Category Archives: demonstration (easy)

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

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!

Thermally driven circulation

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One of my all-time favorite experiments.

The salt group got a bit bored from watching ice cubes melt, so I suggested they look at temperature differences for a change, and they ran the “leaking bottles” experiment.

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Thermally-driven circulation.

Watch a movie combining their four different setups below!

Wave interference in a tank

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Creating waves and watching them interfere. (deutscher Text unten)

You might not have guessed it from reading about our waves meeting over a sandbank experiment, but we weren’t doing in purely for its entertainment value. Our goal was to see how waves interfere, because the theory of interfering waves seems to be counter-intuitive in some cases. A second experiment we have been doing on this topic is shown below. We create waves by dripping water drops on the water surface and film (and in some cases also watch) from below. Movie at the end of this post!

Obwohl es sicherlich nicht danach aussah, haben wir das  Experiment mit den Wellen auf der Sandbank nicht nur aus Spaß veranstaltet, sondern durchaus mit einem wissenschaftlichen Hintergrund: Wir wollten uns ansehen, wie sich mehrere Wellen überlagern.

Von oben werden Wassertropfen in den Tank getropft, das daraus entstehende Wellenfeld wird von unten gefilmt (und in einigen Fällen auch beobachtet).

 

Thermally-driven overturning circulation

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Cooling on one end of the tank, heating on the other: A temperature-driven overturning. [deutscher Text unten]

Always one of my favorite experiments – the overturning experiment (and more, and more).

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Final preparations. – Letzte Vorbereitungen

Unsere “Klima und Strömungen”-Gruppe hat heute ausprobiert, wie man in einem Tank eine Umwälzströmung erzeugen kann, indem man an einem Ende wärmt und am anderen Ende kühlt. Einige Versuche waren nötig, bis das Experiment perfektioniert war: Am Anfang fehlte die Wärmequelle am einen Ende, was aber erst auffiel als das kalte Wasser am Boden schon das Ende erreicht hatte. Dann war die Wärmequelle zwar vorhanden, aber von außen am Plexiglastank angebracht.

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Warm red surface current, cold blue deep current. – Warme rote Oberflächenströmung, kalte blaue Tiefenströmung.

Letztendlich wurden ein rotes Wärmepack erhitzt und ein blaues Kühlpack eingefroren, und beide in den Tank gesteckt. Und voila! Eine tolle Zirkulation!

Ganz gegen Ende des Experiments haben wir dann noch Farbkristalle in den Tank fallen lassen, und wie man im Bild unten sehen kann, sind die super, um die Zirkulation zu visualisieren. Aus den anfangs senkrechten Streifen formt sich schnell ein Strömungsprofil: Am Boden von kalt nach warm (links nach rechts) und an der Oberfläche in die entgegengesetzte Richtung.

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Dye streaks as circulation tracers. – Blaue Farbstreifen, um die Zirkulation zu verdeutlichen.

Und wenn man ganz genau hinschaut: Salzfinger! :-)

Capillary effects

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When hydrostatics just doesn’t explain things.

Occasionally one notices water levels in straws that are slightly above the water levels in the glass. And of course – even though we always talk about water seeking its level and hydrostatics and stuff – we know that that’s how it should be because of the capillary effects. And then we probably all did that experiment in school where we had a very thin glass tube and the water rose really really high. But have you ever wondered how heights between straws with different diameters would differ? (Really? Only me?)

Anyway, here is how:

I do realize that the diameter of “typical” straws differs from country to country, but these are the Norwegian – and German – typical straws, so I herewith define this as universally typical. Anyway, from left to right: 8mm, 4mm and 3mm diameter on the outside. Unfortunately I don’t have the tools to measure the inner diameter. Plus I really need to get clear thin straws! Sorry the water level is so hard to see in the yellow straw – I even dyed the water for you!

But even with the imperfect materials I have – isn’t this quite an impressive result?

Btw, this is what it looked like when I did the experiment in my kitchen.

When in doubt, pile higher. And deeper.

Marsigli’s experiment

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Density-driven flow.

The experiment presented in this post was first proposed by Marsigli in 1681. It illustrates how, despite the absence of a difference in the surface height of two fluids, currents can be driven by the density difference between the fluids. A really nice article by Soffientino and Pilson (2005) on the importance of the Bosporus Strait in oceanography describes the conception of the experiment and includes original drawings.

The way we conduct the experiment, we connect two similar tanks with pipes at the top and bottom, but initially close off the pipes to prevent exchange between tanks. One tank is filled with fresh water, the other one with salt water which is dyed pink. At a time zero we open the pipes and watch what happens.
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Two tanks, one with clear freshwater and one with pink salt water, before the connection between them has been opened.

As was to be expected, a circulation develops in which the dense salt water flows through the lower pipe into the fresh water tank, compensated by freshwater flowing the opposite way in the upper pipe.
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The two tanks equilibrating.

We measure the height of the interface between the pink and the clear water in both tanks over time, and watch as it eventually stops changing and equilibrates.
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The two tanks in equilibrium.

Usually this experiment is all about density driven flows, as are the exercises and questions we ask connected to it. But humor me in preparation of a future post: Comparing the height of the two pink volumes and the two clear volumes we find that they do not add up to the original volumes of the pink and clear tanks – the pink volume has increased and the clear volume decreased.
How did that happen?

Surface tension and office supplies.

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Lots of stuff an be made to float on water just because of surface tension.

This morning, I was taking pictures of heaps of waters on coins. I was planning to follow up on that post with pictures of a dome of water on a full mug. So far so good.

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Surface tension preventing this over-full mug from overflowing.

Then, I was planning on putting paper clips on top to show how surface tension would keep them afloat.

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More surface tension.

Except it DID NOT WORK. Maybe there was dish soap residue in the glass? Maybe I was too clumsy? I have no idea what was wrong. Anyway, I was on the phone with my mom later that day, and within half an hour I had the picture below in my inbox.

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Paper clips and other stuff floating on the surface of a mug filled with water. All because of surface tension.

I guess you can make almost anything float on the surface if you put your mind to it… ;-)

Surface tension – heaps of water.

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The classical way of demonstrating surface tension.

When talking about surface tension, the classical thing to do is to talk about the shape of drops of water.

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Water drop on a coin.

As seen before in this post, the drops of water act as lenses.

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It is pretty amazing how much water you can pile on a single coin!IMG_6533

If you can’t see it from the photos, here’s a video. But rather than watching the video, you should try it yourself. It’s fun!