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My five-year old daughter was asking about astronauts the other day and why they float in space. After me showing her a few bits on the kids section on the NASA web site I started explaining about the planets and how the sun was a ball of gas and you couldn't stand on it. I then had to explain to her what a gas was, using ice, water and steam as examples for the three main states.

A few hours later she asked me if the bubbles from the bubble-bath in the bath water were a liquid and if they were why didn't the gravity make them run off her hands like water. That had me stumped.

I'm not a scientist but I've been doing my best to try and figure this out and explain it to her but I'm not fully sure, either.

Firstly, are bubbles from bubble-bath a liquid or have I made a bad assumption?

I thought it might be down to friction but I assume bubble-bath is a detergent and shouldn't encounter much friction.

I then thought it might be because the bubbles had a large surface area relative to their weight and would suffer wind resistance as a result.

My best guess now is that it's because the bubbles were more viscous than the water.

Are any of these right, if not can someone explain it to me in simple terms so I can tell her, please?

UPDATE

Wow, thanks for all the responses! I'm usually on Stackoverflow and I don't think that in two and half years on there I've asked anything that's generated this level of response. Thank you to everyone who took the time to answer or comment - I've read all that's been written here.

I explained this to my daughter last night saying that the bubble bath liquid was sticky and as a result would stick to things and slow down gravity's pull. I drew a bubble and explained about the liquid stopping the air in the middle getting out and by standing opposite and holding hands both I explained how we were like the liquid of the bubble squeezing the air in the middle which she understood.

I expect that if she's asking questions like this now I'll be on here again soon asking something else. I've certainly learnt a lot! Thanks again.

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Adhesion: mainly. –  mehfoos Aug 8 '13 at 10:50
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Bubbles are a two phase system: gas on the inside with a liquid skin (the gas on the inside is important to balance the surface tension of the liquid and stabilise the bubble). The liquid skin is light and thin enough that surface tension and aerodynamic forces easily overwhelm gravity most of the time. That's why you see blown bubbles floating in the light breeze, and also why bubbles are good at sticking together. –  Michael Brown Aug 8 '13 at 11:07
    
But some bubbles do run like water. They do not have to be full of gas. Liquids are generally to heavy on earth, but not in space. –  babou Aug 9 '13 at 1:48
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I see you're in the UK - you might want to watch the rather brilliant "POP! The Science of Bubbles" bbc.co.uk/programmes/b01rtdy6 –  Widor Aug 9 '13 at 12:34
    
Be a bit wary of what you read, however scientific it may seem. Answers are often a bit too assertive (not necessarily wrong) while things are not as clear cut as they make them. The way the site works and rewards is an incitement to answer a bit too fast, not always taking enough time for a moderate presentation of facts and their limits. Here is a picture of a typical rigid and spherical soap bubble which is flowing. Is it running as your daughter wants it ? And soap bubbles can be cut in pieces by appropriate action. –  babou Aug 9 '13 at 19:02

8 Answers 8

up vote 24 down vote accepted

The reason fluids flow off your hand while solids don't, is that fluids can change shape and solids can't. The molecules in a fluid want to stay together, but they don't care about the shape they're in, so gravity will cause them to spread out over your hand and flow off the sides. Solids can't change shape so they just stay on top off your hand, held in place by friction.

A bubble is a thin sphere of a water/soap mixture filled with air. The water/soap mixture has surface tension. This means that the molecules are pulling on each other to try and reduce the size of the bubble. But the air inside the bubble has air pressure. If the bubble gets smaller, the air pressure increases, pushing back on this thin layer of water and soap. This will result in a stable situation: the surface tension is pulling inwards, and the air pressure is pushing outwards, resulting in a specific size and shape. If the bubble somehow got smaller the air pressure would restore its size, and if it got bigger the surface tension would. If the bubble is deformed to something other then a sphere, the surface tension and air pressure are no longer regular and equal, and they will keep pulling and pushing until they are again, which, again, makes the bubble a sphere.

So in a sense, a bubble is behaving as if it was a solid, because it has a rigid shape and size. The bubble can't spread out over your hand and flow off the sides, because it wants to maintain its shape and size. And the bubble as a whole doesn't move as easily because of adhesion to your hand (the fluid-counterpart of friction). If you blow against the bubble or tilt your hand, the airflow or the gravity will overpower the adhesion, and the bubble as a whole will slide of your hand. It will never spread out and flow off unless you pop it, at which point there is no bubble to speak of any more, but just the water/soap mixture, which is a fluid.

In summary, a bubble has a somewhat rigid shape because of the combination of surface tension and air pressure. This means it can't flow, but only move as a whole. Adhesion between the bubble and your hand prevents the bubble from simply sliding off your hand.


I'm not great at this, but here's my attempt to phrase it as to be understandable for a child:

If something flows, it has to change shape. Fluids flow because they don't care about what shape they are. Solids, like a die, don't flow because they do want to be in a specific shape. A die is always a cube. Because of this, the die can only move as a whole. The die doesn't fall off your hand because there is friction between the die and your hand. Just like a piece of rubber, or a strip of anti-slip, on a table.

A bubble is a ball with air inside and a thin layer of water on the outside. Everything is made up of tiny things called 'molecules' (let's not get ahead of ourselves here). The molecules in a solid hold each other very tight, that's why solid things can't change shape. The molecules in a liquid pull on each other, but they don't hold each other. Because the molecules are pulling on each other, the water in the bubble wants to get smaller. But, the air inside the bubble also has molecules. Air is a gas. The molecules in a gas don't hold each other at all, they just wan't to get as far away from each other as possible. So the molecules in the air inside the bubble want the bubble to get bigger. If the molecules in the air are pushing harder than the molecules in the water are pulling, the bubble gets bigger. If the molecules in the water are pulling harder, then the bubble gets smaller. After a while, the bubble will become exactly so big that the molecules in the air are pushing just as hard as the molecules in the water are pulling.

Now if the bubble becomes smaller, the air molecules will push it out again. If the bubble becomes bigger, the water molecules will push it in again. So the bubble can't change shape. You can see this in a balloon (thanks to Bobson). Take an empty balloon. It is very small because the rubber is pulling the balloon together, and there is no air in the balloon to push it out. Now if you inflate the balloon, more and more air will get inside. So the air will push out harder and harder, making the balloon bigger. If you poke the balloon, you can feel the air pushing against your finger. And if you take your finger away again, the air pushes the balloon back into shape. This is exactly the same as in a bubble. Except the water will 'break' much easier then the rubber in the balloon. So you can't really poke it.

So just like the die, the bubble and the balloon want to be in a specific shape. This means the bubble can only move as a whole. The die couldn't slide off your hand because of friction. With the bubble something similar is happening:

Hold your hands in a cup and throw some water in. Now open your hands. The water flowed off your hands, but some of the water is still sticking to your hand. This is because the molecules in the water and the molecules in your hand are pulling on each other too. It's called adhesion. Because of this adhesion between the water at the bottom of the bubble and your hand, the bubble can't slide off your hand, just like the die.

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There is an error this answer: the surface tension of the water/soap mixture is not 'very high'. This implies that it is higher than water. However, it is lower than water. Try putting a piece of paper on water that drifts, and then add soap: it will drop. See exploratorium.edu/ronh/bubbles/soap.html. This page incidentally would be very interesting the questioner's 5 year old! Has she ever wondered... why is it so hard to get bubbles from plain water? –  Spork Aug 8 '13 at 15:14
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@PearsonArtPhoto - I think it'd be doable. You just need to add in a balloon to demonstrate with. –  Bobson Aug 8 '13 at 18:14
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@Bobson Thanks for the tip with the balloon :) I added it. –  JSQuareD Aug 8 '13 at 21:11
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Good explanation, but it was difficult to keep reading "dice" when you meant "die". –  Ryan M Aug 8 '13 at 22:02
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"Bubbles aren't just a liquid, they are a liquid with gas in the middle. Bubbles are always round everywhere - if something pushes on them they either pop or bounce back to being round. If they keep their round shape then they can't run off your hands like water does." –  psr Aug 9 '13 at 0:04

I would tell her the bubbles contain water, and that water is sticky. I would remind her that even after she lets water run off her hands by gravity, she still needs to dry them off with a towel (unless you use an electric hand dryer), because some of the water sticks. It's easier to see the foam than it is to see the water, because the foam is puffed up with air so it takes up more space, but surely she can feel when plain water is stuck to her skin.

She's probably not going to push the question at this stage into electrostatic attraction, which is fortunate because then you're at the point Richard Feynman was at when he couldn't explain to a reporter how magnets work. So a consistency "explanation" (i.e. this acts like that because it is like that, and you already take that for granted) will probably suffice.

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I presume you are talking about soap bubbles. girl blowing bubble

Soap bubbles as a whole are a mixture of water and soap(solved in water) and air(which is technically a mixture itself); so they don't have a specific state. However, if we ignore the air, then the solution of soap within water is a liquid; in fact a liquid with appropriately high surface tension(which as people mentioned happens to be less than ordinary water's surface tension).

Surface tension is a result of molecules highly attracting each other. It will cause the surface to tend to minimize its area. The molecules are like people with many hands, grabbing and pulling each other closer to themselves.

But if there is air constrained inside of a (let's say spherical) bubble; then the surface tension wants to shrink the size of the bubble, while the air inside will resist this by increasing its pressure. So they will reach an equilibrium. Now if there is a burst, then some molecules will loose contact with some of their neighbors and the molecules pulling from the other side will cause the bubble to collapse.

Now why don't bubbles slip off hands as water does? There are few things to consider in this case. First the relatively strong adhesion forces between the soap and skin will cause an attractive force between the bubble and the hand(like glue). The other thing to point is that bubbles are fairly light; with average density slightly higher than air(the exact number depends on so many factors) and much lower than water; In fact it's low enough that you can see a tiny breeze(and the tiny friction force between air and the bubble) makes them float in the air. These effects will cause the bubbles to run off the hands way much slower than water.

Good luck explaining these to your brilliant daughter!

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No, soap+water has a much lower surface tension than plain water. That's why you can't make stable bubbles in plain water, you have to reduce the surface tension with soap. –  Jim Garrison Aug 8 '13 at 16:28

Some bubbles run like water

Some fly, some fall down and some fall up.

All the previous answers are correct. The structure of bubbles is the result of surface tension. And that gives them a spherical shape when they stand in isolation. But why a spherical shape ? The reason is that the sphere is the shape that has the least surface for its volume. Since surface tension tends to minimize the surface without changing the volume. it naturally leads to this spherical shape.

If you make bubles attached to a wire frame, you can get strange and beautiful shapes that are the minimal surfaces going through the lines formed by the wire. Many examples can be found on the Internet (look for: bubble wireframe). Bubbles can actually solve some minimisation problems.

One aspect of surface tension is that is is actually a very weak force. It can keep air inside a bubble, without too much pressure, because air is very light, but it cannot do much more. It could not keep together a bubble full of water because water is far too heavy for it, unless you consider very small bubbles like those you se on leaves in the morning or after the rain. Water drops are actually bubbles of water. And they are too heavy to fly.

Things are much different in space, and generally in free fall, as thing no longer have weight. Then you can form large bubbles of water because surface tension no longer has to fight gravity. These bubbles will not collapse as molecules on the surface can be replaced by molecules inside. But they easily change shape if you touch them because the liquid is not compressible (like air) and will propagate the impulse from your finger. They can also flow like any liquid if you try to squeze them in your hand. But when left alone, they return to their round shape.

Several films are available on the web: Water bubble in Space (zero gravity) and Space Physics: The Science of Liquid Spheres in Zero Gravity

Finally there is yet another kind of bubbles: the bubbles of gas created in water. These bubbles contain gas, but are immersed in liquid. Being lighter than the surrounding medium, they "fall up" towards the surface of the liquid (because of Archimedes principle). They also keep a spherical shape because of surface tension. But they can be deformed as the surounding liquid propagate forces. If you create large gas bubbles in water, they will also flow through your fingers if you try to catch them. This can easily be experimented in a bath tub, by blowing air with a rubber pipe, or even a simple straw.

To summarize, there are 3 kinds of bubbles. They all have in common a surface made of liquid, that tends to take a spherical shape because of surface tension.

  • some have air inside and air outside. Thus they are very light and can easily fly in the breeze. They are the most beautiful because of the optical effects of thin transparent films. But they are also the most fragile, and disappear if the thin liquid surface is ruptured for some reason. Since they have only air inside and outside, they stay always close to their spherical shape because the compressibility of air will dampen anny attempt to propagate a force that would change their shape.

  • some have water inside and air outside. They are usually heavy and fall down. These are drops on earth. Some are so small that they are light enough to fly (a problem of mass to surface ratio), and constitute fog and clouds. But they are also too small to be visible to the naked eye. When in free fall in space, on board a satellite, they are no longer subjected to gravity and can be very large. They can lose temporarily their shape when touch because the liquid inside propagate forces.

  • some have air inside and water outside. They are very light, and will "fall up" because they are lighter than the surrounding liquid. However, in space, since there is no weight, they will not fall in any direction, exactly like the water bubbles in air. They to may have their spherical shape altered by the propagation of forces through the incompressible liquid outside.

The latter two kinds (with water outside or water inside) can flow, i.e., can run like water, either to go up or to go down. Bubbles with air inside and outside float and do not need to flow. There is not much to make them flow as they are very light, with very little than can carry forces around, so that surface tension is the dominant force. Furthermore, they are so fragile that applying forces is likely to simply destroy them. Still, I suspect it is very difficult but maybe not impossible to make such a bubble flow, for example around an edge that would cut it in two, provided its parameters (size) are such that the "half-bubbles" would still be viable.

With the help of films, observing water drops on an oily surface, observing bubbles in sparkling water or created in a glass with a straw, there is actually much to tell to a child, and to experiment with him. See for example on MIT site, which include a picture of an insect walking on water.There are also films of Water Striders Walking on Water

Note: I used interchangeably "air"and "gas", or "water" and "liquid", since this answer is not intended to be too technical.

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Phrased for a child:

Water and honey and gasoline and oil are liquids. "Liquids" means that the little molecules we would see if we had very good vision, stick to each other and roll down an incline or over your hand. As another said, like children in a chain holding hands, they can push and pull and can move as a chain wherever the force takes them.

Solids, like wood or stones, have molecules that are strongly tied to each other and can only move as one body.

Bubbles can only form in liquid where there is also a gas. Take bath water and air as an example.

If you blow with a straw through the bath water, bubbles form.

The air from your mouth, through the straw and then the water, pushes the mixed soap-and-water molecules up. The soap has made the solution stretchy, and the mixed molecules of soap and water surround the air from the straw and come out as a thin film trapping the air.

This is like the surface of the water except it surrounds the air of the bubble, and if one is careful the bubble does not break but can separate itself and float on the water or be slowly picked up by the hand.

It does not break because the solution molecules, which make the outside of the bubble stick to each other, make a balloon like surface surrounding the blown air.

Thus, even though the surface of the bubble is liquid, the bubble is mostly gas, and behaves like a gas balloon in the air, floating away.

In the bathtub bubbles can slide like a slow water fall because the gas makes them very light against the resistance of the air, and the liquid surface adheres to the sides.

So bubbles are not a liquid,nor a gas.

They are a balloon with a thin liquid surface and an interior of gas.

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can elastic be replaced or elaborated with "stretchy"? solution with "different things mixed together"? –  antony.trupe Aug 8 '13 at 16:51
    
@antony.trupe Sure. This answer is a sketch –  anna v Aug 8 '13 at 17:26

As all of the answers have mentioned, they key is surface tension. I would recommend demonstrating what surface tension is via experimentation. There are a couple of ways that it could be done (Belly flop vs straight in jumping in to a pool, for instance), but there is a YouTube video that demonstrates surface tension quite well at http://www.youtube.com/watch?v=e1RVOc0VZ30 . The method of demonstrating this is as follows:

  1. Take a bowl full of water.
  2. Place a tissue of water on the top carefully, with a needle above it.
  3. The tissue will absorb water, and sink, but the needle will stay above the water. This is because the surface of water is harder to get in than when you drop below the surface. Once the needle is nudged below the surface, it will sink quickly.

Bubbles are essentially all edge, and as a result, are stronger than they otherwise would be, acting in a similar manner to balloons. But as it is all just at the very surface, then they are much weaker than balloons, explaining their tendency to break apart easily.

I think that's the best you can explain to a 5 year old, but this is actually an active area of research. Generally speaking, mixtures of water/gas and air are called foams, and they have some very interesting properties. But I'll spare your 5 year old those details;-)

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Strangely enough, when I asked her if she knew what a bubble was she said foam, so she's got one up on me already! –  GrandMasterFlush Aug 9 '13 at 13:09

Its like a balloon, just smaller and more fragile.

EDIT: I mean, sure, there is more to it than that. But If your gonna get a five year old to understand all of it, its gonna take you a few years.

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Oh, by the way, you should try to add sugar or syrup to your soap water for your soap bubbles. That way you can bounce them on your jeans and shit. I used to love that when I was little. –  Mathias Madsen Stav Aug 8 '13 at 15:26

Bubbles stay the way they are due to surface tension. They are essentially formed due to a liquid(detergent solution).

This surface tension is a result of cohesive forces in between liquid molecules and adhesive forces with other molecules(Your skin, for eg.). These forces arise because of weak bond-formations at the molecular level between two liquid molecules, or a liquid molecule and another molecule.

These are similar to the forces which give adhesives like glue its stickiness. So basically, a bubble is just a sticky liquid on your hands. It just doesn't feel sticky because there is a very thin layer of it.

You also might wanna look at Slow-Mo videos of bubbles bursting to see droplets of liquid forming, to convince yourself and your daughter bubbles are actually liquid in state.

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