Controllable strain recovery of shape memory polystyrene to achieve superhydrophobicity with tunable adhesion

In this work, different superhydrophobic structures were fabricated on the surfaces of polystyrene (PS) blocks, using oxygen reactive ion etch (ORIE) and controllable strain recovery. The PS used here is a thermal shape memory polymer (SMP). Microhills are first generated on the surface of a PS block by ORIE. The PS block is then heated up, which triggers the strain recovery. During this process of recovering its original shape, the PS block gradually reduces its lateral dimensions, while increasing its thickness. Using different recovery temperatures, the surface morphologies can be controlled, which provides an approach to adjust surface wetting properties, including adhesion. At a temperature of 148 degrees C, microhills deformed to high-aspect-ratio nanowires. The corresponding PS surface has wetting properties similar to those of a lotus surface. The wetting is in Cassie-Baxter state, and a water drop is easy to get off from this surface. When the recovery temperature is increased to 162 degrees C, microwrinkles appear on the PS surface due to the different stiffnesses between the oxygen-treated top layer and the underlying PS substrate. These microwrinkles, together with nanowires located on their tops, form hybrid micro/nanostructures on the PS surface. The corresponding surface has wetting properties similar to those of a rose petal. The wetting is in a mixed state of Wenzel and Cassie-Baxter. A water droplet is stuck on the surface, although its apparent contact angle is as high as 166 degrees +/- 2 degrees.