Daily Practice: Squeaky Clean (October 11, 2011)
Sound is the audible manifestation of vibration which can be caused by friction. Some musical instruments create sound through controlled friction. Musical sound is made by the violin bow drawn across the strings, by the expelled breath of the trumpet player blown through the reed.
Our body hairs have an external layer composed of overlapping horny scales called the cuticle. Your skin also has an external layer composed of horny cells. Sliding your fingertips down the length of a hair causes friction between the two surfaces.
When your hair is dirty, its surface is coated with oil and other substances. This coating reduces the friction between your fingertips and the hair shaft. As a result, there is no audible sound.
But, when you shampoo your hair and remove the coating, you can once again hear the friction caused by your fingertips on hair. We describe this sound as "squeaky," it is heard when hair and fingertips are wet after rinsing out the shampoo. So yes, squeakiness is a good measure of hair's cleanliness.
A good loud squeak is an accurate indicator of clean hair. Continuing to shampoo after that point can be damaging.