Ask Marilyn

I’ve long been fascinated with science, and one thing that puzzles me is why shampoo produces so much more suds during the second application. Do you know the answer? —Joe Farnham, Hornell, New York

Certain substances—in this case, the oil produced by the sebaceous glands on your scalp—reduce the ability of detergents to produce foam. This is why shampoo doesn’t foam much during the first application, assuming your hair actually needed washing.

But the amount of foam isn’t related to cleansing power. Rather, foaming agents are added because consumers like foam and equate it with cleaning action. In fact, your hair is effectively cleaned during the first application. So, as the substances that hinder foaming are removed, the second application produces lots of foam. And no wonder: Your hair is already clean!

The science of the whole process is relatively complex, and different shampoos provide varying results depending on your hair type and goals, what styling products you use, how often you wash your hair, and more. But also interesting, in addition to the reality that foaming is not related to cleaning, are these facts: thickening agents are added because consumers believe viscous shampoo works better than thin shampoo, and agents are used to make shampoo opaque as consumers think translucent shampoo just doesn’t look right!