Yi Zheng, C. L. Su, Jiong Lu, Kian Ping Loh
Water wetting on a hydrophobic surface at ambient conditions is disallowed by the nonpolar nature of the surface and high vapor pressure of water. However, the presence of sub-millimeter sized hydrophilic patches allows the waxy wings of desert beetles to become wettable by morning mist. Here, we show that a sprinkle of graphene oxide nanoflakes (nanoGOs) is effective in condensing water nanodroplets and seeding ice epitaxy on graphite at ambient conditions. By controlling relative humidity and nanoGO density, we are able to study the formation of a complete ice wetting layer on a time scale of 20 hours. This presents an unprecedented opportunity to visualize ice nucleation and growth in real time using non-contact atomic force microscopy. The stages of crystallization, as proposed by Ostwald in 1897, is fully unfolded at a microscopic level for the first time. We obtain real-time imaging of sequential phase transition from amorphous ice to a transient cubic ice Ic stage and finally to the stable hexagonal ice Ih. Most interestingly, we discover that ice nucleation and growth can be influenced by modifying the functional groups of nanoGO, and by intermolecular hydrogen-bonding between nanoGOs. This affords a strategy to control heterogenous ice nucleation and snow crystal formation.
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http://arxiv.org/abs/1307.2673
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