Indentation of ultrathin elastic films and the emergence of asymptotic isometry

TL;DR

This paper investigates the indentation behavior of ultrathin elastic films on liquids, revealing rapid transition to a wrinkle-relaxation regime and a novel isometric response where the liquid absorbs most of the work.

Contribution

It introduces the concept of a nontrivial isometry in the film's deformation and characterizes the transition to the Far from Threshold regime in floating elastic films.

Findings

Rapid onset of wrinkle relaxation regime

Work transmitted mainly to the liquid at large indentations

Identification of a novel isometric deformation state

Abstract

We study the indentation of a thin elastic film floating at the surface of a liquid. We focus on the onset of radial wrinkles at a threshold indentation depth and the evolution of the wrinkle pattern as indentation progresses far beyond this threshold. Comparison between experiments on thin polymer films and theoretical calculations shows that the system very quickly reaches the Far from Threshold (FT) regime, in which wrinkles lead to the relaxation of azimuthal compression. Furthermore, when the indentation depth is sufficiently large that the wrinkles cover most of the film, we recognize a novel mechanical response in which the work of indentation is transmitted almost solely to the liquid, rather than to the floating film. We attribute this unique response to a nontrivial isometry attained by the deformed film, and discuss the scaling laws and the relevance of similar isometries to other systems in which a confined sheet is subjected to weak tensile loads.