Monthly Archives: September 2014

The icy elevator

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Weird things happening when ice cubes melt.

Remember I said that there were weird and wonderful things going on when I last ran the melting ice cubes in salt and fresh water experiment? It is really difficult to see in the picture below (sorry!) but you can probably spot the ice cube floating at the surface and the melt water sinking down, inducing some turbulence? And then there is a small ice bit a bit to the right of the center of the picture. And that ice bit is floating upwards.

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Dyed ice cube floating at the surface, and small ice bits floating up

Watch the melting ice cubes video below to see all the thing in action, it is visible really well as soon as the picture is moving:

So what is going on there? I think the solution to this riddle lies in me forcing ice to freeze even though it contains more salt (or in this case, red food dye) than it is happy with. Remember how dyed ice cubes look?

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Ice cubes frozen from colored water

So basically there is dye trapped in the middle of the cube, because cooling is happening from all sides, hence ice is starting to form from all sides, pushing the dye to the center of the ice cube. In the ocean, cooling would of course only happen from above, so salt is being rejected as brine.

Anyway, since I wanted to dye the ice cubes to make things more visible for this blog, I am adding a dissolved substance to the water that would usually not be there. Hence I am making the ice slightly denser than it would otherwise be. So when small ice bits chip away from the main cube (which still contains large parts of pure fresh water ice from the sides of the cube where, during the freezing, the dye could still be rejected; and which therefore still floats), they are denser than the water and sink. But as they melt, the dye washed out, and eventually the remaining ice is fresh, hence less dense, enough to float up again.

The whole thing looks pretty fascinating.

What do you think, is that the correct explanation? Or can you come up with a better one? Let me know!

P.S.: Everybody I showed this video to was fascinated by how the little piece of ice is floating up. But what I find a lot more fascinating is how it came to be at the bottom of the beaker in the first place! After all, ice is supposed to float on water (or drift up again if pulled down and then released) but how did it get down there???

Melting ice cubes reloaded

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Or why you should pay attention to the kind of salt you use for your experiments.

The melting ice cubes in salt and fresh water is one of my favorites that I haven’t written about in a long time, even though (or possibly: because) I wrote a whole series about it last year (see links at the end of this post).

Now that the EMSEA14 conference is almost upon us and Kristin and I busy preparing our workshop, I thought I’d run the experiment again and – for a change – take the time to finally know how much time to schedule for running the experiment. This is the experiment that I have run most often of all in all kinds of classes, but there you go… Usually I have more time than just 30 minutes, and there is so much other content I want to cover in that workshop!

There are a couple of things that I learned running this experiment again.

  • It takes at least 10 minutes to run the experiment. My water was slightly colder than usual room temperature, my ice cubes slightly smaller, though. And those 10 minutes are only the time the ice takes to melt, not the time it takes to hand out the materials and have the groups settle down.
  • There is a reason it is always recommended to use kosher salt for these kind of experiments. Look at the picture from one of the old posts in comparison to the ones from today: The iodized salt containing folic acid I had in my kitchen dissolves into really milky water. I really should have walked the two extra meters to get the good salt from my oceanography supplies in the other room!
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Ice cubes melting in fresh water (left) and salt water (right) – old experiment

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Ice cubes melting in fresh water (left) and salt water (right) – experiment today

  • Some food dyes are the devil. My whole kitchen is red. Plus the ice cubes didn’t freeze nicely (for a post on ice cubes freezing from salt water click here), the ice chipped when I tried to get the cubes out of the ice cube tray. I definitely can’t have that mess at a workshop. So here is another argument for using non-dyed ice cubes! The more important argument being that you think more if the cubes are not dyed and you don’t immediately see the explanation…

But it is always a fun experiment to run, and there are always new things to spot. Watch the video below and see for yourself! (Explanations on the weird phenomena coming up in a future post!)

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The links to the “melting ice cubes” series:

Ice cubes melting in salt water and freshwater (post 1/4)

Ice cubes melting in fresh water and salt water (post 2/4)

Melting ice cubes – one experiment, many ways (post 3/4)

Melting ice cubes – what contexts to use this experiment in (post 4/4)

Other posts on this experiment:

Dangers of blogging, or ice cubes melting in fresh water and salt water

Guest post: The mystery of the cold room

Why talking to your neighbor might help more than listening to the lecturer

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Why does learning through peer instruction work?

As you might have noticed by now, I’m a big fan of concept questions combined with “talk to your neighbor” peer instruction. And studies show that talking to your neighbor is often more successful in teaching you new things than listening to the lecturer is.

In their paper “Why peer discussion improves student performance on in-class concept questions“, Smith et al., Science (2009), try to separate two possible reasons for the success of peer instruction: Does learning gain through PI result from gains in understanding during discussion, or simply from peer influence of knowledgeable students on their neighbors?

In order to separate those effects, the authors first ask a multiple choice question, let the students vote, use peer instruction, and let students vote again. They then ask a very similar question, which students who didn’t vote correctly the first time for the first question likely wouldn’t be able to answer correctly, either. So if those students answer correctly now, that supports the idea that they gained understanding during discussion rather than being just influenced by the knowledgeable students in the previous case. And their data shows that the third vote consistently gives better results than the first vote, and, surprisingly, often even better results than the second vote after peer instruction.

The power of increasing understanding through conversations with the neighbor is also supported by 47% of students disagreeing with the statement “When I discuss clicker questions with my neighbors, having someone on the group who knows the correct answer is necessary in order to make the discussion productive”. Discussing concepts seems to be the key, not being convinced by someone more knowledgeable.

Learning with fluid toys

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How fluid toys can be used to demonstrate principles of fluid mechanics.

I guess every attempt to hide that I LOOOOVE fluid toys of any kind is futile. So imagine my excitement when my colleague sent me an article titled “Serious Fun: Using Toys to Demonstrate Fluid Mechanics Principles” by Saviz and Shakerin (2014). While their ideas are not really applicable to the kind of courses I usually teach, it is refreshing to see them embrace fluid toys in teaching, and it made me realize that I didn’t post movies that I made of toys that my sister gave me and my dad for our Birthdays back in May.

If you fancy seeing this thing in motion, go watch the videos below!

Creating a continuous stratification in a tank, using the double bucket filling method

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Because I am getting sick of stratifications not working out the way I planned them.

Creating stratifications, especially continuous stratifications, is a pain. Since I wanted a nice stratification for an experiment recently, I finally decided to do a literature search on how the professionals create their stratifications. And the one method that was mentioned over and over again was the double bucket method, which I will present to you today.

Two reservoirs are placed at a higher level than the tank to be filled, and connected with a U-tube which is initially closed with a clamp. Both reservoirs are filled with fresh water. To one of the buckets, salt is added to achieve the highest desired salinity in the stratification we are aiming for. From this bucket, a pump pumps water down into the tank to be filled (or, for the low-tech version: use air pressure and a bubble-free hose to drive water down into the tank as shown in the figure above!); the lower end of the hose rests on a sponge that will float on the water in the tank. When the pump is switched on (or alternatively, the bubble-free hose from the reservoir to the tank opened), the clamp is removed from the U-tube. So for every unit of salt water leaving the salty reservoir through the hose, half a unit of fresh water flows in to keep the water levels in both reservoirs the same height. Thus the salt water is, little by little, mixed with fresh water, so the water flowing out into the tank gets gradually fresher. If all goes well, this results in a continuous salinity stratification.

Things that might go wrong include, but are not limited to,

  • freshwater not mixing well in the saline reservoir, hence the salinity in that reservoir not changing continuously. To avoid that, stir.
  • bubbles in the U-tube (especially if the U-tube is run over the top edges of the reservoirs which is a lot more feasible than drilling holes into the reservoirs) messing up the flow. It is important to make sure there is no air in the tube connecting the two reservoirs!
  • water shooting out of the hose and off the floating sponge to mess up the stratification in the tank. Avoid this by lowering the flow rate if you can adjust your pump, or by floating a larger sponge.

P.S.: For more practical tips for tank experiments, check out the post “water seeks its level” in which I describe how to keep the water level in a tank constant despite having an inflow to the tank.

Making science topics relevant to students’ lives increases interest and performance

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Duh!

That students are more interested, and hence perform better, when they are motivated to learn something sounds not terribly surprising. But did you know that you can actually increase motivation by making the students write about the relevance of the topics you are teaching?

In the study “Promoting Interest and Performance in High School Science Classes” by Hulleman and Harackiewic (2009), 262 high school students taught by seven science teachers were randomly assigned one of the following tasks, to be conducted periodically throughout the semester: either to summarize the content of the lessons, or to write about the usefulness of the course material in their own lives.

At the end of the school year, the authors of this study found that the grades of students writing about the relevance of the material to their own lives were on average a full grade point higher than those of the students only summarizing the material. This effect was especially large for students with low expectations of performing well in class.

Yes, this was only one study on a limited number of high school students and those results are not directly transferable on every other course. But they seem significant enough to warrant considerations in the way we plan our courses. Writing more always seems to be a good idea (at least in the field I teach in). But if tweaking the writing assignment just this tiny little bit can have such an effect on learning outcomes, why not just tweak it and make students think about the relevance of course content in their lives?

What are the ingredients of a rainbow?

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Still collecting materials for our instructional short movies.

A while back I talked about how my colleague and I were experimenting with short instructional screen casts, and I shared some first attempts at movies on how rainbows form. We are still working on a story board for an improved version, but I was lucky enough to see a very pretty rainbow in a fountain the other day.

The picture below is a good demonstration of how rainbows form where there are water droplets in the air (provided there is enough sunlight, too, and we are watching from the right position) – we still see a bit of the rainbow to the right of the fountain, even though the wind direction has changed and the fountain is now blown to the left, visible because of the mist and the lower part of rainbow.

Fascinated as I was I had to film clips of this, too, which are combined in the movie below. There you see the rainbow appearing and disappearing, depending on where the fountain is moved by the wind, i.e. whether it is moved to the part of the sky where all the angles are right for us to see a rainbow, or not.

It was a magical moment – enjoy! :-)

The effects of rotation on a collapsing column

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Comparing a rotating and non-rotating dipole.

I just realized that I never explicitly showed the difference between rotation and no rotation, even though I do have the footage to do so: Two experiments set up to create a monopole, which both turned dipole.

In the non-rotating experiment (which was, by the way, set up carefully in preparation for a rotating experiment, but then the v-belt on the rotation table failed [but luckily this was on the last night of the JuniorAkademie, so we had otherwise run everything we had been planning to run], so we ended up with a non-rotating experiment), the dipole shown below develops within seconds of the central dense column being released.

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A dipole created by releasing a column of dense water in the middle of a non-rotating tank.

In the rotating experiment, however, this is what the dipole looks like after a similar amount of time:

And we see that in the non-rotating case, the eddies are spreading to fill the whole width of the tank within seconds, whereas in the rotating case the eddies mainly stay confined into their respective columns. This is the often quoted phenomenon of conservation of vorticity in a rotating system, where movements happen mostly in the horizontal plane, whereas in non-rotating system, vertical movements happen easily, too (i.e. the dense water from the upper part of the initial dense column can sink to the bottom of the tank in this case, which it could not do in the rotating case), and turbulence can hence develop in 3D and not only 2D.

For videos of both experiments, please check out the posts on the rotating case and the non-rotating case.

Simulations of hetonic explosions

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Because sometimes it’s easier to control a computer than rotation, salinity, water and dye.

After looking at a non-rotating cylinder collapse the other day, it is time to look at proper hetonic explosions (you know? The experiment on the rotating tank where a denser column of water at the center of the tank is released when the whole tank has reached solid body rotation). In Bergen, we used to show this experiment as a “collapsing column” experiment, the tilting of a frontal surface under rotation. For those cases, all the parameters of the experiment, e.g. the rotation rate, the density contrast, the water height, the width of the cylinder, were set up such as to ensure that one single column would persist in the middle of the tank. At JuniorAkademie, we’ve also run it in other setups, to form dipoles or quadrupoles. For a real hetonic explosion, we would typically go for even more eddies than that.

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Students watching the experiment shown below. We put paper on the outside of the tank because all the feet swiping past are kind of distracting on the movie later, but that is obviously really annoying for live observers. But in our defense – we only did this once for one experiment late one evening, and didn’t expect so many people to be interested in the experiment! Plus they got to watch on the tablet which showed the top-camera’s view via WiFi… ;-)

But if you read through all those posts then, you might remember that I’ve been complaining about how it is really difficult to set up an experiment in such a way that you have total control over the amount of vortices that form. Firstly, because the system is inherently chaotic, but let’s forget about that for a minute. But then because the calculations aren’t that easy for school kids to do, and then even when everything is calculated correctly, water has to be prepared with the correct salinity, the rotation has to be set to the correct period, the cylinder has to be completely centered in the tank, the water level has to be just right and when the cylinder is pulled up, this has to happen with a swift movement as to add as little disturbance as possible. Not an easy task, especially when there is a camera mounted on the tank!

To show us what to expect, Rolf did some model simulations for us. This is what a monopole looks like:

Shown is an isoline in density, separating the dense water below from the lighter water above. Superimposed are the horizontal velocities, so you get a sense of the rotation.

For more advanced experimentalists to recreate, here a dipole:

As for the monopole, you see chimneys that are open on top. That is because the density is higher than the one of the isoline inside the eddy, so you get the impression that you can look inside.

But the picture is different for quadrupoles, here the four eddies (that form when the central column breaks up) do not reach the water surface any more, hence they appear closed in the visualization below.

Btw, the time is of course not measured in weekdays, that’s just a glitch in the visualization that we didn’t fix.

Seeing the simulated situations for the three cases above was quite comforting  after having run this experiment a couple of times. When you run the experiment in a tank, there is always a lot of turbulence that you wish wasn’t there. But it really helps to keep your expectations in check when you see that in the simulation there are always little vortices, trying to break away from the main ones, too, and that that is how it is supposed to be.

So now for an attempted experimental monopole, which turned out as a dipole due to turbulence introduced when removing the cylinder, similarly to what happened to us in the no-rotation collapsing column experiment.

When you watch the side views closely, you can see that the tank appears to be wobbling (which we usually can’t see, because this is the only time we taped a camera to the side of a tank – usually when filming from the side, I film from outside the rotating system, holding the camera in my hand). You see it most clearly when the yellow dye crystals are added – the water is sloshing back and forth, and that is most definitely not how it is supposed to be. Oh, the joys of experimentation! But what is pretty awesome to see there is how the vertical dye streaks get pulled apart into sheets as they get sucked into the vortices. Reminds me of Northern Lights! :-)

Collapsing column

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Or: This is what happens to a hetonic explosion experiment without rotation.

I’ve posted a lot while at JuniorAkademie a while back, so it is hard to believe there are still experiments from that time that I haven’t shown you. But I’ve probably only shown you about half the experiments we’ve done, and there are plenty more in the queue to see the light of day on this blog!

Today I want to talk about hetonic explosions (you know? The experiment on the rotating tank where a denser column of water at the center of the tank is released when the whole tank has reached solid body rotation). In Bergen, we used to show this experiment as a “collapsing column” experiment, the tilting of a frontal surface under rotation. For those cases, all the parameters of the experiment, e.g. the rotation rate, the density contrast, the water height, the width of the cylinder, were set up such as to ensure that one single column would persist in the middle of the tank. At JuniorAkademie, we’ve also run it in other setups, to form dipoles or quadrupoles. For a real hetonic explosion, we would typically go for even more eddies than that.

Today I want to show you this experiment under very special conditions: The no rotation case!

For all of you oceanographers out there who know exactly what that experiment will look like, continue reading nevertheless. For all of you non-oceanographers, who don’t know why some oceanographers might be disappointed by this experiment, continue reading, too!

You see, one of the fundamental assumptions we often make when teaching is that what is exciting to us, the instructor, is exciting to the students, too. And the other way round – that experiments that we might find boring will be boring the students, too. But I often find this to be completely wrong!

In case of the hetonic explosion experiment with no rotation, the experts know what will happen. We pull out the cylinder containing the denser water, so the denser water column will collapse and eventually form a layer of denser water underneath the rest of the water. We know that because we are aware of the differences between rotating and non-rotating systems. However, many students are not. And if you don’t have a strong intuition of how the water will behave, i.e. that in this case you will eventually have two layers, rather than a dense column surrounded by lighter water, it is not terribly exciting when you finally do the rotating experiment and – contrary to intuition – the dense water does not end up below the lighter water. So in order to show you in my next post what to be excited about, today I am showing you the normal, non-rotating experiment:

But note that the experiment is not nearly as boring as you might have thought! We had put a lot of vaseline at the bottom of the cylinder to prevent the denser water from leaking out, so when the cylinder was pulled up, it gave an impulse to the dense column, which ended up splitting up into a dipole upon hitting the wall of the tank. Still looks pretty cool, doesn’t it? And for this to be a good teaching video, I really should have continued filming until the layers had settled down. In my defense I have to say that we had a second experiment set up at the other rotating table that we wanted to run, so I had to get the cameras over to the other table… And you’ll see those movies in my next post!