Credit: © 2007 ACS

Liquid droplets containing particles leave behind rings as they dry — an effect familiar to anyone who has tried to clean up a coffee stain. Now, by forcing liquid droplets to assume an elongated shape, researchers at the Massachusetts Institute of Technology in the USA have found a way to exploit this effect to position and align individual nanostructures, such as carbon nanotubes, in electronic devices1.

The MIT group, led by Michael Strano, started by calculating the hydrodynamic forces near the edges of a 'long' drying droplet. In their model, they assume the droplet is shaped like a slice from the side of a tin can and that the area underneath the droplet does not change, even as it evaporates. Their results suggest that convective forces push anisotropic particles suspended in the liquid to line up along the long edges of the drop.

Liquid droplets naturally form a spherical shape, but by patterning a gold surface with alternating strips of polar and non-polar molecules, the group was able to form droplets similar to the shape assumed in their model. With this technique, they have an 84% success rate in lining up individual carbon nanotubes, and expect it will have an impact on producing electronic devices like field-effect transistors and interconnects on a large scale.