Surface Wrinkling of Plasma‐Exposed PDMS is Caused by Water Vapor Sorption: An Optical Environmental Sensor

Abstract Wrinkling of polydimethylsiloxane (PDMS) has unlocked a plethora of technological applications, from tunable surface wetting to photonic response. Surface undulations with prescribed wavelength and amplitude are excited by in‐plane mechanical compression of bilayers comprising a thin, stiff outer skin on a soft elastomer or gel. Plasma oxidation has become ubiquitous for creating such thin (≈10 nm) glassy interfacial layers. Spontaneous wrinkling can occur even in the absence of external mechanical strain fields, which has been rationalized in terms of a thermally‐induced strain that accompanies the expansion‐contraction cycle of such laminate films. It is shown that exposure to water vapor is, instead, responsible for surface wrinkling, due to the swelling the oxidized skin layer. This interpretation of surface wrinkling provides a rationale for the apparent experimental variability of the wrinkling process. This hypothesis is verified experimentally by observing and modelling the spatiotemporal evolution of the reversible wrinkling process under a range of controlled environmental conditions. From a practical standpoint, it is found that this effect provides for a facile approach for humidity sensing through structural color changes arising from the diffractive wrinkled skin.