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While bathing, if you rub soap on parts of your body like the chest or hand, it creates less foam. When you rub the soap for a longer time only then the foam produced will be more.

But, when you rub soap on your head with hair, the foam produced is in more quantity in less time. Why does this happen?

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up vote 5 down vote accepted

One of the more interesting jobs I did as colloid scientist working for Unilever was to look into why our South African company was getting complaints about the amount of foam from our shampoo (life isn't all boring for us colloid scientists :-).

It turns out that the amount of foam created varies with hair type. Curly afro-carribean hair generated much less foam than straight caucasian hair. Even amongst caucasians the thickness of the hairs varies (anything from 60 to 90 microns as I recall) and thicker hair generates less foam than thinner hair.

We didn't have time or funding to do a really thorough job on this (one of the downsides of a job in industry) but it was fairly clear that with straight hair you got high shear rates as the hairs rubbed over each other, and this high shear rate produced small bubbles and lots of rich creamy foam (as our marketing men would say). With thicker hair the increased rigidity made it harder to align hairs with each other and you got lower shear rates and larger bubbles and this produced less foam and it wasn't as creamy. With curly hair you got very little contribution to the shear from hairs rubbing over hairs and it was hard to generate much foam and what foam there was tended to be low quality.

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Interesting... I'm guessing that's why a soap-soaked sponge foams more when squeezed than soap-soaked steel wool when squeezed? – DumpsterDoofus Mar 19 '14 at 21:24
@DumpsterDoofus: yes, and you'll find the finer the pores in the sponge the small the bubble size in the foam. – John Rennie Mar 20 '14 at 6:43

Assuming this is actually true, I would guess it can be explained by available surface areas during the lathering process.

Bubble formation is induced by the incorporation of air into the soap solution (sort of like the physics of how whipped cream forms), and this typically happens at the interface between surfaces being rubbed together. At the hand-skin interface, the surface area is just the area of contact. But when rubbing soap into your hair, there is both the hand-hair interface plus the rubbing together of individual hair strands, which collectively have a very large surface area.

With all the pockets of air between individual hair strands, there is ample opportunity for incorporation of air into the soap when they are rubbed together, generating a quicker lather than that which occurs between two flat surfaces rubbed together,

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