Understanding and controlling the friction of human hair.

Pleasant sensory perception when touching, brushing, and combing hair is largely determined by hair friction. As hair ages and weathers, its friction increases, mainly due to the progressive loss of the protective 18-methyleicosanoic acid (18-MEA) monolayer on its surface. Hair also displays anisotropic friction due to the protruding edges of the cuticles, which can interlock when sliding towards the root of hair. Moreover, certain chemical (e.g. bleaching and colouring), thermal (e.g. straightening and curling), and mechanical (e.g. brushing and combing) processes can dramatically accelerate 18-MEA loss, leading to much higher friction and unsatisfactory sensory perception. Hair care products, and in particular conditioners, have been developed to temporarily repair this damage through the deposition of various chemicals on the surface of the hair. These formulations can reduce friction to levels similar to that measured for virgin hair. Other external factors can also affect hair friction, such as humidity and cleanliness, as well as biological characteristics, such as ethnicity and age. Here, we provide a perspective on the advances made in the field of hair tribology, meaning the friction, lubrication and wear of hair. Historic and state-of-the-art experimental, theoretic and computational techniques for measuring hair friction are reviewed. We discuss different hair friction mechanisms across the scales and review the roles of surface chemistry and surface roughness on hair tribology. The influence of hair care products on hair friction is further discussed. Finally, we highlight open challenges and opportunities for future hair tribology experiments and models.