About neutrally buoyant particles, popcorn, and more bubbles

When you see all our pretty images of currents and swirling eddies and everything, what you actually see are the neutrally buoyant particles that get lit by the laser in a thin sheet of light. And those particles move around with the water, but in order to show the exact movement of the water and not something they are doing themselves, they need to be of the exact same density as the water, or neutrally buoyant.

But have you ever tried creating something that just stays at the same depth in water and does neither sink to the bottom or float up to the surface? I have, and I can tell you: It is not easy! In fact, I have never managed to do something like that, unless there was a very strong stratification, a very dense lower layer in which stuff would float that fell through a less dense upper layer. And in a non-stratified fluid even the smallest density differences will make particles sink or float up, since they are almost neutral everywhere… One really needs stratification to have them float nicely at the same depth for extended periods of time.

But luckily, here in Grenoble, they know how to do this right! And it’s apparently almost like making popcorn.

You take tiny beads and heat them up so they expand. The beads are made from some plastic like styrofoam or similar, so there are lots of tiny tiny air bubbles inside. The more you heat them up, the more they expand and the lower the density of the beads gets.

But! That doesn’t mean that they all end up having the same density, so you need to sort them by density! This sounds like a very painful process which we luckily didn’t have to witness, since Samuel and Thomas had lots of particles ready before we arrived.

Once the particles are sorted by density, one “only” needs to pick the correct ones for a specific purpose. Since freshwater and salt water have different densities, they also require different densities in their neutrally buoyant particles, if those are to really be neutrally buoyant…

Below you see Elin mixing some of those particles with water from the tank so we can observe how long they actually stay suspended and when they start to settle to either the top or the bottom…

Elin experimenting with the buoyancy of our particles

Turns out that they are actually very close to the density of the water in the tank, so we can do the next experiment as soon as the disturbances from a previous one have settled down and don’t have to go into the tank in between experiments to stir up particles and then wait for the tank to reach solid body rotation again. This only needs to be done in the mornings, and below you see Samuel sweeping the tank to stir up particles:

Samuel sweeping particles from the topography that sank to the bottom over night

Also note how you now see lots of reflections on the water surface that you didn’t see before? That’s for two reasons: one is because in that picture there are surface waves in the tank due to all the stirring and they reflect light in more interesting pattern than a flat surface does. And the other reason is that now the tank is actually lit — while we run experiments, the whole room is actually dark except for the lasers, some flashing warning signs and emergency exit signs close to the doors and some small lamps in our “office” up above the rotating tank.

But now to the “more bubbles” part of the title: Do you see the dark stripes in the green laser sheet below? That’s because there are air bubbles on the mirror which is used to reflect the laser into the exact position for the laser sheet. Samuel is sweeping them away, but they keep coming back, nasty little things…

Samuel sweeping particles from the topography that sank to the bottom over night

I actually just heard about experiments with a different kind of neutrally buoyant particles the other day, using algae instead of plastic. I find this super intriguing and will keep you posted as I find out more about it!

On buoyancy

This is an experiment that Martin brought to Ratzeburg and that he let me use on my blog: Using a beam balance to talk about buoyancy.

So at first we have two objects hanging on the beam balance, a heavy one with a large volume, and a lighter one with a smaller volume.

As we lower the beam balance towards the water, the large object starts floating! Whereas the other one does not.

And in fact, the small object sinks and the larger one keeps floating.

What a great experiment to talk about density and buoyancy!

Sink or swim – experiments using tin foil

A pet peeve of mine are books on handcrafts or experiments or any kind of activity that come with drawings instead of pictures, because I always suspect that it was easier to draw whatever the author wanted to show than to take a photo of it. Which, to me, suggests that it isn’t really all that easy to conduct the experiment or build the wicker basket or whatever it is you are attempting to do.

So here is an experiment that I had seen drawings of and that Martin and I went to try: on swimming and sinking.

Step 1: Take two identical pieces of tin foil.

Two identical pieces of tin foil

Step 2: Build a boat out of one of the pieces, and a ball out of the other one.

Two identical pieces of tin foil made into a boat and a ball.

Step 3: Place the boat and the ball on the water surface.

Step 4 to step 9: (And these are the steps that the nicely drawn instructions always omit) Watch the ball float on the surface. With growing annoyance, try to make the ball as compact as possible in order to make it sink.

Step 10: This is what we wanted to see after step 3 already. Even though the boat and the ball are made of identical pieces of tin foil and their mass is the same, the boat floats while the ball sinks.

A boat and a ball made of identical pieces of tin foil. Boat floats, ball sinks. Nice demonstration!

What do we learn from this? Always test experiments before using them as a demonstration, especially those that look extremely simple!

Which displaces more water, a boat with the anchor onboard or in the water?

Not that this is a big effect in the ocean, but still, it’s a nice demo.

A body submerges into the water until it displaces an amount of water that is equal to its own weight. So if you have a ship with an anchor onboard, and then you drop the anchor into the water. Will that affect the water level? And if so, how?

How much water is displaced by a boat carrying the anchor onboard vs a boat and the anchor in the water? It’s the same amount of water in the jars and the same coins in both pictures…

I like this demonstration because it is so easy and also because so few people get it right when you ask them about it (which is actually a bit shocking).

Cartesian divers – theoretical considerations

A bit more reflection on cartesian divers.

When I wrote the two previous posts, I had known cartesian divers for a very long time in many contexts, for example as something that is routinely used in primary school teaching. While I was aware that developing a correct physical description of such a diver is challenging, I assumed that everybody had an intuitive understanding of how a diver would react when pressure was applied on the bottle. To me, this is an experiment that I would use to demonstrate the different compressibilities of air and water, assuming that everybody can imagine what happens if the density of a floating body changes.
Turns out my assumption of what people intuitively understand was way off. In the paper Helping students develop an understanding of Archimedes’ principle. I. Research on student understanding”, Loverude, Kautz and Heron talk about difficulties university science majors have with hydrostatics. Of seven volunteers who were interviewed, who had all completed their instruction in hydrostatics and all reported course grades at or above the mean, all but two predicted that the diver would rise as pressure was applied to the bottle. And none of the students could account for the observation that the diver sank!
Now I’m wondering at which point the students’ difficulties arise. Is it that they don’t know about different compressibilities or is it at a much more basic level? From the study mentioned above it seems that students don’t appreciate the tiny density range (where calling it a range might already be over-stating it) in which a body can float in (non-stratified) water without swimming at the surface or sinking to the ground. In a way this makes sense – most of the time that we look at water in a way comparable to how we look at a cartesian diver (i.e. through side walls so we are looking at a depth section of a non-stratified fluid), we are actually looking at aquaria where fish float in very similar ways to the cartesian divers. But we never stop to think about how floating and adjusting depth in a fluid is actually quite an achievement. Which we see when the fish die and first float at the surface and then sink to the bottom…
In any case. If it is the case that students don’t appreciate how rare it is for something to float in a fluid, then showing a cartesian diver might even be working against us by reinforcing a perception that is harmful to the students’ future understanding of hydrostatics. Or we can use the divers in a different way – have students build them themselves, so that they need to fiddle with them to adjust their initial density until it is just right, before they start working in the way shown in the previous posts. I think this is a thought I want to develop further… So stay tuned!

Cartesian diver – organic version

Using orange peel as cartesian divers.

Guess what my mom told me when we were playing with cartesian divers the other day? That orange peel works really well as a cartesian diver! Who would have thought?

And just because we like playing we tried both orange peel and tangerine peel. Watch!

Funnily enough, they behave very differently. While the thick orange peel works really well, the much less thick tangerine peel very quickly looses all the air bubbles and hence the buoyancy and the ability to adjust buoyancy. So if in doubt (and not interested in extending the experiment to a lesson in contrast and compare) – oranges are the way to go!

Cartesian diver

Compressibility of water and air.

Today I want to talk about the different compressibilities of water and air. Actually, no, I just want to show you an experiment. One way to visualize that air is a whole lot more compressible than water is to look at cartesian divers. You probably know the fancy ones as shown on the far left of the picture below that you typically find at Christmas Markets or high-end (i.e. nerdy) toy stores.

But, as you probably guessed already, this post is about making those divers from scratch. You probably know that you could just use those old-fashioned eye-drop pipettes, or normal plastic pipettes. But how boring is that? (Plus how much material do you need when doing this experiment with a big class!) All you need is shown in the image below: Straws, scissors and paper clips.

Some of the more complicated instructions tell you to cut a piece of straw and put modeling clay on top to seal it, but I’m lazy. A much simpler version is shown here: Bend a straw, cut the long end, close the two ends together with a paper clip (also helps as added weight to adjust the buoyancy of the diver) and there you are!

How does that homemade diver dance? Watch the movie below:

So how do the classical cartesian diver compare to homemade one?

So we see that while both of them dive up and down, they don’t behave exactly the same. And if we were using this experiment in class, we would now talk about how this is due to the different volumes of air in the two divers, and the different densities of the structures themselves. But what I find much more important right now: My diver doesn’t turn as nicely as the conventional one! So what is one to do?

Exactly. Poke a hole in it. Let’s find out if that did the trick?

Almost as nice as the glass diver, no? So now start playing and send me movies of your divers! :-)