Abstract：Complex surface microstructures are ubiquitous in nature and endow biological tissues with superior physical and chemical performances. Inspired by nature, various bionic structures have been well designed and extensively applied in engineering fields such as micro-electro-mechanical system, optical devices, energy materials, biological templates and sensors. Self-organization of surface wrinkles in film-substrate systems driven by stresses is a facile and effective method to achieve various complex surface patterns. The wrinkle phenomena are typically observed in stiff films on elastic substrates when the imposed compression is beyond a critical value and they have been extensively investigated in the past decades. Recently, the surface instabilities of homogeneous stiff films on viscous substrates via stress relief have also received a great deal of attention. However, the instability modes and mechanical mechanisms of heterogeneous or structured stiff films on viscous substrates are still unclear up to now. In this work, metal iron films are deposited on viscous gel substrates by magnetron sputtering and spontaneous wrinkles of the films are investigated. Especially, the effects of constrained boundary and thickness-gradient boundary on wrinkle morphologies are elaborated. It is found that as the film thickness increases, the film surface evolves from networked folds to labyrinth-like wrinkles gradually, and the wrinkle wavelength decreases slightly first and then keeps constant. Near the constrained boundary, the wrinkle wavelength successively changes, exhibiting a hierarchical characteristic. Due to the boundary effect, the stress is anisotropic near the boundary and the wrinkles are always perpendicular to the boundary. In the thickness-gradient region, the wrinkles exhibit herringbone or wavy shapes along the gradient direction while the wavelength is almost unchanged. The morphological characteristics, evolutional behaviors and physical mechanisms of the wrinkles on viscous substrates are analyzed deeply based on the stress theory. This study could promote better understanding of the sputtering effect of metal films on viscous substrates and implement controllable surface microstructures by design of patterned structures.
2022, Vol. 43 
