Adhesion and Nanotribology of Biofibres

Abstract: The fibre probe atomic force microscope is developed, which allows direct measurement of force and friction between single fibres at the nanoscale under various conditions. In this work, polyester and human hair fibres were used as being representative of synthetic and animal fibre. It is shown that the forces between hair fibres and polyester fibres in solution can be well explained by a DLVO interaction and that cationic surfactants modify the interactions in a manner consistent with current views of adsorption behavior. Friction coefficients are obtained from a friction-load relationship for the fibres using Amontons’ law. The variation of friction coefficient of the synthetic fibres with surfactant concentration is well correlated with surfactant adsorption behavior. However the frictional behavior of hair is somewhat unexpected, showing a minimum in frictioncoefficient below the cmc which may be related to surfactant – protein interaction at the surface.An analysis method to distinguish fibre friction in different sliding directions is established to allow discussion of directional friction effects. In contrast to macroscopic measurements, the effect is found to be small for native hair but significant for bleached hair at most of the sliding angles. This mechanism is largely correlated with the higher surface roughness of bleached hair. In addition the friction coefficient of bleached hair is shown to change periodically with the sliding angle, associated with cuticle – cuticle interlocking.A monolayer consisting of 18-methyleicosane thiol (18-MET) is fabricated as a model surface for hair. Friction and force measurements are performed and the results compared with those of the straight chain analogue eicosane thiol (ET) monolayer to discriminate the effect of the branched methyl on 18-MET. The 18-MET monolayer shows a slightly higher surface energy due to the terminal gauche defect of the film, and this can be correlated with a loss of adhesion in MSUD (maple syrup urine disease) hair where the methyl branched surface lipids are largely replaced by the straight chain analogue. While the current view on the function of methyl branched lipids is to reduce the friction of hair, our results shows lower lubricating ability for 18-MET monolayer than that of ET monolayer and the effect may be associated with the surface roughness and film density. It is also shown that the presence of the methyl branch alters surfactant adsorption behavior to the surfaces which has an impact on boundary lubrication.The adhesion mechanisms of the components in gastroliths, a calcium carbonatedbio material from a red claw lobster, are investigated by means of colloid probe AFM. A technique employed to extract the gastrolith and thus vary the composition, enabled a systematic study on the contribution of the various components; calcium carbonate, chitin and gastrolith proteins, to the detachment behavior and adhesion energy. Sequential detachment and large adhesion energies where observed between native gastrolith substrates and colloidal probes consisting of microparticles of heavily demineralized gastrolith and calcium carbonate. The sequential detachment behavior was absent when the soluble proteins have been removed from gastrolith substrates and the adhesive energy is reduced by more than two orders of magnitude. The sacrificial bonds that provide the large adhesion energies are probably related to multifunctional gastrolith proteins that are able to bind to both chitin and calcium carbonate.