Before a woman immerses in the Mikveh, she must thoroughly wash her hair and her body to ensure that there is no Hasisa – substance that obstructs the water from coming in contact with the body. The question was asked whether an immersion is valid if the woman had not completely rinsed off her body the soap suds from the soap or shampoo that she had used when bathing. Hacham Ovadia Yosef was asked about a case involving a woman who, after immersing in the Mikveh, noticed some suds behind her ear or behind her neck, which she had apparently neglected to rinse off after she bathed. Do the suds constitute a Hasisa, such that she would be required to immerse again after removing them, or is her immersion valid despite the presence of suds?
Hacham Ovadia addressed this question in his work Taharat Ha’bayit (vol. 3, p. 243; listen to audio recording for precise citation), where he writes that the immersion in such a case is valid. He explains that since water is able to penetrate the suds and come in direct contact with the woman’s body, the suds are not considered an obstruction. Hacham Ovadia compares this case to the situation of a woman who immerses with oil on her skin, or with honey that had not yet dried, in which case the immersion is considered valid. A liquid substance on the skin does not constitute a Hasisa because it does not block the water from coming in contact with the skin, and this would apply to suds, as well.
Summary: If a woman realized after immersing in the Mikveh that there were still some suds on her body from the soap or shampoo she used when bathing, the immersion is nevertheless valid.