as a few began screaming that it would explode! Fortunately, this was a fake as Mike showed them the hole he had punctured in the bottle cap before the show – but the setup was successful 🙂
We then showed some examples of how liquid nitrogen can be used to change the properties of materials that it comes in contact with. At Fermilab, we use liquid nitrogen (and even colder liquid helium) to change some metals to become superconducting. It is these superconducting materials that are needed to produce the strong magnetic fields used to steer super high-energy protons in the accelerator.
First we used inflated balloons to show how the air inside them could be liquefied by making it cold with LN. This causes the reverse of the expansion we explored earlier, namely a ~700:1 contraction of the volume, so the balloons collapse down. Removing them from the liquid nitrogen, they slowly warm back up, crackling and twisting around like some kind of monster as they grow back to their original size. In fact, to really show the dramatic effect of contraction as the gas liquefies, we go on to pull out about a dozen large balloons we had placed before the show into a tiny dewar that they never could have fit into while fully expanded.
What about solid objects that are cooled down so much with liquid nitrogen? They don’t contract enough for the audience to see (but they certainly do contract some, as all objects do when they get cold), but their properties change in other dramatic ways. To demonstrate this we used a racquetball, a rubber glove, a small (4 inch) inflatable plastic basketball, and bananas. The racquetball goes from super bouncy to a loud crack and thud on the stage floor without much of a bounce. The soft (at room temperature) rubber glove becomes brittle to the point where the fingers can be shattered off. We set this up, of course, with Mike appearing to stick his hand into the LN dewar and me shouting that “It’s the wrong safety glove!”. He quickly pulled his hand out and shattered the fingers off revealing his safely recessed hand, much to the delight of our audience. The plastic basketball not only becomes rock hard like the racquetball, but also the air inside contracts as it liquefies, so there is no air pressure pushing out from the inside. A quick tap with the tongs and the previously rubbery, squeezable ball shatters into 1000 pieces like a light bulb.
The bananas were used to make a hammer. Mike begins to peel a banana claiming that he can use it to drive a nail into a piece of wood. I say that I don’t believe him and begin to peel my own banana which I use, at room temperature, to strike a nail. Of course, the banana breaks apart and bits of mushy banana fly around. Mike dunks his into LN to freeze the banana in a matter of moments making it super hard and successfully drives in a nail.
About time for the grand finale, don’t you think? For this, of course, we used the Cryo Cannon! A steel tub about 3 feet long and 4 inches in diameter was standing up on the concrete just off the stage to the right. We reminded the kids about the pressure built up from the expanding nitrogen when it boils as we placed a small amount into a 20 oz. soda bottle. We explained to them that we would use the release of this pressure to launch one of these small plastic basketballs into the air. “How high do you think we can launch this thing? 20 feet? 50 feet?” It was like an auction where money (or height) was no object to the purchasers – I think one kid offered 1,000 feet. Mike sealed the bottle, this time using a non-punctured cap, dropped it into the vertical metal tube, I dropped the “cannon ball” (plastic basketball) in on top, and about 15 seconds later – BOOM!, the ball was launched somewhere around 150 feet probably (about a 15 story building). There was a slight breeze from the west, so the thing floated east towards Lake Michigan as a dozen kids from the front rows tore off in chase to claim their souvenir.
Several people from the Fermilab Office of Communications were there and soon there should be an article about the event in Symmetry Magazine. Also, Susan Dahl, from the Education Office at Fermilab video recorded the cryo show, so I’ll pass those links along when they are available.