Why is soap foam always white?

10 January 2017

Question

Muhammed's 5-year old kid asks: Why does soap come in different colours but always have white bubbles?

Answer

Chris Smith put Muhammed’s kid's question to Imperial College's Stuart Higgins...

Stuart - It’s an amazing question. and you’d think actually that soaps contain all these different dyes in them that absorb light of different colours so why isn’t that colour reflected in the foam? Things look like a certain colour because when the light hits the object it’s either absorbed or certain wavelengths are reflected, but, actually, in this case we’re getting lots of different wavelengths reflected. So, even though there is dye in the soap it has this certain colour, so much light is being scattered back to us and reflected back to us that actually we perceive it as white.

There’s a great experiment and I don’t know if any of you have done this with Marmite?

Chris - Do you love it or hate it?

Stuart - Oh I can’t stand Marmite itself but this is a great. You can do this at home. If you get a bowl and put some marmite in it and you get a whisk and you start whisking the Marmite. Marmite is this really sticky dark brown substance and you think well lights not being reflected from that. But as you whisk more and more air into it you create a foam of Marmite and then actually it starts to go paler and paler. And then, eventually, it starts to turn this sort of white cream colour.

Chris - And you discovered this how?

Stuart:: Practice…

Chris - It really goes white?

Stuart - Yes, if you whisk it far enough. It’s kind of like a beigey white. But yes, if you really put enough air into it, it will eventually go foamy and it will start to look white.

Chris - Can you make Marmite meringues then?

Stuart - Oh that horrible! Ughh!

Chris - That’s a horrible thought…

Stuart - Yeah.

Chris - But from a physics point of view, would it happen?

Stuart - I guess if you could somehow get the Marmite to spread out and be dispersed, as we say, into a mixture of egg and protein then maybe you could. That sounds like a great idea. I might try that later.

Chris - And just to return to the soap point of view, away from the Marmite. It’s the same principle then that when you look at snow, this is white despite the fact that you’ve got water, which is transparent, that makes it, you end up with white snow because it’s lots of little crystals of ice which are doing pretty much the same thing. Milk probably the same, or Dettol, when you pour Dettol or detergent in water you get it going into a cloudy white colour and that’s little blobs which are bending light in the same way.

Stuart - Yeah. It’s all to do with...does this material we’re looking at have lots of different angles and complex structure that can cause light to scatter and reflect in lots of different angles?

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