Dan Shechtman (2016) - The Science and Beauty of Soap Bubbles

To continue the talk that Doctor Hell just gave, I want to tell you that my son is doing Post Doc in Stanford on the very same subject with a colleague of Doctor Hell. So this is the connection. This is a first lecture in a series that I call 'Science and Aesthetics', and this is number one. I will talk about Science and Beauty of Soap Bubbles. I’m from the Technion in Haifa. So let us start with some aesthetics. Soap bubbles were a subject of art for many years. And not only children play with soap bubbles, also adults. You can see here even one picture by Chardin from the year 1733. And I will show you a few more pictures. But if you look at the soap bubbles you will notice the following: Number one, they reflect light. You can see it here and you can see it here: they reflect light - that’s beautiful. And there are several more examples, here again you see that they reflect light and here and here. This is from the 19th century and 20th century. And here is another one from the 19th century. And they can even be transparent - you can see the roses through the soap bubble here. But what you don’t see in any of these pictures is that soap bubbles are very colourful. In fact the most beautiful aspect of soap bubbles is the colours that they have. And this is the subject of my talk. So what is soap? - Let us define soap. In those days, by the way, people didn’t know what soap is. So what is soap? How does it work to clean and form soap bubbles? In order to understand we need to understand the soap molecules. So here are several presentations of soap molecules. If we start at the top then you see that this is the formula. And then you have one type of presentation and a more detailed type of presentation. And an even more detailed type of presentation. And down here we see a very nice drawing of a soap molecule. And the soap molecule, as you can see, is divided into 2 parts. It has a head, like a little snake, and it has a long tail. So what are these? Here is what it is. The head is a polar end which is charged. And that part, the head, mixes very well with water. The back, the tail part, is non-polar and mixes with fat. Now, the part which is the head is hydrophilic, means water-loving. And the part in the back is hydrophobic, water-hating. But it is lipophilic - it loves oil, it mixes well with oil. So here we have the bridge between oil and water. And this is how it works. So if there is a piece of grease, like here, the soap molecules surround it in such a way that the head stays in the water and the tail links to the grease. And can take it away with it as you flush the water. This shape or this form of the molecules is called micelles. Very nice pictures of micelles have been taken and I’m sure that modern optical microscopy will give even more nice shapes. So here are a few examples. This is just a drawing of how these molecules form micelles. These are 3-dimensional, as you can see. And here with electron microscopy - you can see different fat parts here, oil parts here and here. And they are surrounded by micelles, each one of them is surrounded by those micelles. And they can carry the fat or the oil part in the water. So let’s say a few words about cohesion and surface tension. The cohesive forces within molecules, within a liquid, are shared with all the neighbouring atoms. And you will see it in this picture. So there is a molecule here and it is attracted to all the molecules around it. But the ones on the surface do not have anything above them. So this is what forms the surface tension. And it’s a very strong surface tension in water. The enhancement of the intermolecular attractive forces at the surface is called surface tension. As I said, in water it’s very high. And this is why it is very difficult to form a water bubble: the surface tension is so high that they break. If you try to make a frame like this and try to trap a film of water in it, on earth it can be maximum 1 centimetre. In space it can be much larger because there is no gravitation that pulls it down. Diameter here is about 10 centimetres, but this is in space - on earth it is only 1 centimetre. So you cannot form water bubbles, but you can form soap bubbles that contain water. And this is the story. The surface tension of water-soap mixture. In a soap-and-water solution the hydrophobic ends of the soap molecule migrate to the surface - they want to get away from the water. And squeeze their way between the surface water molecules, pushing their hydrophobic ends out of the water. This separates the water molecules from each other, decreasing the surface tension as it is illustrated soon. The structure of a soap bubble: the skin consists of a layer of water. Soap bubbles are real water bubbles but they are coated with 2 layers, on the outside and on the inside, with soap molecules. And oil added - usually when we make soap bubbles, we add some oil, some glycerine. The glycerine coats everything on the outside to prevent evaporation. So the soap bubble can live longer. How long? - Depends. On earth, when you blow them, they can last for a minute or 2 or something like that. But if you put them in an atmosphere which has a lot of water vapour in it, then they can last for a year. They do not evaporate and they last for a long time. Oil added sticks to the hydrophobic tail. And because it does not evaporate, it protects the water film from evaporation. And if you have a closed container saturated with water vapour, it slows evaporation and allows soap bubbles to last up to a year. So what about soap bubble surface stability? A bubble can exist because the surface layer of the water has a certain surface tension which causes the layer to behave somewhat like an elastic sheet. And this is how it looks like. So what do we have here? We have water molecules in the centre. And we have soap molecules sticking out on the outer surface and on the inner surface like this. And this is stable. What you see here on this side is surface tension, how water molecules are at the surface without any surfactant like soap. And here is one with a soap molecule which really separates the water molecule and reduces the surface tension. Now what about colours? Colour is one of the most beautiful aspects of bubbles. They also provide us with an accurate tool to measure the thickness of the soap film and, indeed, an estimate of the size of the molecule. The first person who did this was Jean Baptiste Perrin, a French scientist, and this is him. For his measurement of soap molecules he got the Nobel Prize in 1926. So this was one of the first trials to measure the size of molecules. And the concept of molecules, by the way, was not clear those days. People knew about atoms ok - it’s solid, so there are atoms. But what about liquid? What about in a gas? Can atoms form molecules? It was debatable. And he was the one who measured molecules. Interference of light ray reflects from soap film is the following. This is something that, I am sure, you all know. That if you have a reflection, if you have a beam that comes here and is reflected from a surface in which this has higher index of refraction, then it reverses. So the beam comes like this and the sign returns like this. And the one that reflects from the lower side is not reversed. And consequently what happens here is the following: Reflected light will experience 180 degrees phase change when it reflects from a medium of higher index of refraction, which means from the outer surface. And no phase change when it reflects from a medium of smaller index, which means from here. And what happens here is interference. So we have complementary colours. If one of the colours that makes white light is subtracted from white light by interference, we see the complementary colour. For example, if blue light is subtracted from the white light, then we see yellow and this will give us a measure of the thickness of the film. Bands of light and thickness contour. Here we have an example of soap film in a frame - soap bubble, if you want to call it that, in a frame. And you see the thickness contour. You see that they are spaciously separated at the top, there are large distances between them. And as you go down they are closer and closer and closer. And this is because of the topography of this film that looks like that. Why does it look like that? Because of gravity. This is filled with water and the water is pouring down. Let’s go back. The alternating bands of light and dark in this soap film are actually bands of colour, produced by reflection and interference of light waves. The colour depends upon the film's thickness, as you have seen before. The film shown here is thinnest at the top, becoming thicker towards the bottom. As the film's thickness changes, the colours also change, forming regular bands. So those of you who know topographic maps realise that it is just like in a topographic map. You have a large slope from a mountain then these lines are closer to each other. And if you have a plateau,they behave like this. The shape of a domed soap bubble is like that. This is what a soap bubble really looks like. There is water inside. There is a lot of water at the bottom. And there is much less at the top. And by the way this is why the bubble will soon pop. When this becomes very, very thin the bubble pops. The soap layers, the blue soap layers which protect from top to bottom. And this bubble is sitting on the table or on a piece of glass. Gravity pulls the water down. And when the thickness at the top of the bubble reaches a certain minimum the bubble pops. And in space, where you don’t have this problem, a soap bubble can exist for a very long time. Especially, if it is in a closed container which has humidity in it, it can last forever. I will now give you a few examples of soap bubble colours. Here is, these are pictures that I took for you. You'll notice a few things. Number one, you notice that you have a series of colour changes here. And you can estimate the thickness as you go down this. Here is another example. As you go down they are spaced here, but they become more dense down here. Here is another example. And here you start to notice something interesting: If you look down here, you notice that there is a turbulence. The soap bubble is not quiet. There is a lot of commotion in a soap bubble because the water in the soap bubble moves. And they move extremely fast in the soap bubble. So although you may see such pictures, and you feel that when you look at this that there is something happening there. I will show you soon what really happens in there. Here is another example. And here is something that I want to dwell on a little bit. First of all, you see these black spots here and here and here. You see this one and here. Black spots meaning that the 2 soap films touch each other. And so it’s like reflection from one surface and there is no light coming out. This means that we are starting to reach the end of the bubble. Here is another example in which the bubble looks very quiet. And here is another one and now here is another - look at the commotion down here. A lot of things are happening here. The changing of the bubble thickness: The bubble thickness changes all the time because of the water flow inside, between the 2 films. The water flows very quickly in there, as I said before. And the film thickness changes very fast in every point of the bubble. This mobility is seen as fast moving colours across the surface of the bubble. Let me try to show you what happens here. This is a short movie and you see how fast things go there, very, very fast. So fast that in between frames I lose information. I’ll share a couple more examples. This is real time. You can imagine how fast things change there. Question: What’s the size of the bubble? About 1½ centimetre, the size of the bubble is about 1½ centimetre. One more and then something else. You see there are black spots appearing here and there - zero thickness. Only the thickness of the ... No water, when you see a black spot no water there - the beginning of the end of the bubble. Now I will show you another bubble. If the skin of the bubble is very, very thin, much shorter than the wavelength of visible light, then the 2 reflected rays of light will always meet crest-to-trough and destructively interfere. In this case there will be no visible reflection and the bubble looks black. Then the bubble is only 25 nanometre thick and it will soon pop. And here is an example: This is not a broken bubble, this is a whole bubble, but the top is black. It means that up here - you can see the bubble right here - all this area is so thin that it is barely holding. And this is only soap - what you have here is only soap, no water. The water flows down, all the way down. You can see down here that there are still colours. And here things are changing. Let me show you how this bubble pops. A lot of black things coming up and it’s gone (laughter), dead. So I want to sum up: What are the lessons we learned from this presentation? Not what you think. First of all you learned that the world is beautiful. But you also learned that everything is temporary. Thank you. (Applause.)

Dan Shechtman (2016)

The Science and Beauty of Soap Bubbles

Dan Shechtman (2016)

The Science and Beauty of Soap Bubbles

Abstract

Soap bubbles have been popular children toys since ancient times. Inexpensive, easy to produce and very colorful they became a source of fascination to children and adults alike. Around 1733, artist Jean Siméon Chardin painted Soap Bubbles produced by a young man leaning out a window, and many artists produced lovely paintings depicting children blowing bubbles ever since.

Although soap bubbles can be easily produced, understanding their structure and properties deserves a close examination. Soap bubbles are really water bubbled coated with long soap molecules of which one side is hydrophilic, polar and ionic and the other side hydrophobic and non-polar.

In my talk I will detail the structure of the bubbles and their optical properties

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