Friction dynamics of human hair treated with water or cationic surfactant aqueous solution

Cationic surfactants are adsorbed on the surface of human hair and exhibit a lubricating effect. Here, we evaluated the friction when the hair treated with water or 1 wt% cationic surfactant (cetyltrimethylammonium bromide) aqueous solution was rubbed by a contact probe equipped in a sinusoidal motion friction evaluation system. Because of the rough structure of the cuticle on the human hair surface, an oscillatory phenomenon with a frequency of 50–70 Hz was observed when untreated hair was rubbed with a contact probe. On the other hand, the oscillatory phenomenon was not observed when the hair contained a large amount of water because stick–slip phenomena were inhibited on the soft, swollen hair surface. Furthermore, the change in kinetic friction coefficient and delay time, which is a normalized value of the time difference between the reaction of the friction force to accelerated motion, from untreated hair, Δμ_k and Δδ, was almost zero. However, we found a large difference in Δμ_k and Δδ for the hair treated with cationic surfactant aqueous solution. The treatment with cationic surfactant reduced both friction parameters, Δμ_k and Δδ, indicating that the treatment induced not only the frictional force but also the profile at the beginning of frictional sliding. The significant lubrication is due to cationic surfactant adsorption on the hair surface. These data imply that the smoothness of hair treated with a cationic surfactant is related to a reduction in friction coefficient and delay time δ.