Nanoscale Friction of Biomimetic Hair Surfaces

Abstract

We investigate the nanoscale friction between biomimetic hair surfaces using chemical colloidal probe atomic force microscopy experiments and nonequilibrium molecular dynamics simulations. In the experiments, friction is measured between water-lubricated silica surfaces functionalised with monolayers of either octadecyl or sulfonate groups, which are representative of the surfaces of virgin and ultimately bleached hair, respectively. In the simulations, friction is monitored between coarse-grained model hair surfaces with different levels of chemical damage, where different fractions of grafted lipid molecules are randomly replaced with sulfonate groups. The sliding velocity dependence of friction can be described using an extended stress-augmented thermally activation model. As the damage level increases, the friction generally increases, but its sliding velocity-dependence decreases. At low sliding speeds, which are closer to those encountered physiologically and experimentally, we observe a monotonic increase of friction with the damage ratio, which is consistent with our new experiments using biomimetic surfaces and previous ones using real hair. This observation demonstrates that modified surface chemistry, rather than roughness changes or subsurface damage, control the increase in nanoscale friction of damaged hair. We expect the experimental and computational model surfaces proposed here to be useful to screen the tribological performance of hair care formulations.

Effects of Surfactant Adsorption on the Wettability and Friction of Biomimetic Surfaces

“…This combination has been validated previously for the structure, wettability, and frition of virgin and bleached hair surfaces. 41,56 All bulk systems presented in this work were prepared using PACKMOL 61 for both the pre-assembled micelles and randomly distributed molecules and Moltemplate 62 for assembly of the systems.…”

“…92 The methodology for the NEMD simulations is similar to that previously described for water-lubricated hair surfaces. 41 First, squeeze-out simulations between two hair surfaces are performed in a water bath to establish equilibrium contact thickness and composition for a given contact pressure, σ. The cationic surfactant is preadsorbed on the surfaces in the configurations obtained from the adsorption simulations.…”

“…This range is consistent with our previous estimate of physiologically relevant contact pressures between two hairs. 41 No excess surfactants are present in the bath since we do not expect the surface coverage of surfactants to increase during squeeze-out due to micelle entrapment or further physical adsorption. Very few CTAB molecules were squeezed out from the contact due to the strong affinity to the surfaces.…”

“…[33][34][35][36][37][38][39][40] This is due to the removal of the protective 18-MEA layer and its replacement with anionic sulfonate groups. 41 Atomic force microscopy (AFM) experiments showed that the application of a commercial hair conditioner reduced the friction of chemically damaged and bleached hair to levels comparable to virgin hair through adsorption onto the damaged patches. 36 More recent AFM experiments suggested that friction reduction of chemically damaged hair due to treatment with a commercial conditioner was evident below a normal load threshold of 5 nN (corresponding to a mean Hertz pressure, σ ≈ 120 MPa for a 15 nm SiN tip), above which friction was similar to untreated damaged hair.…”

“…56 We used these models to investigate the friction between two hair surfaces at various levels of chemical damage under dry and wet conditions using non-equilibrium molecular dynamics (NEMD) simulations. 41 Recently, a similar coarse-grained MD framework was used to study the adsorption and friction of large biomacromolecules mixed with ionic surfactants on hair surfaces. 57 In this work we study the adsorption of cationic and anionic surfactants onto virgin and bleached hair surfaces using coarse-grained MD simulations.…”