Control of Surface Wrinkling through Compliant Nanostructured Interfaces

A novel approach for independent control of the wrinkle wavelength and amplitude, with the additional capability for anisotropic wrinkling without the need for multiaxial loading, is presented for applications in flexible electronics. Surface wrinkling occurs due to the mechanical instability induced by the stiffness mismatch between a soft substrate and a stiff film. In a two-layer system comprised of a thick compliant substrate and a thin stiff film, the wrinkle pattern, as described by the wrinkle wavelength, amplitude and orientation, is limited by the material properties and thickness of the two layers. In this work, an interface layer of nanostructures fabricated by Glancing Angle Deposition (GLAD) is introduced to modify the surface wrinkling of polydimethylsiloxane (PDMS) due to a Cu film, thus enabling further control of wrinkling in an otherwise constrained material system. Isotropic (nanospring) and orthotropic (nanochevron) Cu interfaces were GLAD-deposited with different geometric parameters to control the in-plane stiffness of the interface. The isotropic nanospring films provided a novel means to control the scale of wrinkle patterns, namely both the wavelength and the amplitude of surface wrinkles, while maintaining the amplitude-to-wavelength aspect ratio. The anisotropic nanochevron films resulted in anisotropy ratio of ~10 which provided a unique means to modify the wrinkle direction independently of the direction of applied load. The predictions by an experimentally-calibrated analytical model for anisotropic wrinkling were in very good agreement with experiments.