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Phys Rev Lett. 2014 Feb 14;112(6):068103. Epub 2014 Feb 13.

Shapes of a suspended curly hair.

Author information

1
Department of Civil & Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
2
Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
3
Sorbonne Universit├ęs, UPMC Univ Paris 06, CNRS, UMR 7190 Institut Jean Le Rond d'Alembert, F-75005 Paris, France.
4
Department of Civil & Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA and Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.

Abstract

We investigate how natural curvature affects the configuration of a thin elastic rod suspended under its own weight, as when a single strand of hair hangs under gravity. We combine precision desktop experiments, numerics, and theoretical analysis to explore the equilibrium shapes set by the coupled effects of elasticity, natural curvature, nonlinear geometry, and gravity. A phase diagram is constructed in terms of the control parameters of the system, namely the dimensionless curvature and weight, where we identify three distinct regions: planar curls, localized helices, and global helices. We analyze the stability of planar configurations, and describe the localization of helical patterns for long rods, near their free end. The observed shapes and their associated phase boundaries are then rationalized based on the underlying physical ingredients.

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