Spooky science

One of the coolest things I’ve found whilst trolling the internet around Halloween time is Ghost Bubbles.

These bubbles hold their shape quite well and can even be held (above) or bounced. There are two key bits of science at play here: one, is how the bubble holds it shape and, two, why we see the “smoke” inside the bubble.

Why bubbles hold their shape is all about surface tension. Molecules are attracted to one another, and water molecules are no exception. If you try dipping a bubble wand into a glass of water, you won’t have much luck making a bubble. This is because the water molecules are too attracted to one another. If you, however, add a surfactant (like dishwashing liquid) the surface tension of the water is lowered. When you blow a bubble, you’re trapping a layer of water molecules in between two layers of surfactant molecules like this:

The opposing pressures inside and outside the bubble mean that it’s shaped like a sphere. If the bubble comes in contact with dirt or oil, the neat arrangement is broken and the bubble will burst. You can make your ghost bubbles bounce or even hold them by wearing a clean cotton glove or bouncing the bubble on a clean towel.

Second phenomenon: why we can see the “smoke” inside? This has to do with the properties of dry ice. Dry ice is solid carbon dioxide, which goes straight from solid to gas at room temperature (sublimation). Carbon dioxide gas is colder than air, so what you actually see is the water vapour in the air condensing around the carbon dioxide. Like when water vapour condenses on cold glass in the shower.

Try this fun bit of everyday science out for yourself!