Frédéric Restagno, Laboratoire de Physique des Solides, CNRS, Univ. Paris-Sud, Université Paris Saclay, Orsay, France

Abstract: Friction is an old story that is still modern despite of its importance. During this talk, we will present some funny experiments and, in particular discuss the phonebook story.
It is nearly impossible to separate two interleaved phonebooks when held by their spines. A full understanding of this astonishing demonstration of solid friction in complex assemblies has remained elusive. In this communication, we shall report on experiments with controlled booklets and show that the force required increases sharply with the number of sheets.

A model which captures the effect of the number of sheets, their thickness and the overlapping distance has been developed involving one dimensionless amplification parameter [1]. We revealed that the secret to this amplification of friction lies in the angles the sheets make as they approach the overlap region. More precisely, the self-amplification function allowed to collapse all the experimental data onto a self-similar master curve.
In addition to solving a long-standing familiar enigma, this model system provides a framework with which one can accurately measure friction forces and coefficients at low loads, and that has relevance to complex assemblies from the macro to the nanoscale.

[1] Alarcón, H., Salez, T., Poulard, C., Bloch, J. F., Raphaël, É., Dalnoki-Veress, K., & Restagno, F. (2016). Self-Amplification of Solid Friction in Interleaved Assemblies. Physical Review Letters, 116(1), 015502.