Demo idea #1

Note: The following is going to be the first in an anticipated series of demos and/or labs that aren’t conducted much, if ever (or, at least, they’re new to me). Please keep in mind that all of the ideas in this series are based on my first impressions of how an idea may be turned into a demo/lab and are not complete demo/labs. As always, this means that if you do any demos or labs based on these ideas, you assume full responsibility for whatever happens, either good or bad.

Because I occasionally get an email with some variant of the question “Where do you get your ideas?” I figured I’d give you folks a window into how I think about new activities. Some of my musings eventually make it into the classroom while others do not, but all of them help me to think about how I can make my classroom more instructive and interesting. Think of this as a look into how I brainstorm – if I remember, I’ll let you know the eventual result of this consideration if/when I use it.

Today’s idea involves the Leidenfrost effect, which is what occurs when a small drop of liquid skitters around on a hot surface, such as water droplets dancing around on the top of a hot skillet. This effect is caused when the extreme heat of the surface causes the bottom layer of the droplet to vaporize, forming a vapor cushion that causes the droplet to float over the hot surface. Read more about it on Wikipedia here.

Of course, when I was in my kitchen I didn’t look up anything online to see what happens. I just looked at the little drops of water dancing around the top of the skittle and thought to myself “it’s cool that surface tension of the water keeps the little droplets together as they dance around, even though the water is at or near the boiling point.

This led to the idea of a lab or demo or something:

  • Do the demonstration with water droplets and have the students guess why the droplets hold together instead of flying apart.
  • Do the demonstration with a variety of other liquids and have the students guess why these droplets either hold together or blow apart. When surface tension comes up, see if there’s a correlation between surface tension and how much droplets hold together.

Now, there are a few things here that I feel I should mention off the bat:

  1. I don’t know that this will actually work, because I just thought of it yesterday in my kitchen. If it doesn’t work, that doesn’t necessarily mean this is a bad activity – just that this phenomenon is more complicated than just surface tension. I have to think about that a bit to see if it’s a demo-killer if that’s what happens.
  2. Coming up with other liquids might be tough. Isopropanol or ethanol are pretty good choices, but for their flammability. I suppose that if you use a hotplate (no open flame) and a very small droplet of either, it shouldn’t really be a problem. I may try this on my own to see what happens – my gut feeling is that if I prepare for the assumption that all of the alcohol in there will instantly explode in the most violent way possible, I should be OK. A fume hood seems like a good bet, with the kids standing back a little ways.
  3. How to measure surface tension? If we compare surface tension to polarity, then I suppose we’d rank the liquids by dipole moment. I hope there’s a tolerable correlation between droplet size and dipole moment.
  4. How to measure droplet cohesion? If that’s the metric that we’re using to determine how well surface tension and/or polarity holds together the droplets, we really need to find some way of figuring out how to do this. Given that the little particles skitter about so much, I’m guessing this may just have to use qualitative and relative observations rather than absolute sizes.

In any case, that’s the first musing that’s come out of my head for the coming school year. I want to again state that this isn’t something I’ve seriously researched at all, and definitely not something that I’ve played with in the lab, so any work you do in developing this should be undertaken in the knowledge that, by reading this, you’ve fully assumed responsibility for whatever happens.

If you should make this into a lab or demo, I’d appreciate it if you’d let me know so I can share it with others!

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