Fig. 2: Structural design considerations and numerical simulation of compressive deformation of the SEMS.
a Schematics of elastic contact between a cylindrical pillar and a soft substrate before, during, and after the onset of buckling, here Ainitial, Atransition, and Afinal indicate the contact areas of initial contact, transition, and post-buckling, respectively. The difference of areas between initial and final states normalized by the square of radius (R2) are calculated via Hertz Contact Models around the point of Euler buckling load, and plotted onto the color map, with two coordinates being the aspect ratio of cylinder and the ratio of indenter stiffness (Ep) to that of the substrate (Es). Deep blue region indicates the indenter is over-stiff such that Euler buckling is not likely within a reasonable range of skin strengths. b Finite element simulations of relative area change (∆S normalized by the total available surface area S0) under monotonically increasing loads for three different aspect ratios (L/R) of 4, 6, and 8. We assume plain strain condition with 33 pillars in a single unit. Boundary displacements are enforced via prescribing displacement and force feedbacks are averaged along the indenter’s flat surface. c Visual comparison between experimental SEM images and FEA simulation results. The morphologies of deformed pillars throughout buckling show good consistency with experimental observations. Color indicates von Mises stress amplitudes. This experiment is repeated independently for at least three times (SE: sensing electrode). All scale bars are 10 μm.
