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Controlled Free Edge Effects in Surface Wrinkling via Combination of External Straining and Selective O<sub>2</sub> 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 O<sub>2</sub> 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<sub><i>x</i></sub> region (denoted as <i>D</i><sub>1</sub>) and the OP-unexposed “soft” region (denoted
as <i>D</i><sub>2</sub>) is produced. The subsequent full
release of the prestrain (ε<sub>pre</sub>) leads to the selective
wrinkling in <i>D</i><sub>1</sub>, rather than in <i>D</i><sub>2</sub>. It is seen that even in <i>D</i><sub>1</sub>, no wrinkling occurs in the vicinity of the <i>D</i><sub>1</sub> edge that is perpendicular to the wavevector.
Furthermore, the average wrinkle wavelength in <i>D</i><sub>1</sub> (λ<sub><i>D</i>1</sub>) is smaller than that
of the exposed copper grid-free blank area (λ<sub>blank</sub>). This wavelength decrement between λ<sub><i>D</i>1</sub> and λ<sub>blank</sub>, 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<sub>pre</sub>. 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><sub>2</sub>. With respect to <i>D</i><sub>1</sub>, no wrinkling happens or orthogonal wrinkles occur in
this region depending on the applied mesh number, exposure duration,
and ε<sub>pre</sub>. In the above wrinkling process, the combinative
edge effects in two perpendicular directions that are involved sequentially
have been discussed