Hierarchical patterns on laminated composite bilayer films via surface roughness-mediated buckling instability

This study describes buckling instabilities mediated via surface roughness to generate hierarchical structures on a metal (aluminum)/polymer (polystyrene) laminated composite bilayer film on a rigid substrate. Buckling instability is a non-lithographic approach to generate microscale features in thin films. Metals can buckle over the surface due to compressive stress arising from differences in thermal expansion of the individual layers. The topographies of the buckles characterized by wavelength and amplitude can be manipulated by varying the thickness of the layers and the extent of thermal annealing time. Here, the pre-buckled surface on the laminated composite bilayer film formed via two–step molding resulted in hierarchical patterns via surface roughness-mediated buckling instability. The negative replica of the buckles was generated by poly(dimethyl siloxane) (PDMS) and the positive replica of the buckles was fabricated via molding using a PDMS negative replica onto the PS film. The metal layer was subsequently formed on the PS film and the laminated composite bilayer film enabled patterns with hierarchical topography following thermal treatment. The hierarchically structured surfaces exhibit moderately enhanced hydrophilicity and the elastomeric replica with hierarchical buckles showed stretchable hydrophobicity up to 120% stretching due to partial preservation of the pre-structured buckles on the surface.