Niddah 67 - A Woman Immerses in a Mikveh
When a woman immerses in a mikveh, nothing must separate between her body and the water. This is derived from the immersion of a metzora, spiritual leper, about whom the Torah says, "and he will immerse his flesh in water." Since the immersion is done in water as a matter of course, then the word "water" teaches exactly that law, that nothing should interpose between him and the water. Thus, there is a requirement to examine one's body and hair for interposition. In addition, the Sages established that a woman should also scrub, or shampoo, her hair before immersion.
In regards to this, Rava taught six specific laws, such as that while scrubbing the hair, she should not use anything that makes hairs stick together; that only warm water should be used, since cold water stiffens the hair and makes it more difficult to clean; and that she should wash the folds of her body, such as armpits, to remove any interposition. But we learned in the laws of mikveh that water does not need to enter folded areas! - True, but just as with flour offering, where blending is not critical, if it can be accomplished, but critical if it cannot, so here too, water does not have to enter there, but it must be able to, otherwise, the immersion does not work.
Art: Theo van Rysselberghe - Young Woman on the Banks of the Greve River