An India-origin University of Illinois researcher has announced the discovery of the physical mechanism behind the rapid transport of water in carbon nanotubes, thus bringing hope of ultra-efficient devices for drug delivery inside the body closer to reality.
Narayana R. Aluru, a Willett Faculty Scholar and a professor of mechanical science and engineering, says that this discovery may also be beneficial for water purification, and nano-manufacturing.
"Extraordinarily fast transport of water in carbon nanotubes has generally been attributed to the smoothness of the nanotube walls and their hydrophobic, or water-hating surfaces," he said.
"We can now show that the fast transport can be enhanced by orienting water molecules in a nanotube. Orientation can give rise to a coupling between the water molecules' rotational and translational motions, resulting in a helical, screw-type motion through the nanotube," he added.
Aluru and graduate student Sony Joseph used molecular dynamics simulations to examine the physical mechanism behind orientation-driven rapid transport.
Describing their work in the journal Physical Review Letters, the researchers said that the system consisted of water molecules in a 9.83 nanometre long nanotube, connected to a bath at each end.
Nanotubes of two diameters - 0.78 nanometres and 1.25 nanometres - were used, they added.
For very small nanotubes, water molecules fill the nanotube in single-file fashion, and orient in one direction as a result of confinement effects, producing water transport in one direction.
However, the water molecules can flip their orientations collectively at intervals, reversing the flow and resulting in no net transport.
Water molecules are not oriented in any particular direction in bigger nanotubes, and thus do not result in transport.
The researchers point out that though water's net charge is zero, it has a positive side represented by two hydrogen atoms and a negative side represented by one oxygen atom.
They say that this polarity causes the molecule to orient in a particular direction when in the presence of an electric field.
According to them, creating and maintaining that orientation, either by directly applying an electric field or by attaching chemical functional groups at the ends of the nanotubes, produces rapid transport.
"The molecular mechanism governing the relationship between orientation and flow had not been known," Aluru said.
"The coupling occurs between the rotation of one molecule and the translation of its neighboring molecules. This coupling moves water through the nanotube in a helical, screw-like fashion," he added.
The researchers say that their findings also describe a physical mechanism that can be used to pump water through nanotube membranes in next-generation nanofluidic devices.