Category Archives: observation

Fog showing stream lines around cruise liner

Whenever I get out of my house and it looks like this, I am slightly disappointed because it means that the wave watching that morning will not be ideal. I mean, I like colorful sunrises as much as the next person, buuuut…

Today, at least, the fog was kinda interesting, also because there was a large cruise ship driving through.

There was a low layer of fog, but look at what happens as the ship passes through: It lifts up! Visualizing the stream lines around the obstacle. Pretty cool! (And thank you, little police boat, for making at least some waves for me today!)

Even better visible below, but check out the smoke coming from the ship’s chimneys. Do you see how it is propagating forward? Or does it just look like that to me? At least below the fog layer there was pretty much no wind. So what’s going on up there? Anyone care to explain?

Lüneburg — how a few centimetres in distance separate two seemingly completely different bodies of water

Sightseeing is best when it involves a little water watching, like for example last weekend in Lüneburg.

Doesn’t it look intriguing below, the change from a calm, mirror-like surface to something a lot less regular on the other side of the bridge?

Take it in: so peaceful! Although, judging by the plants growing in the water and by how they look like someone took a rake and put them in order, there must be a substantial current going through underneath the bridge.

And turns out there is: The bridge is a weir and there is a waterfall on the other side!

I find it so fascinating how the appearance of water can change literally over the distance of a few centimetre. So calm on one side, and boiling, spraying, turbulent on the other!

And then just a couple meters further downstream, we are back to mainly calm and only a few bubbles floating along give you an indication of what just happened upstream…

And again, no matter how peaceful everything looks here, the water plants tell us that there is still a lot of water moving, bending the leaves with it.

Do you look at this kind of things when sightseeing, too?

Watching the tides cause an hydraulic jump in the Irish Sea!

Looking at the picture above, taken in the South Walney Island Nature Reserve on our walk yesterday, what is the first thing you notice?

For me, it is not the cute little hide which is a perfect spot for seal and bird watching, for me it is — obviously! — what is going on with the waves! So much so that I spent the better part of an hour looking at the opposite direction of where all the seals were frolicking in the waves (except for one that came and played in the most fun part of the sea — more about that later).

Looking at the picture below, do you notice how different the different areas of water surface look? To the left of the wave breaker and going offshore from there, the surface is quite rough, with many waves of different wavelengths. But then going directly offshore from the wave breaker, the surface is smooth(er)! Followed by a rougher stripe, before it becomes smooth again, and a couple of well-defined wave crests reach the shore.

Zooming in on that area right off the wave breaker, you see that there are actually waves breaking towards the smoother area, away from the beach. Any idea what’s going on here, what might be causing those waves? (Hint: Even though there is a boat in the background, it is not some ship’s wake!)

What we can observe here is actually a pretty cool phenomenon, called a hydraulic jump. Due to the tide going out, there is a current developing around the tip of Walney Island, going from left to right in the picture above. This current goes over the still-submerged part of the wave breaker. Since the cross section through which the water has to squeeze is all of a sudden a lot smaller than before and after, the water has to accelerate. And it accelerates so much that waves traveling on it are just flushed downstream and the surface looks smooth(er). Only when the cross section is wider and the water has slowed down, waves become visible again.

The spot where waves are exactly as fast as the current, but running against it, is called “hydraulic jump”. You can spot it right where the waves are breaking: They are trying to go back upstream but don’t manage to, so they stay locked in one place (see here for an analogy of people running up and down escalators to explain this phenomenon). You do see hydraulic jumps “in the wild” quite often, for example in rapids in rivers (and even more so in regulated rivers, very nice example here!). In case of the hydraulic jump right here, there was a seal playing in the current, clearly enjoying the wave action (and quite possibly also feeding on poor fish that suddenly get swept away with the current).

And indeed, 20 minutes later, the same spot looks like this: the surface roughness is a lot higher towards the right of the wave breaker, but all in all there are much fewer, and much smaller waves.

And another 20 minutes later, the formerly submerged wave breaker is revealed!

I find it always so cool when you see a wave field and just from what that wave field looks like, you can deduce what the ground underneath has to be like! In this case from seeing the hydraulic jump, you know that the wave breaker has to continue on offshore.

Wanna see the whole thing in action? Then here is a movie for you!

And the coolest thing is that this spectacle will repeat with every outgoing tide, so pretty much twice a day! And I am fairly confident that it will also happen halfway between, again, when the tide comes in and the current goes in the opposite direction. I would love to go back and check!

Enjoying the South Walney Island Nature Reserve

Wanna come on a walk with me and Astrid around the southern tip of Walney Island?

This is what our parking spot looked like when we arrived (we did park on this side of the gate, obviously).

I find the salt marshes so impressive — all that grass that gets flooded every 12 hours! At low tide I keep taking pictures of crabs and little sea critters that got stranded in grass.

But very nicely visible how important the grass is for coastal protection: Waves get dampened out pretty quickly if they are running through grass!

But let’s start walking. See the high tide lines on the beach? Great markers of some of the last high waters. This kind of stuff — parallel lines in the sand, mirroring the water line — looks very calming to me!

In the South Walney Nature Reserve, there are several hides where you can sit and bird (or seal) watch, or enjoy the shade or shelter. They are so lovingly done, and all include information about the wildlife to be observed. One even has an exhibition of the different sands found in different locations around the island!

…and about other stuff found on the beach: beach shingle, rabbit poo and cow dung! I love this!

Moving on, we got closer to the lighthouse, which, unfortunately, isn’t open to the public any more. Just imagine the kinds of views you would get from up there!

And then, as we were approaching another hide (the red cabin in the picture below), I spotted something else that held me captivated for the better part of an hour. And I don’t mean the seals frolicking in the sea! Can you spot it?

Here is a closer look. Do you see what is going on there?

I’ll publish a blogpost with the explanation later today (it just got too much to put into one post), so stay tuned if you want to look more closely at the water with me!

Walking back, here is a view of an old castle ruin across the bay. See the now exposed salt marshes and gullies?

And here we are, back where we started. Go back to the picture up top of this same gate — isn’t it amazing that all this grassland was flooded only a couple of hours earlier, and will be flooded again in just a couple of hours? I am so used to seeing the german Wadden Sea coast where low tide exposes nothing more but mud

An art & science collaboration with Wlodek Brühl: #dropphotography

If you are here because you saw my title or talk at the Science in Public conference in Manchester and are curious about #dropphotography as a form of art&science collaboration in scicomm — a special welcome to you! If you are here for any other reason — welcome anyway! :-)

One of my favourite pet projects right now is the #scicomm collaboration with the artist Wlodek Brühl.

The idea behing the collaboration is very simple: Wlodek does awesome drop photography like what he showed in his recent exhibition, or the picture below: Drops falling into water, creating sculptures that he captures with digital photography.

I, on the other hand, use the opportunity of having people fascinated and mesmerised by this art, and curious to learn more about it, to talk about — what else? — physics! :-)

For one example of what that collaboration looks like in practice, check out my speech at the opening of his latest exhibition. We have a workshop coming up this autumn that we will run together, as well as other exciting projects which are, unfortunately, still secret. It’s going to be awesome, though!

But back to physics: Creating the exact sculpture you want requires enormous precision. This is what the setup looks like: Reservoirs above a vessel in which the drops fall. And a complex setup of flashlights and a digital camera, all coordinated by a custom-made piece of software.

The most important thing influencing the momentum with which a drop hits the water below is the height it falls from. Below, pictures are taking at constant intervals after drop release, yet you see that the for each new picture, the drop fell a little further than it did in the previous interval — it fell a little faster due to acceleration of gravity. Thus the longer you let a drop fall (i.e. the higher the drop falls from), the more it will accelerate, bringing more energy into producing a fountain.

The next thing that needs to be very precisely controlled are the opening times of the valves that release the drops. Not only when they open, but also for how long. Consider opening times of the valves in the picture below, left to right: 50, 55, 60, 70  milliseconds (The series of pictures on the left you know already from the “acceleration of gravity” pic above).

Depending on how long the valve is opened, different volumes of water are released, forming different drops. Easy to imagine that this will lead to different fountains once the drops hit the water below!

So what happens exactly once the drop hits water? The water surface gets deformed as the drop pulls it downwards. Due to surface tension, it then bounces back up, bringing up a column of water, that then collapses back down. And all these disturbances radiate ripples away from the original point of impact — capillary waves! (Capillary waves are super interesting because they behave very differently from “normal” gravity waves, but that’s a topic for a different post!)

But so that’s how fascinating it is to watch just one drop falling into water. Now imagine several drops falling one after the other, such that a second or third drop hits the column rising up after the first drop already hit the surface? That’s what makes these interesting umbrella shapes:

As you can imagine, there are tons of parameters you could vary now. Not only fall height and drop size, time lag between drops, number of drops that fall, but also viscosity of the fluid, shape of the bowl the drops fall into, all kinds of things. And that’s only using the easiest setup! You could also imagine using several valves, or air pressure to shoot drops with more momentum, or even have water shooting up from below (all of which Wlodek has done!).

And then, of course, depending on when exactly you choose to take a picture of the sculpture, you will see it in very different stages of formation and decay. You see the attempt of surface tension to minimise disturbances, instabilities that still form along the rim of the umbrellas which ultimately burst into many different small droplets…

Do you see the potential to talk about physics pretty much forever here? I love it! :-)

Opening speech for Wlodek Brühl’s art

You might remember that I had the honour of giving a speech at the opening of Wlodek Brühl’s art exhibition back in spring. Preparing my presentation for the Science in Public conference in Manchester next week (that I am immensely looking forward to!), I noticed I never posted the speech. Below is what I sent Wlodek in advance to prepare him for what I might say:

Lieber Wlodek, sehr geehrte Damen und Herren,

es ist mir eine große Ehre, die einleitenden Worte auf dieser Vernissage, dieser Eröffnung der Ausstellung von Werken Wlodek Brühls, zu sprechen und Sie hier Willkommen zu heißen.

Obwohl genau das die Wortbedeutung von „Laudatio“ist, ist mir ausdrücklich untersagt worden, den Künstler zu loben – vom Künstler selbst. Also werde ich heute lieber über Physik sprechen. Wenn Sie die Bilder von Herrn Brühl betrachten, denken Sie dann nicht auch sofort und unausweichlich über Physik nach?

Springbrunnen kennen wir zu Genüge, in Pfützen fallende Tropfen auch. Und letztendlich sehen wir hier genau das, wenn auch mit etwas mehr technischem Aufwand umgesetzt, um die entstehende Skulptur ganz genau beeinflussen zu können. Deshalb kommen uns die in diesen Fotografien gezeigten Formen seltsam vertraut vor – richtig gesehen haben wir sie aber noch nie. Auch wenn genau solche Strukturen um uns herum existieren (und dabei ist anzumerken, dass jede dieser Skulpturen einzigartig ist, wie auch keine Schneeflocke exakt einer anderen gleicht), mit bloßem Auge können wir sie nicht erkennen, weil sie nur für Bruchteile von Sekunden bestehen und unser Gehirn schlicht zu langsam dafür ist. Solche Skulpturen trotzdem bildlich festzuhalten gelingt mit vielen technischen Tricks: Mit computergesteuerten Ventilen, die Tropfen so auslösen, dass sie genau mit anderen Ventilen, dem Blitzlicht zur Beleuchtung und dem Auslöser der Kamera abgestimmt sind. Sichtbar werden dann Skulpturen und nicht nur verwischtes Wasser, weil die Skulpturen in einem stockdunklen Raum für ein Bruchteil einer Sekunde von einem Blitz beleuchtet werden, so dass trotz der längeren Belichtungszeiten der Kamera auf dem Bild nur dieser eine, enorm kurze Moment sichtbar wird, in dem es – exakt zum richtigen Augenblick – geblitzt hat.

Aber kommen wir zur Physik. Wenn Sie diese Ausstellung bei – aus physikalischer Sicht – einfacheren Bildern anfangen und sich dann langsam steigern möchten, würde ich empfehlen, in diesem Raum zu beginnen.

My picture of art by Wlodek Brühl, taken and published with kind permission.

Hier drängt sich mir zum Beispiel die Frage auf, warum die Skulpturen von unten aus dem Wasser nach oben zu wachsen scheinen, bevor sie sich ausbreiten, verzweigen? Der Schlüssel hier ist die Oberflächenspannung des Wassers. Die Wasseroberfläche, durch einen fallenden Tropfen nach unten ausgelenkt, schleudert den Tropfen wie ein Trampolin wieder nach oben und wölbt sich selbst hinterher, bäumt sich auf, bevor sie wieder in sich zusammen fällt. In einigen der Bildern kann man diesen nach oben geschleuderten Tropfen sogar noch erkennen.

My picture of art by Wlodek Brühl, taken and published with kind permission.

Und dann sehen wir im oberen Teil vieler der Skulpturen Formen, die wie Schirme oder Leselampen aussehen, oder wie Vasen. Warum sind diese Strukturen manchmal nach oben geöffnet, manchmal nach unten, manchmal voller filigraner kleiner Ärmchen am Rand? Das sind eigentlich schon zwei Fragen in einer. Die Form der Kelche hängt davon ab, wie schnell sich zwei Tropfen aufeinander zu bewegen und ob ein großer auf einen kleinen trifft oder umgekehrt. Die kleinen Ärmchen sind Instabilitäten, die entstehen, kurz bevor der Schirm zerfällt. Schirme ohne Ärmchen sind also genau in dem Moment belichtet und eingefangen worden, als noch alles stabil war. Bruchteile von Sekunden später wären auch sie instabil geworden.

My picture of art by Wlodek Brühl, taken and published with kind permission.

Und wenn Sie mit diesem Blick durch diese Ausstellung gehen, werden Ihnen noch viele andere Fragen kommen. Manchmal, zum Beispiel, sehen wir Skulpturen, die auf zwei Säulen zu ruhen scheinen. Wie sind diese wohl entstanden? Und dann sind die Skulpturen farbig – und die Farben sind direkt in der Aufnahme entstanden und nicht nachträglich digital eingefärbt. Das wurde in diesem Raum durch farbige Blitze gemacht, in den anderen beiden Räumen durch eingefärbtes Wasser.

My picture of art by Wlodek Brühl, taken and published with kind permission.

Wenn Sie als nächstes dann in den Raum dort hinten weitergehen, sehen Sie Skulpturen, die an der Wasseroberfläche gespiegelt sind. Durch die Spiegelung hat man auf ein mal zwei unterschiedliche Perspektiven auf die Skulptur und kann jetzt Strukturen noch genauer beobachten, um über sie nachzudenken.

My picture of art by Wlodek Brühl, taken and published with kind permission.

Im dritten Raum sehen Sie die neuesten Kunstwerke von Herrn Brühl, die vor wenigen Wochen erst entstanden sind. Hier wird die Physik noch komplexer. Zusätzlich zu all dem, was ich gerade schon über fallende Tropfen erzählt habe, kommen hier noch mit Druckluft angetriebene Fontänen hinzu. Und zwar einfache, die in der Mitte der Skulptur gerade nach oben schießen, und dann auch solche, die aus einer sich drehenden Turbine nach oben und außen geschleudert werden, erst einen Kelch bilden und dann in einzelne Tentakel zerfallen. Und in diesen Bildern sieht man manchmal auch die Schlieren und Pigmente der verwendeten Farben!

Wenn man wollte, könnte man an jedem einzelnen Kunstwerk stundenlang beobachten, grübeln und diskutieren. Wie sähe eine Skulptur wohl aus, wenn ein Tropfen größer gewesen wäre als er war, oder etwas später gefallen, oder vielleicht aus einer anderen Höhe? Oder wenn das Bild Sekundenbruchteile eher oder später gemacht worden wäre und uns damit einen anderen Zeitpunkt der Entwicklung und des Zerfalls der Skulptur gezeigt hätte? Was, wenn anstelle von Wasser zum Beispiel mit Honig gearbeitet würde? An Ihren Gesichtern sehe ich, dass diese Fragen Sie schon jetzt faszinieren. Das ist genau die Physik, die ich speziell bei den Tropfenskulpturen von Herrn Brühl so fesselnd und aufregende finde!

Ich wünsche Ihnen viel Spaß in dieser Ausstellung – dass Sie die beeindruckende Kunst von Herrn Brühl als Kunst genießen können, aber dass Sie sich vielleicht an manchen Stellen auch fragen, wie genau er es wohl geschafft hat, solch ein Meisterwerk entstehen zu lassen. Ich bin mir sicher, dass Herr Brühl Ihnen gerne Rede und Antwort stehen wird! In diesem Sinne: Herzlich Willkommen!

Algae as tracers for currents in Parsteiner See

Remember how I described a breeze creating a wave field on Parchsteiner See?

Well, it did not only cause a wave field, it also set up a circulation! Which I might not have noticed, had nit not also started deforming the algae patches! At first, it looked like above, and we could walk into the lake without having to wade into the green, like so.

But then a little later, there were algae everywhere, and you could see the swirls in the current traced out in green! Pretty cool passive current tracer, aren’t they?

Soap bubble musings

I have too many soap bubble pictures from last weekend’s trip to Kleinwaabs to not write a post about soap bubbles. So let’s get right into it!

First thing I never actually thought about: Why do you want soap to make soap bubbles? Soap lowers water’s surface tension (and see my favourite surface tension experiment here!), so wouldn’t that make bubbles more fragile than just using water? Turns out that without soap, there are hardly any bubbles because water’s surface tension is so high that it tends to lump water together into compact round shapes: so just drops, no bubbles. Which I should have known right away, obviously. So we need the soap as surfactant to keep the insides of the soap bubble apart and prevent collapse into drops.

So let’s look at how soap bubbles form. When someone (Frauke in this case) blows at the soap bubble wand, at first something resembling a wind sock forms (see above). Only after a little while it detaches and closes off bubbles that float away.

Soap in soap bubbles also produces the surface films that make soap bubbles look so pretty. And if you look at them closely, you can even see currents on soap bubbles as water and soap are flowing around on the surface!

Those currents are also one of the mechanisms that will ultimately make the bubbles pop: As gravity pulls the denser water to the bottom of the bubble, the soap concentrates on top. The more soapy the water, the lower its surface tension, so at some point the surface tension becomes too low to keep the bubble together — it pops.

Another mechanism making bubbles pop is just evaporation: As bubbles have a large surface, water evaporates fairly quickly from it, thus leaving more and more soapy water in the bubble. Until, you guessed it, the surface tension becomes so low again that the bubble pops.

A third reason for bubbles popping is also them floating into something which then breaks the surface. If bubbles float into other bubbles, though, this usually doesn’t result into them popping — they stick together and form interesting shapes of round segments and straight dividing walls. Surface tension always tries to minimize the surface area, balancing inside and outside pressure, so these are the energetically best shapes.

Interesting how that sometimes happens, while other times bubbles float nicely their separate ways, sparkling and shimmering in the sun.

And funny how difficult it is to take pictures of soap bubbles. Thanks for your patience, Frauke! :-)

A “Siel” – the valve in a dyke that lets freshwater out but no salt water in

Ok now, after complaining about how I dislike mud and the “no water” (i.e. low water) times in the Wadden Sea yesterday, today I’ll tell you about some stuff I really love about the North Sea coast. For example, the pretty little villages full with shrimp trawlers everywhere!

Picture above is Neuharlingersiel, below is Dornumersiel.

And the pictures below are from my favourite “…siel” so far, Greetsiel. Even though I unfortunately didn’t take any pictures of the village itself! I was so fascinated with all the shrimp fishing vessels.

The boom trawlers with all the colorful nets and ropes and everything are just too pretty, even on an otherwise fairly miserable day!

But what I find fascinating in all the “…siel”s is how they bring together a nautical atmosphere (after all, most of these ships have been out on the open North Sea all night!) and a very inland-pretty-old-town vibe. And here is how they do that: with the “Siel”. The “Siel” is basically a valve that lets freshwater out into the sea, but which doesn’t let salt water come back in. The way that is done by doors that are opened if the pressure on the inside (i.e. the land side) is higher than on the outside, and that close if the pressure on the outside is higher than on the inside. Pretty smart, ey? That way the land gets drained during low water, but no salt water comes inland to mess up the agricultural land. But I find it fascinating how this one little door in a dyke can separate two completely different habitats that coexist peacefully just one step away from each other.

Above, you are looking from the inland on such a “Siel”, which is currently closed because the water is higher on the outside than on the inside.

Oh, and what I always love: Light houses!!!

Yep, so I definitely have to go back. Stay tuned! :-)

Who enjoys playing with water clearly even more than me? An eager beaver!

Work took me to Brodowin (a village northeast of Berlin that is situated in a biosphere reserve and does amazing organic farming). The public transport, however, only took me to Chorin, which is a 1h 5min walk away from Brodowin — if you walk at the pace suggested by my app. Not, however, if you come across stuff like this:

This was the first time I’ve spotted evidence of beavers in the wild, and this time I was just walking all by myself through the forest and couldn’t believe my eyes! When I talked to locals later, they were all like “yeah, it’s a real nuisance around here, they flood acres and are really annoying!” but I was absolutely fascinated seeing their dams, and imagining what even better dam they will be able to build if that tree falls in the right direction, across that little stream. Really hope it does!

The water on the left in the picture above looks so weird, btw, because there is a lot of dust and pollen and stuff floating on it, that is stopped by the dam, while some water clearly manages to filter through. And ideally I should have jumped across the little stream to take better pictures, but there is a limit to what I am willing to do for a good picture. If only they had already gnawed that tree down, there would have been a convenient bridge across… But I will be back to check on their progress! :-)