Superhydrophobic carbon nanotube forests.

ABSTRACT The present study demonstrates the creation of a stable, superhydrophobic surface using the nano-scale roughness inherent in a vertically aligned carbon nanotube forest together with a thin, conformal hydrophobic polytetrafluoroethylene (PTFE) coating on the surface of the nanotubes. Superhydrophobicity is achieved down to the microscopic level where essentially spherical, micrometer-sized water droplets can be suspended on top of the nanotube forest. 3 Kenneth K. S. Lau, Nano Letters Such superhydrophobicity can be understood by observing nature. In certain plant leaves, such as the lotus leaf (Nelumbo nucifera), rain droplets are known to roll or bounce off these leaves, removing dust particles and surface contaminants. This self-cleaning or Lotus effect 6 is caused by both the hierarchical roughness of the leaf surface from micrometer-sized papillae having nanometer-sized branch like protrusions and the intrinsic material hydrophobicity of a surface layer of epicuticular wax covering these papillae 7 . A very rough, heterogeneous surface allows air to be trapped more easily underneath the 5 Kenneth K. S. Lau, Nano Letters water droplet so the droplet essentially rests on a layer of air. A significantly higher surface area compared to the projected area in the case of a rough surface requires a greater energy barrier to create a liquid-solid interface. Coupled to this, when the surface energy of the surface material is intrinsically low, the combined effect is the surface will repel any water that comes into contact with it. Likewise, our PTFE-coated carbon nanotube forests aim to mimic nature's design. By growing a forest of nanotube pillars, an organized, heterogeneous surface is synthesized on a nano-scale. This makes it easy even for extremely small water droplets to be suspended on a surface approaching that of a perfect air-water interface (contact angle of 180°8). By coating this forest template with a PTFE layer, the water comes into contact with a material having one of the lowest surface energy (18 mN/m) and thus a high contact angle (108° on a smooth PTFE surface 9 ). By combining these two elements, the PTFEcoated nanotube forest can prevent water penetration down to the microscopic level, creating an enhanced superhydrophobic effect. Such modified carbon nanotubes may potentially be used in microfluidic devices, anti-soiling or anti-fouling surfaces, efficient heat transfer areas, or non-binding biopassive surfaces. We deposited the vertically aligned carbon nanotube forest with a plasma enhanced chemical vapor deposition (PECVD) technique 10-12 . Although a variety of different methods are also currently available, the PECVD process is the only technique that produces perfectly aligned, untangled (i.e. individually standing) carbon nanotubes whose height and diameter can be conveniently controlled. The PECVD process can be summarized in two main steps. First, the formation of nickel (Ni) catalyst islands on an oxidized (20 nm) silicon substrate through the sintering of a thin (5 nm) Ni film at 650 °C