Towards Bioinspired Superhydrophobic Poly(Lactic Acid) Surface Using Dip-Coating in Xylene/Titanium Dioxide Suspension

Abstract

Poly(lactic acid) (PLA) was injected into test sheets and then dipped in a suspension of xylene and trimethylol propyl silane (95/5) containing titanium dioxide (TiO₂) nanoparticles (NPs) at 65 and 85 °C for 3, 7, 10 and 15 s. TiO₂ NPs aggregates coated the sheet surfaces after dipping. The camera photos and the static water contact angle (WCA) showed that the dip-coating transformed all the hydrophilic surfaces into hydrophobicity, and especially that the sheet with retention of 85 °C and 7 s exhibited superhydrophobicity, while the water droplets on the 65 °C sheets were in pinning state. The water droplets rolled off the superhydrophobic surface at tilt angles below 3°, showing self-cleaning. The retention period of 7 s was suitable, as it achieved the highest surface hydrophobicity regardless the retention temperature. The images of scanning electron microscope (SEM) and optical microscopes demonstrated the deposition of TiO₂ NPs aggregates on the sheet surface and the formation of the porous structure on the surface. The combination of the aggregates with nanoscale protrusions and the pores with nanoscale pore walls constituted a hierarchical structure. The retention temperature of 65 °C made the pores shallow and wide, and the TiO₂ concentration of 2% instead of 1% caused the excessive TiO₂ NPs aggregates to cover the PLA substrate, reducing the WCA. The wetting models of water droplets on the surfaces of the 65 and 85 °C sticks were classified as Wenzel and Cassie states, respectively.