Tag Archives: salinity

Demonstration: Nansen’s “dead water” in a tank!

A ship that is continuously pulled with a constant force suddenly slows down, stops, and then continues sailing as if nothing ever happened? What’s going on there? We will investigate this in a tank! And in order to see what is going on, we have dyed some of the water pink. Can you spot what is going on?

The phenomenon of “dead water” is probably well known to anyone sailing on strong stratifications, i.e. in regions where there is a shallow fresh or brackish layer on top of a much saltier layer, e.g. the Baltic Sea, the Arctic or some fjords. It has been described as early as 1893 by Fridtjof Nansen, who wrote, sailing in the Arctic: “When caught in dead water Fram appeared to be held back, as if by some mysterious force, and she did not always answer the helm. In calm weather, with a light cargo, Fram was capable of 6 to 7 knots. When in dead water she was unable to make 1.5 knots. We made loops in our course, turned sometimes right around, tried all sorts of antics to get clear of it, but to very little purpose.” (cited in Walker,  J.M.; “Farthest North, Dead Water and the Ekman Spiral,” Weather, 46:158, 1991)

When observing the experiment, whether in the movie above or in the lab, the obvious focus is on the ship and the interface between the clear fresh water layer (the upper 5cm in the tank) and the pink salt water layer below. And yes, that’s where a large-amplitude internal wave develops and eats up all the energy that was going into propulsion before! Only when looking at the time lapse of the experiments later did I notice how much more was going on throughout the tank! Check it out here:

The setup for this experiment is discussed here and is based on the super helpful website by Mercier, Vasseur and Dauxois (2009). In the end, we ended up without the belt to reduce friction, and with slightly different layer depths than we had planned, but all in all it works really well!

Accidental double-diffusive mixing

When setting up the stratification for the Nansen “dead water” demo (that we’ll show later today, and until then I am not allowed to share any videos, sorry!), I went into a meeting after filling in layer 4 (the then lowest). When I came back, I wanted to fill in layer 5 as the new bottom layer. For this experiment we want the bottom four layers to have the same density (so we would actually only have one shallow top layer and then a deep layer below [but we can’t make enough salt water at a time for that layer, so I had to split it into four portions]), and I had mixed it as well as I could. But two things happened: a) my salinity was clearly a little fresher than the previous layer, and b) the water in the tank had warmed up and the new water I was adding with layer 5 was cold tap water. So I accidentally set up the stratification for salt fingering: warm and salty over cold and fresh! Can you spot the darker pink fingers reaching down into the slightly lighter pink water? How cool is this??? I am completely flashed. Salt fingering in a 6 meter long tank! :-D

 

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.

Melting ice cubes experiment — observing the finer details

If you don’t know my favourite experiment for practically all purposes yet (Introduction to experimenting? Check! Thermohaline circulation? Check! Lab safety? Check! Scientific process? Check! And the list goes on and on…), check it out here. (Seriously, of you don’t recognize the experiment from the picture below, you need to read up on it, it’s awesome! :-))

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Susann and I got funding from PerLe (our university’s project to support teaching innovation) to add a couple of cool new features to Susann’s “intro to meteorology” lecture, and doing a hands-on experiment with 50 students in a lecture theatre in their second lecture was only one of the first of many more to come.

We used the experiment to introduce the students to oceanic circulation, and this experiment is, in my experience, very engaging and sparks curiosity, as well as being very nicely suited as a reminder that things are not as easy as they seem to be when you see those nice plots of the great conveyor belt and all the other simplified plots that you typically see in intro-level lectures. Especially understanding that there are many different processes at play simultaneously, and that they have different orders of magnitude and might act in different directions helps counteract the oversimplified views of the climate system that might otherwise be formed.

I usually use dye to make it easier to observe what’s going on in the experiment (either by freezing it directly into the ice cubes as shown in the picture on top of this blog post, or by dripping it onto the melting ice cubes when students have started to observe that — counter to their intuition — the ice cube in the fresh water cup is melting faster than the one in the salt water cup).  We had dye at hand, but I decided on the spur of the moment to not use it, because the students were already focussing on other, more subtle, aspects that the dye would only distract from:

The shape of the ice cubes

In many of the student groups, the most prominent observation was that the shape of the melting ice cubes was very different in the fresh water and salt water case. In the fresh water case, the ice cube melted from the sides inwards: as a cylindrical shape with a radius that was decreasing over time, but in any instance more or less constant for all depths. In the salt water case, however, the ice cube melted upwards: The top did not melt very much at all, but the deeper down you looked the more was melting away. Why?

Condensation on the sides of the cup

Another observation that I prompted was in what regions the cups showed condensation. In the fresh water case, there was a little condensation going on everywhere below the water line, and sometimes there were vertical streaks down from where the ice cube was touching the wall. In the salt water case, there was only a small band of intense condensation close to the water level.

This, not surprisingly, looks very similar to what a thermal imaging camera sees when observing the experiment (as described in this post).

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Taken together, those two observations are quite powerful in explaining what is going on, and it seemed to be a fun challenge for the students to figure out why there was condensation on the outside of the cups in the first place (does condensation occur in warmer or colder places?), what it meant that different places ended up being warmer or colder, and how all of that is connected to global ocean circulation. Definitely an experiment I would recommend you do! :-)

Evaporating sea water

How much salt is there in sea water? What concentration do you need before crystals start forming? What will those crystals look like? I am sure those are the kind of questions that keep you awake at night!

Of course this can easily assessed experimentally. On a visit to the University of Bergen’s Centre for Science Education just now, I was shown the result of such an experiment: A litre of water was mixed with 35 grams of salt to simulate sea water with its typical salinity. Below, you see what the beaker looked like after sitting out for three months.

You can see that salt crystals are forming at the walls of the beaker, but that their structure depends on depth below the initial water level (see the 1000 ml mark on the beaker).

When there is still a lot of water in the beaker, crystals look like ornate flowers. Then, the less water is left in the beaker, the more square the crystals become. And at the bottom of the beaker, you see the typical salt crystals you would expect.

 

Actually, even though they look like the kind of salt crystals I would expect, apparently someone who knows about crystallography commented that there must be other stuff in there than just cooking salt since the crystals don’t look the way they should. I need to read up on this! :-)

Anyway, this is an experiment that I want to do myself, so maybe in three months time there will be more pictures of this!

Thanks for a very nice lunch, Olaug, Frede, Andreas, Morven and Elin! Looking forward to working with you a lot more in the future! :-)

P.S.: with this blog post I am testing to blog pretty much “real time” from my mobile phone, so if you notice anything odd, please let me know!

Taking the hydrostatic paradox to the next (water) level

How well do people understand hydrostatics? I am preparing a workshop for tomorrow night and I am getting very bored by the questions that I have been using to introduce clickers for quite a lot of workshops now. So I decided to use the hydrostatic paradox this time around.

The first question is the standard one: If you have a U-tube and water level is given on one side, then what is the water level like on the other side? We all know the typical student answer (that typically 25% of the students are convinced of!): On the wider side the water level has to be lower since a larger volume of water is heavier than the smaller volume on the other side.

Clearly, this is not the case:

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However, what happens if you use that fat separator jug the way it was intended to be used and fill it with two layers of different density (which is really what it is intended for: to separate fat from gravy! Your classical 2-layer system)?

Turns out that now the two water levels in the main body of the jug and in the spout are not the same any more: Since we filled the dense water in through the spout, the spout is filled with dense water, as is the bottom part of the jug. Only the upper part of the jug now contains fresh water.

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The difference in height is only maybe a millimetre, but it is there, and it is clearly visible:

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Water level 1 (red line) is the “main” water level, water level 2 (green line) is the water level in the spout and clearly different from 1, and water level 3 is the density interface.

We’ll see how well they’ll do tomorrow when I only give them levels 1 and 3, and ask them to put level 2 in. Obviously we are taking the hydrostatic paradox to the next (water) level here! :-)

Ice cubes melting in fresh water and salt water. By Mirjam S. Glessmer

Using the “melting ice cube” experiment to let future instructors experience inquiry-based learning.

Using the “melting ice cube” experiment to let future instructors experience inquiry-based learning.

I recently (well, last year, but you know…) got the chance to fill in for a colleague and teach part of a workshop that prepares teaching staff for using inquiry-based learning in their own teaching. My part was to bring in an experiment and have the future instructors experience inquiry-based learning first hand.

So obviously I brought the ice cubes melting in fresh water and salt water experiment! (Check out that post to read my write-up of many different ways this experiment can be used, and what people can learn doing it). On that occasion the most interesting thing for me was that when we talked about why one could use this — or a similar — experiment in teaching, people mainly focussed on the group aspect of doing this experiment: How people had to work together in a team, agree to use the same language and notation (writing “density of water at temperature zero degree Celsius” in some short syntax is not easy when you are not an oceanographer!).

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And this experiment never fails to deliver:

  • 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 documentation 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 at the bottom of the beaker. Today, 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 blog post.

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So yeah. Still one of my favorite experiments, and I LOVE watching people discover the fascination of a little water, ice, salt and food dye :-)

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Photo taken by Ulrike Bulmann

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Photo taken by Ulrike Bulmann

Btw, when I gave a workshop on active learning last week and mentioned this experiment, people got really really hooked, too, so I’ll leave you with a drawing that I liked:

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Water seeks its level. U-tube experiment. By Mirjam S. Glessmer

When water doesn’t seek its level

Last week we talked about misconceptions related to hydrostatic pressure, and how water always seeks its level. Today I’m gonna show you circumstances in which this is actually not the case!

We take the fat separator jug we used last week. Today, it is filled with fresh water, to which we add very salty water through the jug’s spout. What is going to happen? Watch the movie and find out!

Turns out that now the two water levels in the main body of the jug and in the spout are not the same any more: Since we filled the dense water in through the spout, the spout is filled with dense water, as is the bottom part of the jug. Only the upper part of the jug now contains fresh water.

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The difference in height is only maybe a millimetre, but it is there, and it is clearly visible.

Do you see the opportunities for discussions this experiment provides? If we knew the exact volumes of fresh water and salt water, and the exact salinity, we could measure the difference in height of the water levels and try to figure out how much mixing must have taken place when the fresh water was added to the jug. Or we could use the difference in height to try and calculate the density difference between fresh water and salt water and then from that calculate salinity. So many possibilities! :-)

My favorite demonstration of the coolest mixing process: Salt fingering!

I am updating many of my old posts on experiments and combining multiple posts on the same topic to come up with a state-of-the-art post, so you can always find the best materials on here. And today I would like to present you my favorite experiment: Salt fingering!

Check out the new page I made for salt fingering!

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Self-portrait with salt fingers

As you guys might have noticed, I’ve been playing around with my site a quite bit. My blog has moved to mirjamglessmer.com/blog in order to make room for static pages of my favorite experiments or teaching tips right at the landing site mirjamglessmer.com. What do you think? Good idea? Did you notice anything that isn’t quite working yet or do you have advice or wishes? Let me know!