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The differential friction effect of keratin fibres

The frictional characteristics of keratin fibres are anisotropic. The most obvious characteristic feature is that the frictional work required to slide one fibre over another is greater in the direction tip-to-root than the converse; this is the so-called differential friction effect (DFE). The authors describe a series of experiments where fine polymer (polyethylene terephthalate) and carbon fibres are slid in the two directions with a range of normal loads over cleaned human, camel, guanaco and llama hairs in order to quantify the DFE. These hair fibres exhibit a range of asymmetrical cuticular structures which have been characterised using electron microscopy. It is suggested, in keeping with established wisdom, that it is this topographical asymmetry which is responsible for the DFE. An analytical model is developed and evaluated in order to rationalise the origins of the DFE. The model combines various precedents from contact mechanics, the adhesion theory of friction and earlier studies of friction. The predictions of the model are consistent with the experimental data.