Wrinkled, Dual-Scale Structures of Diamond-like Carbon (DLC) for Superhydrophobicity

Yudi Rahmawan, Kyung-Jin Jang, Myoung-Woon Moon, Kyung-Suk Kim, Kwang-Ryeol Lee, Kahp-Yang Suh
 

We present a simple two-step method to fabricate dual-scale superhydrophobic surfaces by using replica molding of poly(dimethylsiloxane) (PDMS) micropillars, followed by deposition of a thin, hard coating layer of a SiO_x-incorporated diamond-like carbon (DLC). The resulting surface consists of microscale PDMS pillars covered by nanoscale wrinkles that are induced by residual compressive stress of the DLC coating and a difference in elastic moduli between DLC and PDMS without any external stretching or thermal contraction on the PDMSsubstrate. We show that the surface exhibits superhydrophobic properties with a static contact angle over 160o for micropillar spacing ratios (interpillar gap divided by diameter) less than 4.Atransition of the wetting angle to approximately 130o occurs for larger spacing ratios, changing the wetting from a Cassie-Cassie state (C_m-C_n) to a Wenzel-Cassie state (W_m-C_n), where m and n denote micro- and nanoscale roughness, respectively. The robust superhydrophobicity of the Cassie-Cassie state is attributed to stability of the Cassie state on the nanoscale wrinkle structures of the hydrophobic DLC coating, which is further explained by a simple mathematical theory on wetting states with decoupling of nano- and microscale roughness in dual scale structures.