Tag Archives: kitchen oceanography

Stuff you can (and should!) observe in your kitchen: circulation in the water when boiling eggs

Now that I have introduced the new tag “kitchen oceanography: food related” to my blog, it’s time to add a couple new posts to that category. And today I have a fun post for you!

But first, what does “kitchen oceanography” even mean?

Kitchen oceanography

/ˈkɪtʃɪn ˌəʊʃəˈnɒɡrəfi/
noun

The benefits of “kitchen oceanography”

It’s pretty apparent why “kitchen oceanography” is a great alternative to regular tank experiments: because you can do it with stuff you have at home rather than needing access to a lab with a tank, and then a lot of water, salt, dye, other resources to conduct the experiments. Doing kitchen oceanography, we use a minimal amount of resources.

But the second, even larger benefit to me is that you can do these kinds of experiments and observations basically everywhere and at any time. So you can fit in a quick session of kitchen oceanography while sitting in front of the fire place on a skiing trip with friends, or while doing the dishes with your godchild. And you can inspire others who might not have access to labs to still do cool oceanography experiments, at home or wherever they like!

Kids who have cooked with their parents are more likely to be interested in STEM

Apparently, the biggest predictor of future interest in STEM topics is whether people as kids often cooked with their parents! No literature source for this, but that’s what my educational research colleagues next door told me… So playing in the kitchen, whether on kitchen oceanography or with food, is a good thing!

It’s not like watching paint dry: Observing boiling eggs

Observing boiling eggs might not sound like a super exciting activity to engage in, but sometimes it is. Last year we did observe interesting foam pattern when boiling eggs (I still can’t explain where the foam is coming from! Can you?).

Foam pattern in a pot of boiling eggs

Foam pattern in a pot of boiling eggs. P.S.: The “black egg” sings different songs to let me know how hard-boiled my eggs are at any given moment. I love this because they are songs I learned from my godson and it always reminds me of him and his family :-)

Foam pattern show circulation within the pot

The pattern in the foam show the convection pattern of the boiling water around the eggs which act as obstacles. Water is raising from the bottom of the pot to its surface, bringing up foam. But the eggs are located so close below the water’s surface that the circulation above them (if there is one) is pretty much disconnected from the convection happening all around the eggs.

But then if you throw out the water…

Limescale deposits at the bottom of an empty pot after boiling eggs in it

Limescale deposits at the bottom of an empty pot after boiling eggs in it

Even the empty pot still shows you what the circulation pattern must have been like!

But then the next cool thing happens when you throw out the water: There are limescale crystals on the bottom of the pot! And, interestingly enough, they show the former locations of the eggs. And I think they are forming in exactly those spots because just as there is (hardly any) circulation above the eggs, the circulation below is also inhibited, water has longer residence time (because it isn’t whipped away by convection) and those crystals can form.

An alternative explanation might be that there is more limescale below the eggs because calcium carbonate gets dissolved from the egg shells and gets deposited as limescale right below the eggs because the concentration is highest closes to the eggs.

Which explanation do you think is more likely? Or do you have another one entirely?

Layered latte: A great real-life example of double-diffusive mixing!

Sometimes sitting in a café for a work meeting with #lieblingskollegin Julia can lead to unexpected discoveries of oceanographic processes — in my latte! It’s those little things that inspire blog posts…

“Kitchen oceanography” brings the ocean to your house or class room!

Oceanography is often taught in a highly theoretical way without much reference to students’ real life experience. Of course a sound theoretical basis is needed to understand the complexity of the climate system, but sometimes a little “kitchen oceanography” — doing experiments on oceanographic topics with household items — goes a long way to raise interest in the kind of processes that are not easily observed in the real world. I’ve previously written a lot about simple experiments you can perform just using plastic cups, water, ice cubes, and a little salt. But sometimes it’s even easier: Sometimes your oceanography is being served to you in a cafe!

Oceanic processes can be observed in your coffee!

Have you ever looked at your latte and been fascinated by what is going on in there? Many times you don’t just see a homogenous color, but sometimes you see convection cells and sometimes even layers, like in the picture below.

Layers in a latte.

Layers in a latte.

But do you have any ideas why sometimes your latte looks like this and other times it doesn’t?

When you prepare latte in the right way, many layers form

Layers forming in latte (and in the ocean or in engineering applications) are an active research field! In the article “laboratory layered latte” by Xue et al. (2017), the authors describe that the “injection velocity” of espresso into the warm milk has to be above a critical value in order for these pretty structures to form in a latte. They even provide a movie where you can watch the layers develop over a period of several minutes.

The homogeneous layers with sharp boundaries are caused by double-diffusive mixing

Double-diffusive mixing, which is causing the formation of these layers, is the coolest process in oceanography. In a nutshell, double diffusive mixing is caused by two properties influencing density having different rates of molecular diffusion. These different rates can change density in unexpected ways and an initially stable stratification (high density at the bottom, low density on top) can, over time, become statically unstable. And static instability leads to adjustment processes, where water parcels move in order to reach the position in the fluid where they are statically stable — the fluid mixes.

Layers in half a glass of latte.

Layers in half a glass of latte.

But there are more fascinating things going on with the latte. Would you expect this stratification to remain as clearly visible as it is in the picture above even though the glass is now half empty? I did not! And then check out what happens when you move the glass: Internal waves can travel on the boundaries between layers!

You can use this in class to teach about mixing!

Mixing in the ocean is mostly observed by properties changing over time or in space, and even though (dye) tracer release experiments exist, they are typically happening on scales that provide information on the large-scale effects of mixing and not so much on the mixing itself. And they are difficult to bring inside the classroom! But this is where kitchen oceanography and experiments on double-diffusive mixing come in. If you need inspiration on how to do that, I’ve recently published an article on this (unfortunately only in German), but there are plenty of resources on this blog, too. Or shoot me an email and we’ll talk!

P.S.: Even though the coffee company is displayed prominently in the pictures above, they did not pay for my coffee (or anything else). But if they’d be interested and make me a good offer, I’d definitely write up some fun stuff on learning oceanography with coffee for them ;-)

Experiment: Double-diffusive mixing (salt fingering)

On the coolest process in oceanography.

My favorite oceanographic process, as all of my students and many of my acquaintances know, is double-diffusive mixing. Look at how awesome it is:

Double-diffusive mixing happens because heat and salt’s molecular diffusion are very different: Heat diffuses about a factor 100 faster than salt. This can lead to curious phenomena: Bodies of water with a stable stratification in density will start to mix much more efficiently than one would have thought.

In the specific case of a stable density stratification with warm, salty water over cold, fresh water, finger-like structures form. Those structures are called “salt fingers”, the process is “salt fingering”.

IMG_4233_sehr_klein

Salt fingering occuring with the red food dye acting as “salt”.

Even though salt fingers are tiny compared to the dimensions of the ocean, they still have a measurable effect on the oceanic stratification in the form of large-scale layers and stair cases, and not only the stratification in temperature and salinity, but also on nutrient availability in the subtropical gyres, for example, or on CO2 drawdown.

Over the next couple of posts, I will focus on double diffusive mixing, but less on the science and more on how it can be used in teaching. (If you want to know more about the science, there are tons of interesting papers around, for example my very first paper)

How to easily set up the stratification for the salt fingering process.

Setting up stratifications in tanks is a pain. Of course there are sophisticated methods, but when you want to just quickly set something up in class (or in your own kitchen) you don’t necessarily want to go through the whole hassle of a proper lab setup.

For double diffusive mixing, there are several methods out there that people routinely use.

For example the hose-and-funnel technique, where the less dense fluid is filled in the tank first and then the denser fluid is slid underneath with the help of a hose and a funnel. And a diffuser at the end of the hose. And careful pouring. And usually a lot more mixing than desired.

Or the plastic-wrap-to-prevent-mixing technique, where the dense fluid is put into the tank, covered by plastic wrap, and then the lighter fluid is poured on top. Then the plastic wrap is removed and by doing so the stratification is being destroyed. (No video because I was frustrated and deleted it right away)

Or some other techniques that I tried and didn’t find too impressive. (No videos either for the same reason as above)

But then accidentally I came across this method (as in: I wanted to show something completely different, but then I saw the salt fingers and was hooked):

Granted, this is not a realistic model of an oceanic stratification. But as you can see towards the end of that movie, that turns out to be a blessing in disguise if you want to talk about the process in detail. As you see in the movie, the salt fingers inside the bottle are much smaller than the salt fingers outside the bottle. Because, clearly, inside the bottle the warm water is cooled both at the interface with the cold water inside the bottle, and by heat conduction through the walls of the bottle, since the water is surrounded by cold water. The warm water that flowed out of the bottle and up towards the water’s surface is only cooled at the interface with the water below (the air above is warmer than the cold water). So this gives you the perfect opportunity to discuss the scaling of salt fingers depending on the stratification without having to go through the pains of actually preparing stratifications with different gradients in temperature or salinity.

IMG_9084

Self-portrait with salt fingers :-)

In my experience, the best salt fingers happen when you use hot water with dye (as the warm and salty top layer) and cold fresh water below. Salt fingers develop quickly, you don’t have the hassle of hitting the exact temperatures or salinities to make the density stratification statically stable, yet unstable in salinity, and it ALWAYS works.

 

IMG_9079

Double-diffusive mixing. Scale at the bottom is centimeters.

 

IMG_9054

Salt fingering in a tank. Scale at the bottom is centimeters.

And look at how beautiful it looks! Do you understand why I LOVE double diffusion?

P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 4th, 2015.

I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.

Guest post: Using seawater to make bread!

Last week I got one of the coolest emails I have ever received: Someone had found my blog while googling for the salt content of seawater in order to use it to make bread, and he sent me a couple of pictures the resulting bread! Of course, I asked if I could share it as a guest post on my blog, so here we go (Thanks, Martin Haswell, for this unique and inspiring contribution! See, everybody? Real-world impact of science blogging!):

Making bread using seawater

There is nothing like a challenge from your best friend, to do something that you’ve never done before but might just work. In my case, make bread using sea water.

My friend Mandy had brought me back from New York a copy of Jim Lahey’s book “My Bread”. Jim’s ‘no-knead’ method of bread making uses flour, water, salt (normally) and a tiny amount of yeast – and a lot of time, but no kneading. The dough is left for a long time to rise and is baked very very hot, and makes a tasty and crusty loaf.

Jim has a recipe in his book called  “Jones Beach Bread” in which he uses seawater instead of house water plus salt to make the dough. Knowing that we both used the ‘no-knead’ recipe and that I had access to a beach with clean water, Mandy challenged me to follow this recipe, and this is how it went.

Martin collecting seawater on the beach, far enough out to miss most of the turbidity

Martin collecting seawater on the beach, far enough out to miss most of the turbidity

Martin checking the seawater sample for sand or other impurities

Martin checking the seawater sample for sand or other impurities

Jim Lahey’s book “My Bread” that contains Jim’s 'no-knead' method of bread making used for the bread in this blog post

Jim Lahey’s book “My Bread” that contains Jim’s ‘no-knead’ method of bread making used for the bread in this blog post

Waiting for the bread to raise

Waiting for the bread to raise

The finished result! Doesn't it look delicious?

The finished result! Doesn’t it look delicious?

The bread tasted very good, crusty and tasty. I made two loaves, one with the seawater filtered through a coffee filter and the other with unfiltered seawater. Normally this recipe needs around 12-18 hours rising time but this took 28 hours for the two risings, but it is winter in southern Brasil (Florianópolis, on the coast) and the day temperature was only 72F (22°C) on the day of the experiment. It’s also possible that the greater proportion of salt might have hindered the development of the yeast and held back the rise. This wasn’t a very scientific experiment.

I calculated that Lahey’s original no-knead’ recipe calls for 8g salt to 300g of water which makes 26.66g per litre, whereas sea water (according to Mirjam’s 2013 blog is 35g/litre so this should mean that the sea bread loaf should be around 30% more salty than normal; if I’m honest, it didn’t tasty significantly more salty).

Further experiments: the obvious test would be a sea water loaf vs conventional made, risen and baked at the same time.

Notes:

The Jones Beach in Jim’s recipe is the Jones Beach State Park on Long Island, New York State. The current water cleanliness data is here (PDF), scroll down for the Jones Beach SP results.

The beach that I collected my sea water from is currently ‘própria‘ but I wouldn’t collect after heavy rain (runoff) or heavy seas (turbidity).  As a safety precaution one could boil the sea water and let it cool just enough before using. In fact, when the weather is cold, that would be the best way of giving the bread a good start.

[note by Mirjam: I’ve done a super quick google search and it looks like typical salinities for the Florianopolis area can go down to 30-ish and thus be lower than the typical, open ocean value of 35, but during summer they might go up to 37 (Pereira et al., 2017) but in addition to the seasonal changes, your salinity probably depends very much on which beach you took the water sample at (for example if it was a lagoon-ish beach with a lot of freshwater runoff and not so much mixing with the open ocean). Since you collected the water fairly close to the beach and during winter, it’s likely that the salinity wasn’t quite as high as the 35 I mentioned (which would explain why the bread didn’t taste as salty as you might have expected). If you wanted to know the exact salinity next time you are making bread, an easy method to measure the salinity of sea water would be to boil a liter until all the water has evaporated and weigh the remaining salts. This isn’t very precise for oceanographer-standards, since some of the substances that oceanographers include in their measure of “salinity” in sea water at normal temperatures might actually evaporate with the water, but since the largest constituent of the “salt” in sea water is just normal NaCl, the mistake you’d be making is probably small enough for cooking purposes, and you’d get a general idea of how “typical” your sample is in terms of seawater salinity.]

Bio:

Martin Haswell is an English photographer who loves travel and making bread.

Experiment: Temperature-driven circulation

My favorite experiment. Quick and easy and very impressive way to illustrate the influence of temperature on water densities.

This experiment is great if you want to talk about temperature influencing density. Although it doesn’t actually show anything different from a temperature driven overturning experiment, where circulation is determined by hot water rising and cold water sinking, somehow this experiment is a lot more impressive. Maybe because people are just not used to see bottles pouring out with the water coming out rising rather than plunging down, or maybe because the contrast of the two bottles where one behaves exactly as expected and the other one does not?

Anyway, it is really easy to do. All you need is a big jar and two small bottles. Cold water in one of the small bottles is dyed blue, hot water in the other small bottle is dyed red. Both are inserted in the jar filled with lukewarm water (movie below).

Using bottles with a narrower neck than mouth is helpful if you want to use the opportunity to talk about not only temperature-driven circulation, but also about double-diffusive mixing (which you see in form of salt fingers inside the red bottle in the picture above).

Isn’t this beautiful?

P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on December 2nd, 2015.

I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.

Tides themselves don’t induce (a lot of) mixing, only tides hitting topography do. An experiment.

As you might have noticed, the last couple of days I have been super excited to play with the large tanks at GFI in Bergen. But then there are also simple kitchen oceanography experiments that need doing that you can bring into your class with you, like for example one showing that tides and internal waves by themselves don’t do a lot of mixing, and that only when they hit topography the interesting stuff starts happening.

So what we need is a simple 2-layer system and two different cases: One with topography, one without. And because we want to use it to hand around in class, the stratification should be indestructible (-> oil and water) and the container should be fairly tightly sealed to prevent a mess.

Here we go:

There definitely is a lot to be said for kitchen oceanography, too! Would you have thought that using just two plastic bottles and some oil and water could give such a nice demonstration?

Experiment: Oceanic overturning circulation (the slightly more complicated version)

The experiment presented on this page is called the “slightly more complicated version” because it builds on the experiment “oceanic overturning circulation (the easiest version)” here.

Background

One of the first concepts people hear about in the context of ocean and climate is the “great conveyor belt”. The great conveyor belt is a very simplified concept of the global ocean circulation, which is depicted as a single current that spans the world oceans (see Figure 1 below). In this simplified view of the global circulation, water flows as a warm, global surface current towards the North Atlantic, where it cools, sinks and finally returns southward and through all the world oceans near the bottom of the ocean. Water is transported back to the surface through mixing processes and starts over its journey again as a warm surface current. While in reality some part of the conveyor belt is wind-driven and many processes come to play together, a large part of the circulation can be explained by the water sinking due to cooling at high latitudes.

Conveyor_belt

Figure 1: The great conveyor belt. My sketch on top of a map from http://www.free-world-maps.com

This can be very easily represented in a demonstration or experiment.

Materials

What we need for this experiment:

  • 2 gel pads for sports injuries, one hot, one cold
  • red and blue food coloring
  • a clear plastic container to act as tank
  • a pipette or drinking straws to disperse drops of dye
  • dye crystals to show the circulation. Can also be drops of a different color dye.
Running the experiment

The container is filled with lukewarm water.  On the “poleward” end, we add the cold pad, the warm one at the “equatorward” end of the tank.

Blue dye is tripped on the cold pad to mark the cold water, red dye on the warm pad as a tracer for warm water.

overturning

Thermally-driven overturning circulation: Warm water flowing near the surface from the warm pad on the left towards the right, cold flow from the cool pad at the bottom right to left.

A circulation develops. If you drop dye crystals in the tank, the ribbon that formed gets deformed by the currents for yet another visualization of the flow field.

overturning2

Thermally-driven overturning circulation. In the middle of the tank you see a ribbon of dye, caused by falling dye crystals, being transformed by the currents in the tank.

Here is the video:

What observations to make

Besides the obvious observation, watching, there are a couple of things you can ask your audience to do.

For example, if they carefully slide their fingers up and down the side of the tank, they will feel the warm water near the surface and the cold water at the bottom.

If you have a clear straw, you can use it as plunging syphon to extract a “column” of water from the middle of the tank, showing again the stratification of red, clear, blue.

If you put little paper bits on the surface, you will see them moving with the surface current.

Can you come up with more?

Who can I do this experiment with?

Someone recently asked me whether I had ideas for experiments for her course in ocean sciences for non-majors. Since most of the experiments I’ve been showing on this blog were run in the context of Bachelor or Master oceanography-major courses, she didn’t think that the experiments were as easily transferable to other settings as I had claimed.

So here is proof: You can do pretty complex experiments with non-university level students. To prove my point, let’s go to a primary school.

IMG_3219

Me running the overturning experiment with a primary school class in 2012.

IMG_3214

The overturning experiment as seen by the teacher (2012).

Of course, you can adapt this experiment to different levels of prior knowledge. For example, in the primary school, I introduced this experiment by showing pictures of lions and penguins and other animals that the pupils knew live in warm or cold climates, and we talked about where those animals live. In the end this aimed at how temperatures are a lot colder at the poles than at the equator. This is the differential heating we need for this experiment to work. While this is something that I felt the need to talk about with the primary school kids, this can be assumed as a given with older students (or at least that is the assumption that I made).

With the university-level courses, one of the points that I made sure came up during the discussion are the limitations of this model. For example that we apply both heating and cooling over the full depth of the water column. How realistic is that? Or the fact that we heat at one end and cool at the other, rather than cooling on either end and heating in the middle?

Let me zoom in on something in the picture above.

IMG_3214_2

Curious features in the thermal conveyor experiment. Do you know what this is about?

Do you see these weird red filaments? Do you think they are a realistic part of the thermal circulation if it was scaled up to a global scale?

Of course not. What we see here is salt fingering. This is a process that is caused by the different diffusivities of heat and of the red dye. And while it is pretty large scale in our small tank, you cannot scale it up just like that when talking about the real ocean. And it is also really difficult to get rid of salt fingers for this experiment, in fact I haven’t yet managed. But I am open to suggestions! :-)

Another point that I would talk about with university-level students that I would probably not bring up with primary school kids (- although, why not if I had more time than just those 45 minutes per class?) is that ocean circulation is driven by more than just differential heating. Even when just considering the density-driven circulation, that is additionally influenced by changes in salinity. Put that together with wind-driven circulation and we are starting to talk about a whole new level of complicated…

But anyway. My point is that even primary school kids can benefit from doing this kind of experiments, even if what they take away from the experiments is not exactly the same as what older students would take away.

Discussion
As with every experiment, it is a lot easier for an “expert” to observe what he or she wants to observe, than for their students.
The left column in the figure above is taken from an instruction for educators and parents of primary school kids I wrote a while back. When taking the pictures I was aware that the quality in terms of signal-to-noise was not very good (and in fact people [i.e. my parents] even told me). In my defense: The pictures of this experiment I shared on this blog are all less noisy, and I even explicitly addressed and discussed some of the noise! But still, only when reading that article today I fully appreciated how difficult it might be to see the signal through the noise (especially when the speech bubbles in the picture don’t even point exactly to the right places!), and how distracting it probably is when I implicitly assume that students see the signal and even start discussing the noise more than the signal.

So what we see above are, in the left column, the pictures I originally shared in that manual. In the middle column, I’m showing what I see when I look at the pictures on the left. And then in the right column I’m drawing what people might be seeing when looking at that same experiment. No idea if that really is what students see, but looking at the pictures now, there is actually no reason why they should see what I see. See?
One indicator of the signal-to-noise ratio and of what students actually perceive as important can be found in the three little essays the primary school kids show in the picture above wrote after my visit in December 2012: Two out of the three explicitly mention that I used a yoghurt beaker as heating on the one end of the tank (while the third only refers to a beaker). Clearly that seems to have been a very important observation to them.
So what do we take away from this? I, for one, am going to make sure to pay more attention to the signal-to-noise ratio when showing demonstrations. And if there happens to be a lot of noise, I am going to make it a lot clearer which part of the signal is actual signal, and which is noise. Lesson learned.

P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on January 13th, 2016.

I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.

Experiment: Oceanic overturning circulation (the easiest version)

“The easiest” in the title of this page is to show the contrast to a “slightly more complicated” version here.

Background

One of the first concepts people hear about in the context of ocean and climate is the “great conveyor belt”. The great conveyor belt is a very simplified concept of the global ocean circulation, which is depicted as a single current that spans the world oceans (see Figure 1 below). In this simplified view of the global circulation, water flows as a warm, global surface current towards the North Atlantic, where it cools, sinks and finally returns southward and through all the world oceans near the bottom of the ocean. Water is transported back to the surface through mixing processes and starts over its journey again as a warm surface current. While in reality some part of the conveyor belt is wind-driven and many processes come to play together, a large part of the circulation can be explained by the water sinking due to cooling at high latitudes.

Conveyor_belt

Figure 1: The great conveyor belt. My sketch on top of a map from http://www.free-world-maps.com (used with permission)

The experiment

Since the global conveyor belt is such a basic concept that we come across in many different contexts, it is very useful to have a good demonstration of what is happening in the world ocean. Plus demonstrations and experiments are always fun!

I here present a very simple experiment that can be used for many different purposes. In science outreach, for example on a fair or in a talk, to catch people’s attention and raise an interest in oceanography. In schools to do the same, or to connect the fascination of the ocean to school physics and talk about density, convection, heat. At university to do all of the above, as well as to practice writing lab reports, talk about the scientific method or the validity of simplifications in theoretical or physical models.

Materials needed

All we need to run this experiment is

  • a clear plastic container
  • lukewarm water
  • red and blue food dye
  • an ice cube tray and
  • access to a freezer.

Ideally we’d also have a thermos or some other kind of insulation to keep the ice cubes frozen until we start running the experiment. To prepare the experiment, all we need to do a half a day ahead is mix some blue food dye into the water that we put in the ice cube tray, and freeze the ice cubes.

Running the experiment

To run the experiment, we start out by filling our “tank” with lukewarm water. Let it settle for a bit. Now we decide for one end of your tank to be the “equator” end. There, we add some red food dye (see Figure 1).

overturning-ice-1

Figure 2: Tank with luke warm water. Red food dye added to the “warm” end of the tank.

Then we add the blue ice cubes to the “poleward” end of our tank (see Figure 3).

overturning-ice-2

Figure 3: Blue ice cubes melting at the poleward end of the tank. The cold melt water sinks to the bottom of the tank and then spreads “equatorward”.

The cold melt water from the ice cubes is denser than the lukewarm water in the tank and therefore sinks to the bottom of the tank where it spreads “equatorward”, pushing below the warmer water. This can be seen where the red water is pushed upwards and “poleward”.

Discussion

Of course, the processes at play here are not exactly the same as in the real ocean.

For one, deep water formation is NOT due to ice cubes melting in lukewarm water. In fact, melting of sea ice will in most cases not lead to any kind of sinking of water, since the melt water is fresh and the surrounding ocean water is salty and hence denser than the melt water. Cooling in the ocean happens through many processes at the surface of the ocean, like radiation into space and evaporation.

Heating is also represented in an extremely simplified way in this experiment. Heating in the ocean occurs mainly (with the negligible exception of thermal springs in the ocean) by radiative heating from the sun, and at the surface only. We “heat” throughout the whole depth of the ocean by filling the whole tank with lukewarm water.

Also, the mixing processes that, in the real ocean, bring deepwater back to the surface are not represented here at all. Our tank will eventually fill with a layer of cold water at the bottom (See Figure 4), and the circulation will stop once all the ice has melted.

overturning-ice-3

Figure 4: Blue ice cubes melting at the poleward end of the tank. The cold melt water sinks to the bottom of the tank and then spreads “equatorward”. Slowly, the tank fills with cold water.

Why use the experiment?

Even with all the simplifications described above, this experiment is a great first step in becoming intrigued by the ocean, and towards understanding ocean circulation. Seeing the melt water sink from the ice cubes is fascinating no matter how little interest one might have in the physics that cause it. Sliding a finger up and down the side of the tank lets you experience – feel! – how the temperature changes from warm near the surface to very cold near the bottom. Actually physically feeling this is a lot more impressive than just watching the experiment or even just being shown temperature sections of the ocean. And the experiment invites you to play: What if you added little pieces of paper on the water surface, would you see them move with the flow towards the cold end of the tank? Or if you dropped a dye crystal in the middle of the tank, would the dye ribbon that forms be deformed by the currents in the tank? And what if you added twice as many ice cubes, would the currents be twice as fast?

This is pretty much the easiest experiment you can imagine. If you are afraid of what food dye might do in the hands of your participants, you don’t even need to let them handle it themselves, even when they are working in small groups with individual tanks: just go around dripping the dye in and then add the dyed ice cubes yourself. While someone might still tip over a tank and spill the water, this has yet to happen to me. Especially since, before running the experiment, you will have pointed out that they need to make sure not to bang against the tables as to not disturb the experiment. And now apart from making sure that the ice cubes are frozen when you want to run this experiment, there is nothing that can go wrong. So why not try this experiment next time you want to talk about global ocean circulation?

Watch a video of the experiment here:

What would I do differently next time?

Next time, I would pay attention to which end of my tank will represent the equatorward and poleward side of the ocean. Not that it matters much, but in most graphics of sections through the North Atlantic, the northern end will be on the right side and the southern end on the left. If the experiment is set up the other way round (as on all pictures and movies above) you will need to remember to point it out (or even mark it on the tank with a sharpie or such).

Still scared of the hassle of running experiments?

And for all of you who hesitate doing awesome experiments because it looks like you need so much equipment: No you don’t. Here is a “making of” shot from how I did this experiment on my coffee table while sitting on my couch. The background is the back of an old calendar sheet, clipped to the back of a chair. And that’s it.

Screen shot 2015-11-02 at 3.41.24 PM

The setup for my experiment.

P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 4th, 2015.

I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.

Experiment: Eddy in a jar

Rotating experiments in your kitchen.

Eddies, those large, rotating structures in the ocean, are pretty hard to imagine. Of course, you can see them on many different scales, so you can observe them for example in creeks, as shown below:

IMG_1266

Eddies in the Pinnau river, and their dark “shadows”.

If you can’t really spot them in the image above, check out this post for clues and a movie.

So how can you create eddies to observe their structure?

MVI_0698

Dye spiral caused by an eddy in a jar

I took a large cylindrical jar, filled it with water, stirred, let it settle down a little bit and then injected dye at the surface, radially outward from the center. Because the rotating body of water is slowed down by friction with the jar, the center spins faster than the outer water, and the dye streak gets deformed into a spiral. The sheet stays visible for a very long time, even as it gets wound up tighter and tighter. And you can see the whole eddy wobble a bit (or pulsate might be the more technical term) because I introduced turbulence when I stopped stirring.

Watch the movie below if you want to see more. Or even better: Go play yourself!

P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 27th, 2015.

I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.

Experiment: Ice cubes melting in fresh water and salt water

Explore how melting of ice cubes floating in water is influenced by the salinity of the water. Important oceanographic concepts like density and density driven currents are visualized and can be discussed on the basis of this experiment.

Context

Audience

This hands-on experiment is suited for many different audiences and can be used to achieve a wealth of different learning goals. Audience ranges from first-graders over undergraduates in physical oceanography to outreach activities with the general public. Depending on the audience, this activity can be embedded in very different contexts: For children, either in their physics teaching to motivate learning about concepts like density, or in the context of learning about the climate system and ocean circulation. For college/university students the activity can either be used in physics teaching to get a different view on density; in oceanography/Earth science to talk about ocean circulation and processes that are important there; to motivate the scientific process; or to practice writing lab reports (you can be sure that students will at some point be tasting the water to make sure they didn’t accidentally swap the salt water and fresh water cup – a great teachable moment for a) Never putting anything in your mouth in a laboratory setting, and b) Always documenting exactly what you are doing because stuff that you think you will definitely remember obviously isn’t remembered that clearly after all). For the general public, this is typically a stand-alone activity.

Skills and concepts that students must have mastered

It helps if the concept of density is known, but the experiment can also be used to introduce or deepen the understanding of the concept.

How the activity is situated in the course

I use this activity in different ways: a) as a simple in-class experiment that we use to discuss the scientific method, as well as what needs to be noted in lab journals and what makes a good lab report, or density-driven circulation; b) to engage non-majors or the general public in thinking about ocean circulation, what drives ocean currents, … in one-off presentations.

 

Goals

Content/concepts goals for this activity

Students learn about concepts that are important not only in physical oceanography, but in any physical or Earth science: density in general; density of water in particular, depending on the water’s temperature and salinity; how differences in density can drive currents both in the model and in the world ocean; how different processes acting at the same time can lead to unexpected results; how to model large scale processes in a simple experiment. After finishing the activity, they can formulate testable hypotheses, are able to reason based on density how a flow field will develop and they can compare the situations in the cups to the “real” ocean.

Higher order thinking skills goals for this activity

Students learn about and practice the use of the scientific method: formulation of hypotheses, testing, evaluating and reformulating.

Other skills goals for this activity

Students practice writing lab reports, making observations, working in groups.

 

Description and Teaching Materials

Materials

(per group of 2-4 students):

  • 1 clear plastic cup filled with room-temperature salt water (35psu or higher, i.e. 7 or more tea spoons of table salt per liter water), marked as salt water (optional)
  • 1 clear plastic cup filled with room-temperature fresh water, marked as fresh water (optional)
  • 2 ice cubes
  • liquid food dye either in drop bottle, with a pipette or with a straw as plunging syphon

Description

Before the experiment is started, students are asked to make a prediction which ice cube will melt faster, the one in salt water or the one in fresh water. Students discuss within their groups and commit to one hypothesis.
Students then place the ice cubes into the cups and start a stop watch/note the time. Students observe one of the ice cube melting faster than the other one. When it becomes obvious that one is indeed melting faster, a drop of food dye can be added on each of the ice cubes to color the melt water. Students take the time until each of the ice cubes has melted completely.

Discussion

The ice cube in the cup containing the fresh water will melt faster, because the (fresh) melt water is colder than the room-temperature fresh water in the cup. Hence its density is higher and it sinks to the bottom of the cup, being replaced by warmer waters at the ice cube. In contrast, the cold and fresh melt water in the salt water cup is less dense than the salt water, hence it forms a layer on top of the salt water and doesn’t induce a circulation like the one in the fresh water cup. The circulation is clearly visible as soon as the food dye is added: While in the freshwater case the whole water column changes color, only a thin meltwater layer on top of the salt water is colored (for clarification, see images in the presentation below)

 

Teaching Notes and Tips

Students usually assume that the ice cube in salt water will melt faster than the one in fresh water, “because salt is used to de-ice streets in winter”. Have students explicitly state their hypothesis (“the one in salt water will melt faster!”), so when they measure the time it takes the ice cubes to melt, they realize that their experiment does not support their hypothesis and start discussing why that is the case. (Elicit the misconception, so it can be confronted and resolved!)

My experience with this experiment is that all groups behave very consistently:

  • At least 80% of your audience will be very sure that the ice cube in salt water will melt faster than the one in fresh water. The other 20% will give the correct hypothesis, but only because they expect a trick question, and they will most likely not be able to come up with an explanation.
  • You can be 100% sure that at least in one group, someone will say “oh wait, which was the salt water again?” which hands you on a plate the opportunity to say “see — this is a great experiment to use when talking about why we need to write good documentations already while we are doing the experiment!”
  • You can also be 100% sure that in that group, someone will taste the water to make sure they know which cup contains the salt water. Which lets you say your “see — perfect experiment to talk about lab safety stuff! Never ever put things in your mouth in a lab!”
  • You can also be sure, that people come up with new experiments they want to try.
    • At EMSEA14, people asked what would happen if the ice cubes were held at the bottom of the beaker.
    • At a workshop on inquiry-based learning, people asked what the dye would do if there was no ice in the cups, just salt water and fresh water. Perfect opportunity to say “try! Then you’ll know! And btw — isn’t this experiment perfect to inspire the spirit of research (or however you would say that in English – “Forschergeist” is what I mean!). This is what you see in the pictures in this blogpost.

It is always a good idea to have plenty of spare ice cubes and salt/fresh water at room temperature ready so people can run the experiment again if they decide to either focus on something they didn’t observe well enough the first time round, or try a modified experiment like the ones described above.

A reviewer of this activity asked how easily students overcome the idea that water in the cup has to have just one temperature. In my experience this is not an issue at all – students keep “pointing” and thereby touching the cups, and in the thin-walled plastic cups I typically use the temperature gradient between “cold” melt water and “warm” salt water is easily felt. The (careful!) touching of the cups can also be explicitly encouraged.

Different ways to use this experiment

This experiment can be used in many different ways depending on the audience you are working with.

  • Demonstration: If you want to show this experiment rather than having students conduct it themselves, using colored ice cubes is the way to go (see experiment here). The dye focuses the observer’s attention on the melt water and makes it much easier to observe the experiment from a distance, on a screen or via a projector. Dying the ice cubes makes understanding much easier, but it also diminishes the feeling of exploration a lot – there is no mystery involved any more. And remember in order for demonstrations to increase the learning outcome, they need to be embedded in a larger didactical setting, including forming of hypotheses before the experiment is run and debriefing afterwards.
  • Structured activity: For an audience with little knowledge about physics, you might want to start with a very structured activity, much like the one described above. Students are handed (non-colored) ice cubes, cups with salt water and fresh water and are asked to make a prediction about which of the ice cubes is going to melt faster. Students test their hypothesis, find the results of the experiment in support with it or not, and we discuss. This is how I usually use this experiment in class (see discussion here).The advantage of using this approach is that students have clear instructions that they can easily follow. Depending on how observant the group is, instructions can be very detailed (“Start the stop watch when you put the ice cubes in the water. Write down the time when the first ice cube has melted completely, and which of the ice cubes it was. Write down the time when the second ice cube has melted completely. …”) or more open (“observe the ice cubes melting”).
  • Problem-solving activity: Depending on your goals with this experiment, you could also consider making it a problem-solving activity: You would hand out the materials and ask the students to design an experiment to figure out which of the cups contains fresh water and which salt water (no tasting, of course!). This is a very nice exercise and students learn a lot from designing the experiment themselves.
  • Open-ended investigation: In this case, students are handed the materials, knowing which cup contains fresh and salt water. But instead of being asked a specific question, they are told to use the materials to learn as much as they can about salt water, fresh water, temperature and density.As with the problem-solving exercise, this is a very time-intensive undertaking that does not seem feasible in the framework we are operating in. Also it is hard to predict what kind of experiments the students will come up with, and if they will learn what you want them to learn. On the other hand, students typically learn much more because they are free to explore and not bound by a specific instruction from you, so maybe give it a try?
  • Problem-based learning: This experiment is also very well suited in a Problem-Based Learning setting, both to work on the experiment itself or, as we did, to have instructors experience how problem-based learning works so they can use it in their own teaching later. Find a suggested case and a description of our experiences with it here.
  • Inquiry-based learning: Similarly as with Problem-Based Learning, this experiment can be used to let future instructors experience the method of inquiry-based learning from a student perspective. For my audience, people teaching in STEM, this is a nice case since it is close enough to their topics so they can easily make the transfer from this case to their own teaching, yet obscure enough that they really are learners in the situation.

Pro tip: If you are not quite sure how well your students will be able to cope with this experiment, prepare ice cubes dyed with food coloring and use them in a demonstration if students need more help seeing what is going on, or even let students work with colored ice cubes right from the start. If ice cubes and hence melt water are dyed right away, it becomes a lot easier to observe and deduct what is happening. Feel free to bring the photos or time lapse movie below as a backup, too!

dyed_ice_cubes_01

Dyed ice cubes about to be put into fresh water (left) and salt water (right)

dyed_ice_cubes_02

When the ice cubes start melting, it becomes very clear that they do so in different manners. In the left cup, the cold meltwater from the ice cube is denser than the lukewarm water in the cup. Hence it sinks to the bottom of the beaker and the water surrounding the ice cube is replaced by warmer water. On the right side, the lukewarm salt water is denser than the cold melt water, hence the cold meltwater floats on top, surrounding the ice cube which therefore melts more slowly than the one in the other cup.

dyed_ice_cubes_03

The ice cube in the fresh water cup (left) is almost completely gone and the water column is fairly mixed with melt water having sunk to the bottom of the beaker. The ice cube in the salt water cup (right) is still a lot bigger and a clear stratification is visible with the dyed meltwater on top of the salt water.

And here a time-lapse movie of the experiment.

Another way to look at the experiment: With a thermal imaging camera!

screen-shot-2017-06-11-at-17-12-29

Cold (dark purple) ice cubes held by warm (white-ish) fingers over room-temperature (orange) cups with water

screen-shot-2017-06-11-at-17-12-55

After a while, both cups show very different temperature distributions. The left one is still room temperature(-ish) on top and very cold at the bottom. The other one is very cold on top and warmer below.

screen-shot-2017-06-11-at-17-13-20

When you look in from the top, you see that in the left cup the ice has completely melted (and the melt water sunk to the bottom), whereas in the right cup there is still ice floating on top.

Assessment

Depending on the audience I use this experiment with, the learning goals are very different. Therefore, no one assessment strategy can be used for all different applications. Below, I am giving examples of what are possible ways to assess specific learning goals:

– Students apply the scientific process correctly: Look at how hypotheses are stated (“salt melts ice” is not a testable hypothesis, “similar-sized ice cubes will melt faster in salt water than in fresh water of the same temperature” is).

– Students are able to determine what kind of density-driven circulation will develop: Suggest modifications to the experiment (e.g. ice cubes are made from salt water, or ice cubes are held at the bottom of the cups while melting) and ask students to predict what the developing circulation will look like.

– Students can make the transfer from the flow field in the cup to the general ocean circulation: Let students compare the situation in the cup with different oceanic regions (the high Arctic, the Nordic Seas, …) and argue for which of those regions displays a similar circulation or what the differences are (in terms of salinity, temperature, and their influence on density).

In general, while students run the experiment, I walk around and listen to discussions or ask questions if students aren’t already discussing. Talking to students it becomes clear very quickly whether they understand the concept or not. Asking them to draw “what is happening in the cup” is a very useful indicator of how much they understand what is going on. If they draw something close to what is shown on slide 28 of the attached slide show, they have grasped the main points.

 

Equipment

Don’t worry, it is totally feasible to bring all the equipment you need with you to run the experiment anywhere you want. This is what we brought to EMSEA14 to run the workshop three times with 40 participants each:

EMSEA14_list

What we brought to EMSEA14 to run workshops on the ice cubes melting in fresh and salt water experiment

In one big grocery bag:

  • 4 ice cube trays
  • 4 ice cube bags (backup)
  • 2 thermos flasks (to store ice cubes)
  • 1 insulating carrier bag (left)
  • 4 empty 1.5l water bottles to mix & store salt water in
  • 1 tea spoon for measuring salt
  • 500g table salt
  • 21 clear plastic cups for experiments
  • 10 clear plastic cups to hand out ice cubes in
  • 11 straws (as pipettes)
  • 1 flask of food dye
  • 11 little cups with lids to hand out food dye in
  • nerves of steel (not shown :-))

And if you are my friend, you might also get the “ice cube special” — a pink bucket with all you will ever need to run the experiment! Below is what the ice cube experiment kit looks like that I made for Marisa, with labels and everything…

IMG_4202

An “ice cube experiment” kit that I made for a friend. Want one, too?

References and Resources

This activity has been discussed before, for example here:

I have also written about it a lot on my blog, see posts tagged “melting ice cubes experiment“.

P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 4th, 2015.

I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.