Condensed water on superhydrophobic carbon films
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Brian W. Sheldon and Janet Rankin Engineering Division, Brown University, Providence, Rhode Island 02912 (Received 19 March 2008; accepted 28 April 2008)
Nanostructured carbon materials, including carbon nanotubes, nanofibers, and nanowalls, exhibit a wide range of interesting properties dictated by the many different bonding configurations. Many of these materials can possess superhydrophobic behavior when water drops are placed on their surfaces: these drops have high contact angles and can roll freely on the surfaces, which is desirable for self-cleaning. In this work, we prepared porous carbon films using a microwave plasma enhanced chemical vapor deposition technique. These films showed superhydrophobicity with contact angle of 150°, which was explained by the synergetic effect of the highly rough surface combined with the hydrogen terminated edges of graphene sheets. However, the condensed water drops can behave differently: the drops did not roll readily. This behavior mimicked that of water on lotus leaves and further demonstrated that the reported superhydrophobic behavior is a function of how the water gets on to the surfaces.
I. INTRODUCTION
“Self-cleaning” often refers to water repellency as a result of the rolling behavior of high-contact-angle water drops formed on surfaces. As water drops roll about on such a surface, they can carry away dust particles along their paths. The self-cleaning effect is usually associated with the high contact angle of water drops or superhydrophobicity. The surface of the lotus leaf is perhaps the best-known example of such a superhydrophobic (water contact angle larger than 150°) surface, which is believed to be the result of micro- and nano-scale roughness as well as low surface energy.1 The lotus leaf has inspired many efforts in the past decade to mimic its micro- and nanostructures to create superhydrophobic surfaces.2,3 However, recent studies found that the lotus leaves may not be truly superhydrophobic, depending on how the water gets on to their surfaces.4 Specifically, if water vapor can infiltrate the hair-like structure of the lotus leaf and make the wetted hair-like regions more hydrophilic, a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www. mrs.org/publications/jmr/policy.html DOI: 10.1557/JMR.2008.0260 2174
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J. Mater. Res., Vol. 23, No. 8, Aug 2008 Downloaded: 21 Mar 2015
water drops can become “sticky” drops, which do not roll readily at any inclined angles. As a result, the lotus leaf may lose its self-cleaning ability in the presence of water vapor. Presently, little research has been conducted to study condensed water on artificial superhydrophobic surfaces, although condensation could help reveal the mechanisms responsible for achieving self-cleaning. The relationship between contact angle and interface
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