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?
Best Answer
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.