Spontaneous biaxial pattern generation and autonomous wetting switching on the surface of gold/shape memory polystyrene bilayer

In this paper, shape memory effect induced initiation and evolution of surface patterns (wrinkles and cracks) were studied on the surface of gold/shape memory polystyrene (PS) bilayer, alongside with their impacts on autonomous surface wetting effects. Surface wrinkling was generated as a result of in-plane compression in the gold film where the thermal-induced shape memory effect occurred on the foundation layer. Cracks were generated on gold surface when the wrinkle pattern was further developed at a higher strain. The in-plane surface morphological bifurcation was observed when the crack patterns were developed perpendicular to the wrinkles direction, which is induced by biaxial stress transformation within the gold thin film because of the lateral Poisson’s effect. The experimental mechanics investigations describe the relationships of the initiation/evolvement of surface morphology upon gold/shape memory PS bilayer with respects to various settings, such as the thicknesses of gold films, the applied strain on polymer layer, etc. The associated impact on surface wetting condition brought by the generated biaxial pattern on gold surface was studied. The water contact angle fluctuates within a narrow range according to the pre-strain for the samples after heating under the same plasma treatment times, which indicates that the biaxial pattern (cracks and wrinkles) in this paper have a little effect on the hydrophobicity of the gold film surface when the heated samples were treated by plasma for same times. After the surface plasma treatment, the surface hydrophilicity of the samples after post-annealing is significantly higher than that of the sample after deposition. And the contact angle decreases steadily as the air plasma treatment time is increased, the controllable surface hydrophobicity of gold coated PS bilayer can be achieved by tuning the plasma treatment time.