Thurs-Demo: The one that goes boom

It's Thursday. That means another demo. This week, I've been stricken with one of the various plagues that burns through college campuses at the start of the semester. All these viruses with thousands of brand new vectors. It's like some kind of frickin' holiday for them, or something. But since I was producing almost as much mucous as a netted hagfish, I didn't create a new demo this week. But, I have put together a little more information behind a demo I did last fall.



This demo came from an article and web page done by one of my former professors. Their write-up is well detailed and documented, so I'm not going to add much to what's in the journal article. If you really want to do this demo yourself, read what I have to say about my experience and then go read what Harpp et alia have to say. Pay special attention to the safety info. If you've never handled liquid nitrogen (LN2), you really want to pay attention. The stuff can be fun to experiment with, but only if handled safely.

I have embedded the video mashups from our demo runs below. Some things to look for include:

• Our first attempt exploded almost immediately. To make the bottle sink to the bottom, we poured it about 2/3 full of sand. This reduced the available gas volume and forced the bottle to burst too quickly.



• One attempt just sank to the bottom and then bubbled out. This was because I placed a "tall" cap on a "short-stem" bottle. Pay attention to this - some bottles have a shorter threaded neck and the taller caps won't seal properly.
  



• If you experiment with larger or smaller bottles, be sure that there is enough weight attached to the bottle. One of the runs shows the top of a bottle poking out of the water. This isn't very safe, because if you can see the bottle, there's nothing between you and a bursting pop bottle. The explosion was very loud. Fortunately, the bottle failed in a way that propelled it up into the air about 50 feet. Quite impressive, but loud and a little more hazardous than you want for an audience.



• The article suggests placing "ejecta" into the barrel. We tried practice golf balls. Ultimately, the surface we used was too uneven to allow for post-eruption particle tracking, but this has some interesting potential for enriching the demo with additional learning outcomes (plus, it gives students something to do while you're refilling the garbage can). I want to grab some whiffle balls of baseball and softball size and add them to the barrel. But you'll really need enough to completely cover the surface of the water, otherwise there won't really be much to see. As an added bonus, you can use the balls for particle tracking and estimate speed and size of the eruption column.



• I concur with Harpp et al. in their recommendations for a heavy-duty style, plastic garbage can. There's too much flexing and pressure to use metal or thin plastic, unless a burst and laterally-directed blast is what you want. I've used one of these garbage cans for about 10 explosions and it seems fine. The downward pressure does push the can into the ground, so avoid having sharp rocks or something like that underneath the container. They might puncture the can.



• Finally, go beyond the "big bang" and think about the physics behind this demo. Grab your ideal gas law and video tracking software. These demos used "about" 50 mL of LN2 for each explosion (the boiling LN2 makes it hard to get a real good estimate). And the can is the 32 gal size (filled with about 30-32 gal. of water each time). We can estimate height of the water column (and its approximate center of mass) and the time it took to get there.


• Some questions to consider:




• How much energy was released in the explosion


• Why does the garbage bin jump upward? Is it rebounding of the explosive force, or friction from the water moving upward, dragging the bin up with it?


• How could we rig the demo to create the largest possible eruption? (In our first series of tests, there didn't seem to be any obvious difference in column height between a 20 oz (0.6 L) bottle and a 2 L bottle.

Okay, enough thinking. On to the explosions!