Fabrication of Optically Transparent PDMS Artificial Lotus Leaf Film Using Underexposed and Underbaked Photoresist Mold

Abstract

We report an extremely simple method of making an optically transparent superhydrophobic polydimethyl-siloxane (PDMS) thin film using underexposed and underbaked positive photoresist (PR) mold. Significant under soft-bake condition makes PR retain good amount of solvents, which greatly increases the dissolution rate of the PR during developing. A combination of optimal underbaking and underexposure condition created a unique hierarchical micro-/nano- structure on the PR mold, and inverse image of the PR mold was replicated to create a hierarchical micro/nano structured broccoli-shaped PDMS thin film. A 100-nm-thick fluorocarbon film was optionally deposited on the broccoli-shaped PDMS film to further enhance superhydrophobicity. Advancing and receding angles of the film for 4- <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm L}$</tex></formula> water droplet were found to be <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$161.33\pm 0.763^{\circ}$</tex></formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$133.33\pm 0.29^{\circ}$</tex></formula> , respectively, showing superhydrophobicity. It was also found that the optical transmittance at around 550 nm for this PDMS film was approximately 90%. [2012-0381]