- 26 March 2007 -

Nanotube membranes for better flow control

Researchers at Rensselaer Polytechnic Institute, New York, USA have found a more precise way to control the flow of water through carbon nanotube membranes by fusing wet and dry nanotechnologies.

The research could help improve technologies designed to desalinate drinking water or to separate specific strands of DNA.

Nanotube membranes can currently filter out very small impurities and other organic materials such as DNA and proteins from materials with high water content. The problem is that nanotube arrays are hydrophobic, strongly repelling water. Now researchers, led by Nikhil Koratkar, associate professor of mechanical engineering at Rensselar, have found a way to use low-voltage electricity to manipulate the flow of water through nanotubes with a level of precision that has never been demonstrated before, it is claimed.

The researchers discovered that when the nanotube's membrane is given a small positive potential of 1.7 volts, and the water is given a negative potential, the nanotubes quickly switch from repelling water to pumping water through the tube. When the charge on the water is raised, the water flows through at an exponentially faster rate. When the experiment is reversed with a negatively charged nanotube, it takes much higher voltage (90 volts) to move the water through the tube.

By reversing the polarity of the nanotubes, the team found that they could start and stop the flow of water through the tube. When a small positive charge is administered, the water moves through the tube, and when that charge is reversed water flow stops.

The researchers determined that the nanotube walls had been electrochemically oxidized as a result of water electrolysis, meaning that oxygen atoms had coated the surface of the nanotubes, enabling the movement of water through the tube. Once the charge is reversed, oxidation stops and the water can no longer flow through the unoxidized portion of the tube.