5, Surface tension

Perfect bubbles

Do you know why bubbles burst when you touch them? It’s because of oil on your skin. It changes the surface tension across the bubble, destroying it. Light cotton gloves protect the bubble from your skin, and so you can hold them.

Check out Spangler on bubbles (NOTE: this video is age restricted for some reason that I don't understand. It's very good IMO)

If you have an hour to spare, there's an entire documentary on bubbles and how important they are:

Fun fact: If you make a large bubble on a plate, and cover your hand with bubble liquid, you can push your hand right through the bubble. Try it.

Making bubbles

Now, of course to do anything with bubbles, you need a bubble recipe. Here's something that should work:

6 cups water (distilled is best, but tap water will work) 1/2 cup or 120 ml of dishwashing detergent (Morning Fresh works well), you want cheap, non scented detergent. No fragrances or special additives. 1 tbsp baking powder (not baking soda see the difference) tbsp Glycerine or glycerol. This is harder to get. Use light vegetable oil as a substitute. optional: 1/2 cup or 60g of cornstarch(US) or corn flour (EU). They’re the same thing, just slightly different names. Once you have your bubble mixture, you need to make bubbles. There's a great video on making bubbles here:

So what’s going on?

A water molecule is small, super small. You just have no idea. There are around 1,390,000,000,000,000,000,000 H2O molecules in a single drop of water. OMG!

These water molecules are attracted to each other (referred to as cohesion) which causes "surface tension", and this creates a sort of skin on top of water. Check out this great article on surface tension.

Surface tension is why pond skaters can sit on top of pond water and how you can balance a paper clip on top of water (try it).

Washing up liquid changes things

We add washing up liquid to water to lower the surface tension. It makes the water stretchy and wibbly-wobbly which allows you to blow bubbles.

Then we add glycerine or vegetable oil (which is our special ingredient). Glycerine stops bubbles from drying out so they don’t pop as quickly.

Finally the starch makes the water even more stretchy and it evaporates slower.

Remember, bubbles love humid days. When it’s hot and dry they don’t last as long. Indeed I live in Andorra where the air is super dry. Bubbles really don’t last here. This is because the water forming the skin of the bubble evaporates and eventually breaks.

Bubbles popping

Here's a great video from the slow mo guys showing a bubble bursting.

The science

You may have noticed that this is more of an activity or craft exercise than a science experiment, but bubbles are all about surface tension or lack thereof. What's the science behind them?

Bubbles are created when air is blown or wrapped in a thin film of soapy water.

Bubbles cannot be blown with water alone due to water’s high surface tension, an important property of water. Surface tension is the strong cohesive forces on the surface layer of a liquid due to molecular bonds that works to hold the liquid together and resist external forces. In other words, surface tension helps create a kind of skin on the surface of the liquid to help keep the molecules together, like with raindrops.

However, when soap is added, it creates a new solution. This soapy water solution has a weaker surface tension than pure water, which allows us to blow bubbles. When air is blown through the soapy water solution, a bubble forms due to a 3-layer film of soap and water molecules around the air. Similar to a sandwich, the outside and inside film layers (the bread) are soap molecules with a thin layer of water molecules in-between.