Tribological Properties between Taut Hair Fibers in Wet Conditions: A New Shampoo Formulation for Eliminating Stick-Slip Friction

The tribological properties between taut hair fibers in wet conditions were investigated to evaluate the tangling/detangling performance of shampoo formulations during washing (particularly rinsing). A new setup for friction measurements between taut hair fibers was established. Using the setup, tribological properties of hair fibers treated with two shampoo formulations, whose tangling performances were different, were investigated. The base formulation for the shampoos was the same; the only difference was the type of amino-modified silicones used as conditioning agents. Shampoo (Sp) A (poor detangling performance) incudes water-insoluble silicone, and Sp B (excellent detangling performance) includes water-soluble silicone. The tribological behaviors between taut hair fibers treated with the diluted solutions of the Sp were very different; the Sp A-treated hair exhibited stick-slip sliding, whereas the Sp B-treated hair exhibited smooth sliding. This difference presumably comes from the different coacervation behaviors in the diluted solutions and resulting structure of conditioning film formed on hair surfaces. The smooth sliding (ideal for detangling) of Sp B-treated hair comes from the two-layer structure of the conditioning film on hair; positively charged amino-modified silicone dissolved in water adsorbed immediately on negatively charged hair surface, followed by the physical adsorption of coacervates on the adsorbed silicone layer. The outer coacervate layer is easily removed during sliding, and smooth sliding comes from the low friction of the silicone layer. The relationship between the type of amino-modified silicones, conditioning film formation on the surface, and tribological properties was discussed, which gives insights into designing Sp formulations with excellent detangling performance.