Strategies for Superliquiphobic/Philic Surfaces

Liquid repellent surfaces can be used for self-cleaningSelf-cleaning and antifoulingAntifouling from organicOrganic and biological contaminantsContaminants both in air and underwater applications and can reduce fluid dragFluid drag (Bhushan 2009). As a model surface in living natureLiving nature for a liquid repellent surface in air, the upper side of the lotus leafLotus leaf surface repels water (superhydrophobicSuperhydrophobic) and is useful for self-cleaningSelf-cleaning and low adhesionLow adhesion applications (Barthlott and Neinhuis 1997; Bhushan and Jung 2011). As discussed in Chap.  4 , the superhydrophobicSuperhydrophobic properties of the leaf surfaces are achieved due to the presence of a hierarchical structureHierarchical structure created by a microstructureMicrostructure formed by papillose epidermal cells covered with three dimensional (3-D) epicuticular hydrophobicHydrophobic wax nanotubules, shown in Fig. 8.1a. The wax layer makes the surface hydrophobicHydrophobic and the hierarchical structureHierarchical structure makes the surface superhydrophobicSuperhydrophobic. This structure causes water droplets to roll off the leaf surface and take contaminantsContaminants with them to keep the leaf clean. The lower side of the lotus leafLotus leaf does not contain 3-D wax crystals (Neinhuis and Barthlott 1997), and consists of rather flat, tabular, and slightly convex papillae (Koch et al. 2009). Therefore, the bottom surface is hydrophilicHydrophilic, but superoleophobicSuperoleophobic in water, with a contact angleContact angle of 155° with n-hexane oil, Fig. 8.1b (Cheng et al. 2011). The lotus leafLotus leaf exhibits a so-called “Janus interfaceInterface” (named for the two-faced Roman god), with superhydrophobicitySuperhydrophobicity on the upper side, and superoleophobicity under water on the lower side (Cheng et al. 2011).