Controlled Free Edge Effects in Surface Wrinkling via Combination of External Straining and Selective O₂ Plasma Exposure

Abstract

Herein the edge effect from the traction-free boundary condition is utilized to direct the spontaneous surface wrinkling. This boundary condition is attained by a simple combination of mechanical straining and selective exposure of polydimethylsiloxane (PDMS) substrate to O₂ plasma (OP) through a copper grid. When the strained PDMS sheet is subjected to selective OP treatment, a patterned heterogeneous surface composed of the OP-exposed “hard” oxidized SiO_<i>x</i> region (denoted as <i>D</i>₁) and the OP-unexposed “soft” region (denoted as <i>D</i>₂) is produced. The subsequent full release of the prestrain (ε_pre) leads to the selective wrinkling in <i>D</i>₁, rather than in <i>D</i>₂. It is seen that even in <i>D</i>₁, no wrinkling occurs in the vicinity of the <i>D</i>₁ edge that is perpendicular to the wavevector. Furthermore, the average wrinkle wavelength in <i>D</i>₁ (λ_<i>D</i>1) is smaller than that of the exposed copper grid-free blank area (λ_blank). This wavelength decrement between λ_<i>D</i>1 and λ_blank, which can be used to roughly estimate the edge-effect extent, increases with the applied mesh number of copper grids and exposure duration, while decreases with the increase of ε_pre. Meanwhile, there exists a decrease in the amplitude of the patterned wrinkles, when compared with that of the blank region. Additionally, hierarchical wrinkling is induced when the strain-free PDMS substrate is selectively exposed to OP, followed by uniaxial stretching and the subsequent blanket exposure. Consequently, oriented wrinkles perpendicular to the stretching direction are generated in <i>D</i>₂. With respect to <i>D</i>₁, no wrinkling happens or orthogonal wrinkles occur in this region depending on the applied mesh number, exposure duration, and ε_pre. In the above wrinkling process, the combinative edge effects in two perpendicular directions that are involved sequentially have been discussed