Tales from the Prep Room: Making Sand Swim

Today, I'm going to be doing a demo with a pretty classic piece of gear that we know has been around at least since Lawrence Bragg used it in the early '60s and probably even a bit longer than that. So it's up there, which means I'm going to have to get my ladder out. So while I do that, have a look at the archive.

[VIDEO PLAYBACK]

-Here's a real big pot of sand. It's behaving like a solid. I've got two balls resting on it quite happily. They're really heavy balls.

-And as you'd see at the moment it's quite solid, about to be fluidized. Strange things happen. We now have a bath.

[APPLAUSE]

-If, in fact, if you do this on the Sahara desert, you could float boats across the Sahara desert.

-If only we could get enough air blowing up under the Sahara, we could sail across it in the Queen Mary. And we'd have a real ship of the desert instead of having to fall back on a camel.

[END VIDEO PLAYBACK}

OK, this is it. This is called a fluidised bed. And as you can see, it's just a little tray with a few kilos of really fine sand in it. There's one more thing I need to make this work. And that's some air. I don't think I got quite enough in here. So I need to go and get some more.

OK, so that's everything set up. Let's have a bit of a tidy up. Get these out of the way so they are not distracting. OK, so what I'm going to do is to pass the compressed air from the cylinder up through the sand from underneath. So let's get this air turned on. And you can see pretty much straight away that the sand, first of all, kind of inflates the, so the air, rather, inflates the sand. It's already gone up an inch or so.

But more than that it's doing something pretty funny. It's kind of turning it, basically, turning it into a liquid. So it will slosh around all over the place. This process is called fluidisation. And to show you that it is a lot like a fluid you can bob things around. It's really nice. You can see the ripples moving out just like on the surface of a pond. It's beautiful. Chuck a few ducks in there - see how they get on. They tend to capsize. Yeah, there we go. A little boat. Give the whole thing a bit of a stir.

So what's going on here is that as the air bubbles up through the sand it sort of gets in between all the sand particles and kind of stops them sticking to each other and just sort of lubricates the whole mixture. So the sand particles are able to flow over each other interspersed with the air. So the whole lot behaves like this two-part two-component fluid. Oop, yeah, sufficiently like a fluid that the boat has capsized. I don't think any of these things are really very water tight. This is now completely full of sand.

So the way that we've used the fluidised bed at the Ri over the years it's quite often to sort of make an analogy between a solid melting into a liquid. It's a slightly abstract analogy I think in that the sand, if I just turn this off, the sand, when it's still, isn't a great analogy for a real solid. It's not really a great analogy for anything except sand to be honest. So you need to be a bit careful how we use that analogy. But as long as we are careful I think it's worth doing. And it's just a lot of fun to play with. So I'm going to turn it back on again. See how long I can keep my boat afloat.

Yeah, it's not very good for asthmatics.

So there you go. That's sort of a solid behaving a lot like a fluid.