The Science and Beauty of Soap Bubbles
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Transcript: English(auto-generated)
00:19
Okay, so to continue the talk that Dr. Hell just gave,
00:27
I want to tell you that my son is doing post-doc in Stanford on the very same subject with a colleague of Dr. Hell. And so this is the connection. Okay, this is a first lecture in a series
00:47
that I call Size and Aesthetics, and this is number one. And I will talk about science and beauty of soap bubbles. I'm from the Technion in Haifa. So let us start with some aesthetics.
01:02
Soap bubbles were a subject of art for many years. And not only children played soap bubbles, also adult, and you can see here even one picture by Chardin, Chardin from the year 1733.
01:22
And I'll show you a few more pictures. But if you look at the soap bubbles, you'll notice the following. Number one, they reflect light. So you can see it here. And yeah, and you can see it here, they reflect light. Okay, that's beautiful.
01:40
Now, and there are several more examples. Here again, you see the reflect light, and here, and here. And this is from the 19th century and 20th century. And here is another one from the 19th century. Okay, and they can even be transparent. So you can see the roses through the soap bubble here.
02:05
But what you don't see in any of these picture is that soap bubbles are very colorful. In fact, the most beautiful aspect of soap bubble is the colors that they have, and this is the subject of my talk. So what is soap?
02:21
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 presentation of soap molecules.
02:42
If you start at the top, then you'll see that this is the formula, and then you have one type of presentation and more detailed type of presentation, and even more detailed type of presentation. And down here, you see a very nice drawing
03:03
of soap molecules. And the soap molecules, as you can see, is divided into two parts. It has a head, like a little snake, and it has a long tail. So what are these? So here is what it is. The head is a polar end, which is charged,
03:22
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,
03:40
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,
04:03
the soap molecules surround it in such a way that the head stays in the water and the tail links to the soap, to the grease, and can take it away with it as you flush the water. Okay.
04:21
So this, let's go back. This shape, or this form, of the molecules are called micelles, and very nice pictures of micelles have been taken, and I'm sure that modern optical microscopy will give even more nice shapes.
04:41
So here are a few examples. This is just a drawing of how these molecules form micelles. These are three-dimensional, as you can see, and here is electron microscopy, and you can see different fat parts here, oil parts here and here, and they're surrounded by micelles.
05:01
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 force between molecules within a liquid
05:21
are shared with all the neighboring atoms, and you'll see it in this picture. So there's an atom here, or a molecule, and it is attracted to all the molecules around it, but the ones on the surface do not have anything above them.
05:40
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 in surface is called surface tension, and as I said, in water, it's very high, and this is why it is very difficult
06:01
to form a water bubble. The surface tension is so high that they break, and if you try to make a frame like this, and try to trap freedom of water in it on Earth, it can be maximum one centimeter.
06:22
In space, it can be much larger because there's no gravitation that pulls it down. So diameter here is about 10 centimeter, but this is in space, and on Earth, it is only one centimeter. So you cannot form water bubbles, but you can form soap bubbles that contain water, and this is the story.
06:41
So the surface tension of water-soap mixture, in the 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 molecule, pushing their hydrophobic ends out of the water,
07:03
and this separates the water molecule from each other, decreasing the surface tension, as it is illustrated soon. So the structure of soap bubble, the skin consists of layer of water. Soap bubbles are real water bubbles, but they're coated with two layers,
07:21
on the outside and on the inside, with soap molecules. And oil added, usually when we make soap bubble, we add some oil, some glycerin. The glycerin coats everything on the outside to prevent evaporation, so the soap bubble can live longer.
07:40
How long? Depends. On Earth, when you blow them, they can last for a minute or two, or something like that. But if you put them in an atmosphere which has a lot of water vapor in them, then they can last for a year.
08:00
They do not evaporate, and they can last for a long time. Oil added stick to the hydrophobic tail, and because it does not evaporate, it protects the water film from evaporation. And if you have a closed container, it separates, we saturate it with water, vapor, it allows evaporation, it slows evaporation,
08:22
and allows soap bubbles to last for 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?
08:41
We have water molecules in the center, and we have soap molecules sticking out on the outer surface and on the inner surface like this. And this is stable. Okay, what you see here on this side is surface tension, how water molecules
09:01
are at the surface without any surfactant like soap. And here is one with a soap molecule which really separates, it separates the water molecule and reduces the surface tension. Now what about colors? Color is one of the most beautiful aspects of bubbles,
09:23
also provides us with an accurate tool to measuring the thickness of the soap film, and indeed an estimate of the size of the molecule. And the first person who did this was Jean-Baptiste Perrin, a French scientist,
09:40
and this is him. And for his measurement of soap molecules, he got a Nobel Prize in 1926. So this was one of the first trials to measure size of molecule. And the concept of molecule, by the way, was not clear those days.
10:01
People knew about atoms, okay. But any solid, okay, 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.
10:20
Interface of light ray reflects some soap film is the following. This is something that I'm sure you all know, that if you have a reflection, if you have a beam comes here and is reflected from a surface in which this has higher
10:43
index of refraction, then it reverses. So the beam comes like so and the sign returns like so. And if it comes, the one that reflects it from the lower side is not reversed. And consequently, what happens here is the following. Reflected light will express 180 degrees phase change
11:02
when it reflects from a medium of higher index of refraction, which means from the outer surface. And no phase change when it reflects it from a medium of smaller index, which means from here. And what happens here is interference. So we have complementary colors.
11:22
If one of the colors that makes white light is subtracted from white light by interference, we see the complementary color. 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.
11:41
So bends of light and thickness contour. Here we have an example of soap film in a frame, soap bubble if you want to call it in a frame. And you see the thickness contour. You see that they are sparsely separated at the top, the large distances between them.
12:01
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 flowing down. Let's go back. The alternate band of light and dark in this soap film
12:23
are actually bends of color produced by reflection and interference of light waves. The color depends upon the film thickness, as you have seen before. The film shown here is sinisterly torn, becoming thicker towards the bottom. As the film thickness changes, the colors also change, forming regular bends.
12:44
So those of you who know topographic maps realize that it's just like in a topographic map. If 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.
13:03
Okay, so the shape of the old soap bubble is like that. This is how 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.
13:23
When this becomes very, very thin, the bubble pops. And the soap layers, the blue soap layers, which are protected from top to bottom, and this bubble is sitting on a table or on a piece of glass. Gravity pulls the water down, and when the thickness at the top of the bubble
13:41
reaches a certain minimum, the bubble pops. And in space, when 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. So I'll now give you a few examples of soap bubble colors.
14:05
So here is, these are pictures that I took for you. You notice a few things. Number one, you notice that you have a series of colors changes here, and you can estimate the thickness as you go down this.
14:24
Here is another example. As you go down, they become closed, these are spaced here, but become more dense down here. And here is another example. And here you start to notice something interesting.
14:41
If you look down here, you notice that there's a turbulence. It's not, a soap bubble is not quiet. There is a lot of commotion in a soap bubble because the water in the soap bubble move, and they move extremely fast in the soap bubble. So although you may see such pictures,
15:01
and you feel that when you look at it that there's something happening there, I will show you soon what really happens in there. Okay, oops. So here is another example, and here is something that I want to dwell on a little bit. First of all, you see this black spot,
15:22
and here, and here, and here. See this one? And here, yeah. Black spots meaning that the two soap films touch each other. And so it's like reflection from one surface, and there is no light coming out.
15:43
Okay, this means that we are starting to reach the end of the bubble. And here is another example in which the bubble looks very quiet. And here is another one.
16:00
And now, here's another. Look at the commotion down here. There's a lot of things are happening here. Okay, so the changing of the bubble thickness. The bubble thickness changes all the time because of the water flow inside between the two films.
16:20
The water flows very quickly in there, as I said before. And the film thickness changes very fast in every point of the bubble. The mobility is seen as fast-moving colors. Of course, the surface of the bubble. So let me try to show you what happens here.
16:44
So this is a short movie. And see how fast things go there. Very, very fast. So fast that in between frames, I lose information.
17:03
Okay, I'll show you a couple more examples.
17:22
This is real time. You can imagine how fast things change there. About one and a half centimeter. The size of the bubble is about one and a half centimeter.
17:42
Oh, sorry. 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.
18:01
When you see a black spot, no water there. The beginning of the end of the bubble. Okay, now I will show you another bubble. If the skin of the bubble is very, very thin, much shorter than the way of visible light,
18:22
then the two reflected rays of light will always meet crest to throw and destructively interface. And in this case, there will be no visible reflection, and the bubble looks black. And then the bubble is only 25 nanometers thick, and it will soon pop. And here is an example.
18:41
So 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 like here. All this area is so thin that it's barely holding, and this is only soft. What you have here is only soft, no water. The water flew down, all the way down.
19:03
You can see down here that there are still colors, and here things are changing. Let me show you how this bubble pops.
19:23
A lot of black things coming up, and it's gone, dead. Okay, so I want to sum up. So what are the lessons we learned from this presentation?
19:40
Not what you think. First of all, we learned that the world is beautiful. But we also learned that everything is temporary. Thank you.