Carbon nanotube based functional superhydrophobic coatings

The main objective of this dissertation is synthesis of carbon nanotube (CNT) based superhydrophobic materials. The materials were designed such that electrical and mechanical properties of CNTs could be combined with superhydrophobicity to create materials with unique properties, such as self-cleaning adhesives, miniature flotation devices, ice-repellant coatings, and coatings for heat transfer furnaces. The coatings were divided into two broad categories based on CNT structure: Vertically aligned CNT arrays (VA coatings) and mesh-like (non-aligned) carbon nanotube arrays (NA coatings).;VA coatings were used to create self-cleaning adhesives and flexible field emission devices. Coatings with self cleaning property along with high adhesiveness were inspired from structure found on gecko foot. Gecko foot is covered with thousands of microscopic hairs called setae; these setae are further divided into hundreds of nanometer sized hairs called spatulas. When gecko presses its foot against any surface, these hairs bend and conform to the topology of the surface resulting into very large area of contact. Such large area of intimate contact allows geckos to adhere to surfaces using van der Waals (vdW) interactions alone. VA-CNTs adhere to a variety of surfaces using a similar mechanism. CNTs of suitable diameter could withstand four times higher adhesion force than gecko foot. We found that upon soiling these CNT based adhesives (gecko tape) could be cleaned using a water droplet (lotus effect) or by applying vibrations. These materials could be used for applications requiring reversible adhesion. VA coatings were also used for developing field emission devices. A single CNT can emit electrons at very low threshold voltages. Achieving efficient electron emission on large scale has a lot of challenges such as screening effect, pull-off and lower current efficiency. We have explored the use of polymer-CNT composite structures to overcome these challenges in this work.;NA-CNTs were used to create stable superhydrophobic coatings on steel. As compared to VA-CNT, mesh-like structures could sustain large thermal and mechanical stresses without loosing their superhydrophobic properties. A process was developed to reinforce these coatings using an elastomer. Flotation behavior of these coatings was tested. When pressed on water surface, a large hydrostatic pressure acts on the coatings. Optimized mesh-like structures had a very high stability and were important in creating these flotation devices. NA coatings on steel were also used for increasing heat transfer efficiency of heat transfer furnaces. CNTs are known to have two orders of higher conductivity than copper. When combined with superhydrophobicity, these coatings could be used for efficient heat transfer. Stainless steel pipes coated with these coatings were demonstrated to have higher thermal transfer properties as compared to uncoated pipes.