Boundary curvature effect on the wrinkling of thin suspended films

A relation between the boundary curvature kappa and the wrinkle wavelength lambda of a thin suspended film under boundary confinement is demonstrated. Experiments were performed with nanocrystalline diamond films of approximate thickness 184nm grown on glass substrates. By removing portions of the substrates after growth, suspended films with circular boundaries of radius 30-811 mu m were fabricated. Due to residual stresses, the portions of the film bonded to the substrate are of approximate compressive prestrain 11 x 10 - 4 and the suspended portions of the film are azimuthally wrinkled at their boundary. Measurements show that lambda decreases monotonically with kappa, and a simple model that is in line with this trend is proposed. The model can be applied to design devices with functional wrinkles and can be adapted to gain insight into other systems such as plant leaves. A method for measuring residual compressive strain in thin films, which complements standard strain characterization methods, is also described.