May 5 2006
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Tyler Boyes / Shutterstocl
Geckos are capable of climbing on all kinds of natural surfaces. This unique ability has inspired the development of an innovative dry adhesive that can imitate the hairs present on these animals’ feet using vertically aligned multi-walled carbon nanotube, or CNT, array.
If this CNT array is used as a dry adhesive material, then biological adhesion structures can possibly be reproduced and relatively strong adhesion strength (over 100 N/cm2) could be generated. This is because vertically aligned multi-walled CNT arrays possess a structure that mimics biological adhesive surfaces with even tinier feature sizes.
Synthetic Hair Patches
Polymer materials like polyimide and rubber have been used to fabricate synthetic hair patches. These polymer-based synthetic structures have diameters measuring a few micrometers. The polymer hairs generate adhesive strengths that are usually less than 1 N/cm2, with 3 N/cm2 being the highest adhesive strength reported so far with the use of polymer-based micro-pillar structures. Conversely, the large-scale production of such structures is not possible because of the costly and time-intensive electro-beam lithography involved in defining the pillar patterns.
How the Vertically Aligned Multi-Walled Carbon Nanotube Arrays Work
CNT arrays can be grown directly on a solid surface, which can be ideally used as the synthetic material imitating the structure of a gecko foot. When the CNT arrays contact a target surface with specific roughness, their fine structures ensure their potential of filling in the cavities at the interface and make effective contact at mating surfaces.
Forces of Attraction
The adhesion between a target surface and the CNT array is predicated on van der Waals interaction, which is inversely proportional to the structure’s feature size, and a rough estimation indicates that adhesive strengths of over 100 N/cm2 can possibly be generated, considering that all the tubes make effective contact with the target surface.
The present measurement indicates that there are about 10% of CNTs that make effective contact at interfaces—a number (approximately 1010 CNTs per cm2) that is still big. An optimum contact at the interface can be reached by changing the substrate compliance and the tower height of the CNT array, and that could additionally enhance the adhesion strength.
Technical Data
| . |
. |
| Density of CNTs |
~1011 cm-2 |
| Adhesion force generated at interface (with glass) |
~10 N/cm2 |
Adhesion force after 7 detach-reattach cycles
(with various surfaces including glass, GaAS, Si) |
~4 N/cm2 |
| Thermal conductance at interface (mating to glass) |
~105 W/m2.K |
Potential for the Vertically Aligned Multi-Walled Carbon Nanotube Arrays
Measurements on the adhesion of the CNT array across numerous surfaces suggest that adhesion strength of more than 10 N/cm2 (~14 lb/in2 or 1 kg/cm2) can be realized, which is comparatively better than other polymer-based dry adhesives and is analogous to the one generated by a gecko’s foot.
Carbon Nanotubes
It is known that CNTs have unusual electrical, thermal, and mechanical characteristics. They can be bent repeatedly with huge deflection without failure, which inherently makes them well suited as a re-attachable or reusable dry adhesive.
Advantages of Carbon Nanotubes over Synthetic Hair Structures
- One extraordinary feature of the CNT array dry adhesive is its excellent electrical and thermal conductivity.
- The fabrication method involved in the polymer-based synthetic hair structures limits the density and size of the hair patch. By contrast, CNTs’ diameters are of the order of a few tens of nanometers and a high density of the array (>1010 cm−2) can be easily obtained with the help of the existing technology.
- CNTs’ highly hydrophobic property makes them free from the clumping issue, which is often faced by polymer-based synthetic hair structures.
- A more significant benefit of CNT arrays over other polymer-based hair structures is that they can function under certain adverse environments, for example, vacuum and very high and very low temperatures.
- The growth technique for dense CNT arrays is controllable and well developed.
Applications for the Vertically Aligned Multi-Walled Carbon Nanotube Arrays
The applications cover any kind of climbing devices that require both detachable and re-attachable adhesions. Attachment devices in space are another promising application because adhesive materials that are presently available do not have the ability to perform effectively in the space environment (extremely low temperatures and vacuum). Due to CNTs’ electrical and thermal properties, possible industry applications include utilizing them as electrically or thermally conductive tapes.
The CNT array may be used to fasten a diamond coating onto certain parts of a metal surface. It could even allow the development and assembly of electronics without solder. In addition, the CNT array can produce a joint that is stronger compared to several conventional assembly techniques and can be assembled at room temperature. This innovative material can also produce a novel electronic assembly process that eliminates the need for a solder reflow or stencil solder.