Superhydrophobic surfaces by laser ablation of rare-earth oxide ceramics
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esearch Letters
Superhydrophobic surfaces by laser ablation of rare-earth oxide ceramics Gisele Azimi, Hyuk-Min Kwon, and Kripa K. Varanasi, Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 Address all correspondence to Kripa K. Varanasi at [email protected] (Received 13 May 2014; accepted 1 August 2014)
Abstract Robust superhydrophobic surfaces can improve the performance of various applications. Considerable research has focused on developing superhydrophobic surfaces, but in these studies, superhydrophobicity was attained using polymeric materials, which deteriorate under harsh environments. Recently, it has been shown that rare-earth oxide ceramics are hydrophobic and since they are ceramics, they withstand harsh environments including high temperature. Here we fabricate a superhydrophobic surface by texturing a ceria pellet using laser ablation. We demonstrate water repellency by showing an impinging water droplet bouncing off the surface. This study extends the possibility of producing robust superhydrophobic ceramics using accessible techniques for industrial applications.
Introduction Suitably textured hydrophobic surfaces achieve superhydrophobicity, and the fabrication of superhydrophobic surfaces has been extensively studied.[1–6] Such surfaces have been used to improve performance in various applications, including drag reduction,[4,7–10] anti-icing,[11–18] and dropwise condensation.[19–26] In order to be widely applied to actual industrial processes, textured superhydrophobic surfaces have to be robust in harsh environments,[4] but the robustness of these surfaces remains a challenge. One reason is that many textured superhydrophobic surfaces inevitably rely on hydrophobic modifiers,[27,28] most of which are polymers that readily deteriorate under extreme conditions. Although it has been shown that some superhydrophobic surfaces can be created by directly texturing hydrophobic solids,[29–31] the state-of-the-art hydrophobic solids are also limited to polymeric materials.[4,28] Common durable materials, such as metals and ceramics, are generally hydrophilic,[28,32] and they can be rendered hydrophobic using polymeric modifiers. Recently, we have shown that the class of ceramics comprising the entire rare-earth oxide (REO) series is hydrophobic and have demonstrated their robustness under various harsh conditions.[33,34] We explain that the hydrophobicity of REOs arises from the unique electronic structure of the metal atoms where the unfilled 4f orbitals are shielded from interactions with the surrounding environment by the full octet of electrons in the 5s2p6 outer shell. This shielding effect minimizes polar interactions, inhibits hydrogen bonding with interfacial water molecules and renders them hydrophobic. Although few other studies have also reported high contact angles of ceria and
other REOs, the mechanism behind the hydrophobicity of REOs was not explained.[35–38] We showed that REOs sustain their intrinsic hydrop
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