Decoupling the effects of surface roughness and chemistry on the wetting of metallic glasses
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Bio-inspired superhydrophobicity has attracted significant attention over the last two decades. Efforts have been made to unravel the physics of different wetting regimes and to understand the effects of two governing parameters - surface chemistry and topography - on wetting. The majority of this research has been focused on polymeric materials due to ease in the fabrication of textured surface over multiple length scales. In contrast, the effect of surface texture on wetting of metals remains ambiguous because of inability to controllably fabricate micro- and nano-scale metal structures and isolate the chemical and topographic contributions. This ambiguity is amplified by the complex surface patterning techniques for metals, which often alter the structure and the chemistry of metals. To overcome this barrier, we used metallic glasses as model materials, which can be patterned by thermoplastic techniques without affecting the chemical and the structural state. We developed two new thermoplastic patterning methods for metallic glasses to study the effects of topography on wetting. The first method allows the patterning of metallic glasses without use of any chemicals that may modify the surface chemistry. The second method enables to build multitier (nano to macro) patterns on metallic glass surfaces to study the effectiveness of bio-inspired approach in metallic materials. Our results show that single scale surface patterns can render inherently hydrophilic metallic glasses hydrophobic when the surface chemistry is preserved. Combining micro- and nano-scale patterns results in superhydrophobic metallic glasses as observed in biological systems such as petal leaf and pigeon feather. These results suggest that inconsistencies reported in previous studies on wetting of textured metals stem from undesirable chemical changes which are avoided in the present work.