Controlled free edge effects in surface wrinkling via combination of external straining and selective O2 plasma exposure.

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 SiOx region (denoted as D1) and the OP-unexposed "soft" region (denoted as D2) is produced. The subsequent full release of the prestrain (ε(pre)) leads to the selective wrinkling in D1, rather than in D2. It is seen that even in D1, no wrinkling occurs in the vicinity of the D1 edge that is perpendicular to the wavevector. Furthermore, the average wrinkle wavelength in D1 (λ(D1)) is smaller than that of the exposed copper grid-free blank area (λ(blank)). This wavelength decrement between λ(D1) 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 D2. With respect to D1, 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.