# 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!

# Getting rid of bubbles in our jet

Sometimes the devil is in the details…

On our first day at the Coriolis platform in Grenoble, I took a picture of the “source” in our experiments (see above): The plastic box that is fed by a hose from above and that has one open side with a “honeycomb” (or: a make-the-outflowing-water-nice-and-laminar thingy, technical term) that introduces the water into the tank that we want to follow around Antarctica.

This source is sitting against our topography, and will be partly submerged so that we introduce the jet at water level and below (instead of having a waterfall going in). The idea is to get a nice and bubble-free flow because — as we talked about yesterday — bubbles reflect the laser very strongly and disguise the signal that we are actually interested in.

So when we were doing our first tests last night, the first step was to flush out all the bubbles from all the pipes and hoses that supply the water to our source. Except that bubbles kept coming. And coming. Until, at some point, we realised that this was the problem:

Sketch of “the bubble issue”

The inflowing water was free-falling through air before hitting the water inside the source box, thus entraining a lot of air bubbles directly inside of the source. Good luck flushing them out… The solution was to add an extra piece of hose to just below the water surface so no air can be entrained.

So when we arrived at the lab this morning to an empty tank* we were delighted to see that the amazing Samuel and Thomas had already fixed the source!

“Antarctica” in the tank which is empty yet again…

*Yes, the tank really was empty again. Turns out that the reflection of the laser off the topography is so strong that it’s both a problem for data quality and that too much of the light gets scattered out of the water to be safe when we use the laser at it’s real setting for the experiments rather than at the super low setting we used for the tests… Disappointing, yes, but we were so surprised and pleased when we arrived this morning and the topography had already been painted AND the source  been fixed! We are super impressed with and grateful to the awesome team here in Grenoble! :-) And we are happy to report that there is water in the tank again and we can start measuring after lunch!

# A string of bubbles

Have you ever noticed champagne bubbles that form as a string right in the middle of the glass and hardly anywhere else? This leads to the very cool pattern you see here:

Astrid and I recently happened to notice how differently bubbles in champagne and in mineral water behaved. In the mineral water, bubbles formed in random spots along the sides of the glass. In the champagne, they mainly formed in the middle; and formed a string of rapidly forming bubbles.

So now I was hoping for a really interesting explanation of why the bubbles behave so differently. They form at different rates, but that makes sense if the partial pressure of CO2 in both drinks is different. After a bit of research on the web it turns out that fancy champagne glasses have tiny scratches right in the center of the glass to serve as condensation nuclei — in other words: to cause exactly what we observed: A nice string of pearls instead of bubbles forming randomly along the sides of the glass. So theoretically, if we had had our mineral water from the same glasses, we would have observed the same thing in mineral water. What a disappointing explanation!

[vimeo 146250051]

# Reading the water

Just because it’s fun! :-)

I’ve mentioned before that I tend to stare at water when nobody else seems to find anything interesting to look at. So just because I’m weird, let’s look at some more water.

For example here. What could have caused waves like those below?

What could have caused this pattern?

Yes. These guys went past and what we see are both the circular waves caused by the oars and the stern wave of the boat.

Rowing boat. Seriously, why would anyone want to go backward all the time???

Ok. So on to the next riddle: What could cause what we see below?

Bubbles on water. What could have caused them?

Right, that was him:

Alsterdampfer!

And this?

More waves.

Yes! Him again!

Alsterdampfer.

Does anyone see where we are going with this?

Correct. Here.

Hamburg town hall.

And a last glimpse on the way back:

Lombardsbrücke.

Isn’t this the most beautiful city in the whole wide world? :-)

# Bubble size depending on pressure

More playing with a vacuum pump.

In this post, we talked about how decreasing the pressure on water can make dissolved gases come out of solution. But what happens if you suddenly increase the pressure again?

This is the same movie as in the previous post, just to remind you of what we did: We decreased the pressure and then let it increase again quickly (you hear the ssssssssssss when the air is streaming back into the bottle).

So to show it in one picture, what happens is basically this:

Bubbles under low pressure (top) and high pressure (bottom). Screen shots from the movie above.

The lower the pressure, the larger the bubbles. When you let the air back into the bottle, the bubbles collapse (or shrink, if you want to be less dramatic).

That reminds me that I really need to film a movie similar to the one below where one can clearly see how bubble size increases the closer the bubbles come to the surface.

Isn’t it awesome to realize that the more you film and write and think about adventures in oceanography and teaching, the more ideas you have of what you want to do next? :-)

# Gases dissolved in water

A simple experiment to show that there are really gases dissolved in water.

Luckily, my parents like to play at least as much as I do. So when I got back from doing “real science” in Bergen the other day, they picked me up at the airport and showed me their latest toys: Vacuum pumps! [edit: Not really vacuum vacuum, but at least much lower than atmospheric pressure. And apparently those pumps are sold with the original purpose of re-sealing wine bottles]

Vacuum pumps are great to show that there are actually gases dissolved in water, because oftentimes that isn’t all that obvious. But when the pressure of the head space of a bottle is decreased, gases that were happily dissolved under atmospheric pressure start coming out of solution.

Gas being bubbled out of water by decreasing the pressure of the head space of the bottle.

Here is a comparison of normal tap water and sparkling water (sparkling water obviously containing much more dissolved CO2 than tap water, hence more bubbling).