Cottenden, D.J.; (2011) A multiscale analysis of frictional interaction between human skin and nonwoven fabrics. Doctoral thesis , UCL (University College London).

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Abstract

Various hygiene products, notably incontinence pads, bring nonwoven “topsheet” fabrics into contact with individuals’ skin. This contact can damage the skin in various ways, including abrading it by friction, a mechanism enhanced by the presence of moisture. In recent years skin-nonwoven friction has been the subject of significant experimental study in the Continence and Skin Technology Group, UCL, in the course of which methods have been developed which can detect differences in friction between a chosen nonwoven and equivalent skin sites on different individuals under fixed conditions. The reasons for these differences are unknown; their elucidation is one focus of this work. The other is to establish the influence of coarse geometry on the dynamics of a tense nonwoven sheet sliding over a substrate and interacting with it by friction. The first part of this work (“microfriction”) is primarily experimental in nature, and involves two separate experiments. The first involves using a microscope with a shallow depth of field to determine the length of nonwoven fibre in contact with a facing surface as a function of pressure; the second consists of measuring friction between chosen nonwovens and a skin surrogate at a variety of pressures and speeds whilst simultaneously observing the behaviour of the interface down a microscope. Both techniques were extensively validated, and the data from the two experiments were then compared. It had originally been intended to conduct the friction experiment on skin (the other experiment does not require it), and though all equipment was developed with this in mind and all relevant permission was sought and obtained, it was not eventually possible. Instead, a skin friction surrogate (Lorica Soft) established in the literature was used. Data from this show that Amontons’ law (with respect to load) is obeyed to high precision (R2 > 0.999 in all cases), though there is the suggestion of sublinearity at low loads. Detailed consideration of the friction traces suggests that two different friction mechanisms are important, and comparison with the contact data suggests tentatively that they may correspond to adhesion between two different populations of contacts, one “rough” and one “smooth”. Further work applying these techniques to skin is necessary. The second aspect of the work is “geometric friction”; that is, the relationship between the geometry of a surface (on the centimetre scale and upwards) and the friction experienced by a compliant sheet (such as nonwoven topsheet) laid over it in tension. A general equation of motion for slippage between sheet and surface has been derived which in principle allows for both objects to deform and interact according to any plausible friction law. This has then been solved in integral form for Amontons’ law and a low density strip exhibiting no Poisson contraction sliding over any surface with zero Gaussian curvature; closed form solutions for the specific cases of a prism and a circular cone have then been derived and compared. Experimental verification has been provided by a colleague, which shows very good agreement between theory and experiment. It has also been shown that, taking a naïve approach, the classic model for a rigid cylinder can be applied even to a quite extreme cone with experimentally negligible error. NB All prior copyrighted material (diagrams in all cases) has been removed from this edition to facilitate electronic distribution. They have been replaced with boxes of the same size, so pagination is identical with the complete version.

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