Biologically inspired tunable hydrophilic/hydrophobic surfaces: a copper oxide self-assembly multitier approach

In this study, a fabrication method for biologically inspired superhydrophobic micro- and nano-structured tier surfaces, each made of a self-assembled copper oxide, is presented. The method is controllable and applicable to bulk production when compared to existing high-end fabrication techniques. By modulating wet chemistry, different shapes and scales of tier structures were created. We demonstrated that their wetting behaviors are closely related to morphological information such as pitch, height and shape. To characterize their wetting behaviors, several experiments were designed and executed. In static water contact angle (WCA) measurements, morphological modulation led to wide WCA range (17°-95°). After hydrophobic self-assembly monolayer of 1-dodecanethiol, their WCA was escalated into superhydrophobic regime. In dynamic WCA, the contact angle hysteresis is greatly reduced by hybridizing the micro- and nano-tier (multiple tiers) when compared to utilizing a single tier. Also, the modification of the surface structure influences the rate of evaporation. In an analytical approach, the multiple tiers show a lower surface free energy compared to that of the single tier. By hybridizing different scales and shapes of tiers-such as hemispheric and conic shapes-the multiple tiers can efficiently reduce the surface energy barrier. Eventually, these manipulations lead to a subtle WCA hysteresis during the liquid motion testing. The analytical results are consistent with the dynamic WCA measurements. The multiple tiers also stabilize the Cassie regime and result in an increased hydrophobicity, which is more than when a single tier is employed.