4.6 Article

Maximizing the Giant Liquid Slip on Superhydrophobic Microstructures by Nanostructuring Their Sidewalls

Journal

LANGMUIR
Volume 25, Issue 21, Pages 12812-12818

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/la901824d

Keywords

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Funding

  1. NSF [0103562]
  2. Div Of Civil, Mechanical, & Manufact Inn
  3. Directorate For Engineering [0103562] Funding Source: National Science Foundation

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In an effort to maximize the liquid slip on superhydrophobic surfaces, we investigate the role of the nanoscale roughness on microscale structures by developing well-defined micro-nano hierarchical structures. The nonwetting stability and slip length on the dual-scale micro-nano structures are measured and compared with those on single-scale micro-smooth structures. A force balance between a liquid pressure and a surface tension indicates that hydrophobic nanostructures on the sidewall of microposts or microgrates would expand the range of the nonwetted state. When a higher gas Fraction or a larger pitch can be tested without wetting, a larger slip length is expected on the microstructures. An ideal dual-scale structure is described that isolates the role of the nanostructures, and a fabrication technique is developed to achieve such a microstructure-smooth tops and nanostructured sidewalls. The tests confirm such micro-nano structures allow a nonwetted state at a higher gas fraction or a larger pitch than the previous micro-smooth structures. As a result, we achieve the maximum slip length of similar to 400 mu m on the dual-scale structures, an increase of similar to 100% over the previous maximum reported on the single-scale (i.e., micro-smooth) structures. The study ameliorates our understanding of the role of each scale on hierarchical structures for a wetting transition and a liquid slip. The resulting giant slip is large enough to influence many fluidic applications, even in macroscale.

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