Imagine trying to control the placement of something as small as a carbon nanotube. Researchers working with nano-materials have to device creative ways to place and align these new materials. Carbon nanotubes are exceptionally strong and have fantastic electrical properties but these properties function along their length. Simon Beyer is a Master’s Student working with Konrad Walus in the Walus printed Micro devices Lab. Simon is studying the use of nano-materials in sensors, particularly strain sensors. To make use of the conductive capacity of the nanotubes in a sensor the tubes that are only 2 nanometres in diameter have to be aligned.
Carbon nanotubes are capable of what is referred to as ballistic electron transport. In a copper wire the electrons travel until they hit an atom and bounce, being deflected from a straight path, creating heat. In a carbon nanotube the electrons are not deflected making them very highly conductive for their size. This is a result of the quantum structure of the nanotube related to the particular alignment of electron orbitals. Electrons in the nanotube are only free to move on the outside of the tube and the structure is smooth with no defects or points of deflection.
Simon is developing a method of alignment by depositing a film of nanotubes along a surface. To do this nanotubes have to be suspended in a liquid in high concentration. In solution the nanotubes will behave like a liquid crystal (long molecules that self orient energetically). Simon then uses evaporation to control the direction of their alignment. 
There are two ways evaporation can occur. In one case a droplet can shrink in all directions at once.This occurs in pure water. In the second process of evaporation, common with liquids that contain particles in suspension, the edges of a droplet get pinned. The pinning of the droplet edge is caused by the particles themselves creating a surface roughness.The droplet remains the same size and the evaporation occurs from the upper, exposed surface. To imagine the difference in these evaporation processes think of washing a car. If the car is rinsed with clean water containing no particles the edges of each droplet will shrink as the surface dries leaving the surface without marks.If the car is rinsed with water containing particles of dirt the droplets will dry by evaporating from abovedepositing dirt at the edge of each droplet.
In an evaporating droplet that contains particles there is an internal flow pushing particles to the edges.We can see the results of this effect in coffee rings, ink drawings and salt stains. Simon uses this movement of particles to push nanotubes into alignment. The long, thin nanotubes get pushed against the edge and orient in parallel. Simon describes this as a nanotube log jam. With a specialized inkjet printer Simon prints a long bead of liquid. As the liquid evaporates there is a long line of nanotube left in alignment.
Strain Gauge Development
If the nanotube is stretched or twisted its resistance changes making it an excellent senor for these movements. The Walus Lab anticipates that the nanotube strain gauge will be much more sensitive than a conventional gauge. Experiments have shown that when one nanotube is isolated it is approximately one thousand times more sensitive to strain than conventional material.
Post Doctoral Fellow Lisheng Wang, who also works in the Walus Lab, is pursuing zinc oxide nanowires as an alternate strain gauge material. Zinc oxide nanowires are particularly sensitive to compression but will respond to bending. These nanowires have piezoelectric properties generating a voltage when compressed. It is possible that the best gauge would include both zinc oxide nanowires and carbon nanotubes.
The Walus Lab is pursuing a number of different types of strain gauge in tandem. The goal of these projects is to measure small cracks in buildings or bridges both on the surface and deep within the structure. When they occur each of these cracks sends out a pressure wave. Sensors could be embedded in the material prior to construction. If highly sensitive sensors could register internal strain on structures before the strain registers on the surface, the health and longevity of these structures could be improved.
Learn More:
The Walus Lab