For my last Science by Email activity, I have always planned to do one of my favourite activities - something that was fun as well as containing a lots of science and maths.
Let's blow some bubbles.
You will need
An airtight container, such as a plastic container with a lid.
Measuring cups
biodegradable, concentrated dishwashing detergent. I have found "Morning Fresh" works well.
warm water.
sugar
Something to stir the bubble mixture with.
Some bubble blowing frames. You can make some using pipe cleaners or wire twisted into a ring.
Making bubble mix
What to do
Mix the following in your container:
3 cups dishwashing detergent.
7 cups warm water
1 cup glycerol or sugar
Mix it very thoroughly (I stirred for about 10 minutes).
You may find it easiest to start by dissolving the sugar into the water, then add the detergent.
The amounts above will produce around 2.5 L of bubble mix. You can make more or less bubble mix by multiplying the amounts given. Just make sure you keep them in proportion.
Once you have mixed them, dip your bubble blowing frames in and start blowing bubbles.
What's happening
Bubble films form because of surface tension. Water molecules are attracted to each other. This attraction is particularly strong at the surface, where it makes the surface of the water act like a stretched-out balloon skin. This effect is called surface tension. (If you have ever done a 'belly-flop' into a pool, you have felt surface tension.) The surface tension of ordinary water is so great that it can't form bubble films, the tension pulls them apart. When you add the detergent, the surface tension is reduced just enough that it can form the films, but not tear apart. The sugar helps slow the rate that the bubble mix evaporates, so it takes longer to dry out and burst.
Bubble blowing tips
Bubble mixture works better as it gets older. Bubble mix that's more than about two weeks old will work better than a brand new batch. I used to do bubble demonstrations when I worked with the Questacon Maths Centre and we used one batch of bubble mix for over a year. When you are finished with your mix for one day, seal the container. Then you can try it again a few weeks later.
Bubbles burst because they dry out, so your bubbles will last longer when they won't dry out. Bubble-blowing works best at night or on a cool, humid day. Undercover on a rainy day is ideal.
Try not to stir the mixture up with your bubble frame. Froth on the surface makes it harder to form bubbles.
Unless you have a small bubble frame, it is easier to make bubbles by waving it gently through the air instead of trying to blow through it with your mouth.
Bubble blowing can be messy, so don't do it on a carpet. The bubble mix will wash out of clothes.
Water quality can affect bubble mix as well. Hard water (water with mineral salts dissolved in it) won't produce bubbles as well as soft water. If you live in an area with hard water, you may find using distilled water makes better bubbles.
Anything acidic, like lemon juice or vinegar, will help break down the bubble mix. You can use them to help clean up.
Bubble colours
What to do
Blow some bubbles or make a bubble film on your bubble frame and hold it vertically.
Look at the colours in the bubble film.
You should see swirling colours on the surface of the bubble film. Over time, these colours may organise into horizontal stripes.
What's happening?
White light is made up of all the colours of the rainbow, mixed together. Imagine the blue part of the white light that hits the bubble film. Most of the light will go all the way through, but some of it will reflect off the outside surface of the bubble film and some of it will reflect off the inside surface. When the two reflected beams of light meet up with each other, two things can happen:
The light waves will make each other stronger, and you will see blue light bouncing off that part of the bubble film.
The light waves will cancel each other out and you will see no blue light bouncing off that part of the bubble film.
When light goes through a bubble, some of the light is reflected by each surface. These reflected beams of light produce the colours you see on the bubble.
Whether you will see a particular colour reflected depends on two things: how thick the bubble film is and the colour of that part of the light that you're considering (its wavelength). In some places, the film may be just the right thickness that you will see a bit of blue light reflected back. In other places, it may be a bit of red light, or yellow, or a mix of blue and red. Over time, the liquid in the bubble flows down, so it becomes thinner on the top and thicker on the bottom, so you get the different coloured stripes.
Bouncy Bubbles
What to do
Use a bubble-frame with a round hole about 10 cm wide.
Dip the frame in the bubble mix and pull it out, but don't blow a bubble with it.
Hold the frame so the bubble film is horizontal.
Move the frame gently up and down.
You will find the film will bounce up and down like a trampoline.
Hold the frame still and the bubble film will go back to being flat.
What's happening
A bubble film may have less surface tension than water, but it still has some surface tension. When you bounced the bubble film up and down, it was stretched, but the attraction between the water molecules kept pulling them together, so when you stopped bouncing, it was pulled flat again. The attraction between the molecules makes a bubble film always pull in to make the smallest surface it can.
Bubble Shapes
What to do
Blow some bubbles in the air. Blow some single bubbles, some double bubbles and some bunches of bubbles. (It helps to have some bubble frames with several holes)
Look at the shapes of the bubbles and how they fit together. You may notice some things about the shapes of the bubbles:
When you blow a single bubble, it will always form into a sphere.
When you blow a double bubble, it will always form a sort of snowman shape, with a film between the two bubbles.
When you blow a bunch of bubbles together, they can form all sorts of different shapes, but there are some rules they always follow. You will never see more than four bubbles touching at a single point or more than three touching along a line. Whenever you have three bubble films coming together, the angles between them are always the same, 120 degrees.
Try experimenting with different bubble frames and see what shapes you can make.
What's happening
As we saw before, bubble film is under tension. This means that the shapes the bubbles make always tries to make the shape with the smallest possible surface area, so they have to stretch as little as possible. This simple rule determines all the shapes you can make with your bubble frames. For example, a sphere has the smallest surface area of any shape of a particular volume. So if you blow a single bubble, it will pull itself into a sphere no matter what shape the bubble frame is. Similarly, a double bubble will always have that snowman shape, because the bubble films would have to stretch more to make any other shape.
Mathematicians have studied the shapes bubbles make for centuries. Although they have found some of the rules that bubbles follow and a variety of strange shapes they can form, it has not been easy. In fact, it wasn't until the year 2000 that mathematicians were able to prove for certain that the snowman shape is the only shape that a double bubble could possibly make.
