Search

International audience; Indentation is a standard, widely used technique in mechanical assays and theoretical analysis. It unveils the fundamental modes of deformation and predicts the response of the material under more complex loads. Here we present an experimental setup for testing thin-film materials by studying the lateral indentation of a narrow opening cut into a film, triggering a cascade of buckling events. The force response F is dominated by bending and stretching effects for small displacements and slowly varies with indenter displacement $F\sim d^{2/5}$ , to finally reach a wrinkled state that results in a robust nonlinear asymptotic relation, $F\sim d^4$. Experiments with films of various thicknesses and material properties, and numerical simulations confirm our analysis and help to define an order parameter that accounts for the different response regimes observed in experiments and simulations.

While important research has been focused on developing surfaces that do not wet, some textures with high-wetting resistance are familiar in other applications in which the opposite is needed. A multivalued surface, common in most fabrics or meshes, allows the invading gas-liquid interface to support relatively high imposed pressures and plays a key role in producing topographic elements that avoid wetting. Here we study experimentally and theoretically the critical pressure needed to move a liquid through a network of pores and show that, for small aperture size, wetting and leaking are typical first-order transitions, with a singular behavior at the omniphobic/omniphilic limit (θc = π/2).

Thin, doubly curved shells occur commonly in nature and their mechanical properties and modes of deformation are very important for engineering structures of all scales. Although there has been substantial work on the stability and modes of failure of thin shells, relatively little work has been done to understand the conditions that promote continuous large scale deformations. A major impediment to progress in this direction is the inherent difficulty in obtaining analytical expressions for the deformed shapes. In this work we propose a new integrable solution which describes the behavior under load of a thin spherical shell with an opening (aperture) of n-fold axial symmetry. We derive a two-parameter family of approximately isometric, constant positive Gaussian curvature shapes that is in excellent agreement with our experimental results of deformed shells (3D scans of compressed ping-pong balls) and simulations (tethered membrane simulations minimizing the stretching and bending energy). The integrable solutions that describe those shapes have n symmetrically arranged curvature singularities which correspond to cusps of the folded shape. We examine the properties of the folded shells and observe that in the analytic solutions isometric closure is more easily achieved when the singularities lie away from the center of the aperture. We find that when allowed by the geometry of the aperture and the nature of the load, physical shells expel the curvature singularities into the aperture.

Controlling the fracture propagation in a film is an important element in the design of better packaging seals that must resist handling and yet provide a tearing mechanism for easy opening. Here we show that under simple initial setup conditions a divergent tear can be obtained that follows the path of a logarithmic spiral over two decades in length scale. We study the general rules leading to the “spiral growth” of a tear and connect its geometry to the specific material properties of the film.

Interfaces are ubiquitous in nature and absolutely key for life as illustrated by such complex interfaces as the cell membrane and the endothelial and epithelial linings of tissues. The mechanical properties of these interfaces play an important role in their biological functions. In this highlight, we describe our recent work (Pocivavsek et al., Science, 2008, 320, 912) using geometry as a tool for studying the behavior of complex interfaces. General scaling laws emerging from studying the shape of elastic interfaces can in turn be used in their characterization. Interfacial wrinkling is a well known phenomenon, however, the geometric patterns seen at biological interfaces are often far from idealized sinusoidal wrinkles. We show how more complex and non-linear patterns naturally emerge from wrinkles and how material properties can be extracted from these non-linear geometries.