Controlled wavelength reduction in surface wrinkling of poly(dimethylsiloxane)

In this paper, we report that the wavelength of a wrinkled PDMS structure decreases upon annealing by combinative treatments of ultraviolet/ozone irradiation and oxygen plasma (case I). The wavelength of wrinkles also decreases when the PDMS surface is subjected to repeated exposure to a short-period oxygen plasma (case II). As opposed to other studies in which the wrinkling wavelength grows with processing time or temperature, some notable characteristics of wavelength reduction are observed in this study. Two routes toward decreasing the wavelength are associated with two different conditions of the PDMS surface. The former can be ascribed to a strain increase during the post-annealing procedure on a viscoelastic intermediate layer and for the latter, on the other hand, the selective hardening of an elastic intermediate layer controls the wavelength reduction. Elemental depth profiles obtained from Auger electron spectroscopy and nano-indentation of the surface by atomic force microscopy are employed to confirm these explanations. We also find that an externally applied stress can cause secondary wrinkling on a global scale, leading to a hierarchical structure with two wavelength scales. The proposed experimental results provide a new means of tailoring the surface morphology of wrinkled structures on demand.