Friction and Force Between Human Hair Studied by AFM Hiroyasu Mizuno 1 , Gustavo Luengo 2 , Mark Rutland 1 1) Surface Chemistry, School of Chemical Science and Engineering, Royal Institute of Technology, 100 44 Stockholm, Sweden. 2) L’Oréal Recherche, 1 Avenue Eugène Schueller, 93601 Aulnay-Sous-Bois, France Atomic force microscopy has been used to measure frictional forces between two crossed native hairs. The frictional and normal forces were measured both in air and solutions of cationic surfactant C 14 TAB at different concentrations. The normal force curve provided no evidence that the hair surface has negative charges in water. The friction coefficient reduced with added surfactant up to 0.5 CMC but increased until it reached a plateau at 2 CMC. We speculate that this irregularity of the change in the frictional coefficients might be attributed to the fact that the lipids bound to the hair surface were removed after exposure to surfactant solutions. Keywords: surface force, AFM, Nanotribology, Hair, Cationic surfactant, friction, colloid probe 1. INTRODUCTION The nature of hair is of great interest for applications involvng hair care products and the importance of hair research is increasing as the needs of cosmetics and hair care products with better function grows. Knowledge about the interaction between hairs, which is not yet understood, is important to all branches of hair care research. Recently we have developed the fibre probe technique using AFM which has made it possible to investigate nanotribological behaviour of two crossed fibres. 1 In this study we report the friction and forces between two crossed native hair fibres under different condition. 2. METHOD Human hair was sonicated in 1% SDS solution for 2 minutes and rinsed with copious amounts of milli-Q water then dried with nitrogen 2 . Cleaned hair was cut to the desired length and a hair fibre with a length of 80-100 μm was glued on a cantilever while a relatively longer one (length of about 400 μm) was mounted on the edge of a used AFM chip perpendicular to the upper fibre (See Fig.1). Etched tungsten wires attached to a micromanipulator (Eppendorf) were used to position the glue and the fibre pieces respectively under a stereo microscope. The upper fibre was positioned parallel to the cantilever and glued such that it covered approximately 60 μm of the cantilever from the tip. After curing of the glue, the protruding upper hair fiber was cut precisely using a focused ion beam (FIB). The hair fibre diameter was measured by microscope and ranged from 53 to 83 μm. Additionally a slight increase in the diameter was observed in water. The surface free energy per unit area was calculated using the Derjaguin approximation and the measured radius. The experiments were performed using an Atomic Force Microscope (Nanoscope IIIa, Picoforce, Veeco). The normal force curves and friction force curves were measured in air (under ambient conditions with relative humidity of 20 - 40%), in 1 mM NaCl solution and a series of trimethyl(tetradecyl)ammonium bromide solutions, (C 14 TAB) with a 1mM NaCl background concentration. The CMC of C 14 TAB at 1mM mono-salt is 3.2 mM 3 . The AFM