Fast or superfast water transport?

(Phys.org) —There were high hopes of using carbon nanotubes, particularly for ultra-fast water transport to desalinate seawater. However, a simulation now reveals that these ultra-fast transport rates might have not been properly grounded after all. Researchers who work with experiments and computer models have been at odds over the capabilities and governing physics of the material ever since.

Carbon nanotubes (CNTs) have stirred up a lot of interest in the world of science since their discovery in 1991. After all, the material has unusual properties that make it ideal for various applications. For instance, CNTs are used in electrical engineering as tips for powerful scanning tunnelling microscopes, as strengthening fibres in synthetic materials or for specific aircraft components. CNTs have triggered the wild imaginations of some that ropes made of these nanotubes may one day carry lifts from Earth into space.

Computer scientists of ETH Zurich simulated the flow of millions of water molecules through long Carbon Nanotubes. Credit: Petros Koumoutsakos / ETH Zurich

Read more at: http://phys.org/news/2013-05-fast-superfast.html#jCp

The use of CNT membranes as a filter medium is perhaps more realistic. Experiments and simulations indicate that water molecules flow through such membranes extremely quickly, which makes them interesting as filters for cost-efficient seawater desalination plants: water molecules pass through the ultra-narrow pores, salt ions don't. This potential for CNT membranes is thus being researched intensively.

Limit identified theoretically greatly exceeded

Water transport through pipes is based on a well-established formula from fluid dynamics. The formula describes flow rate, which is calculated using the length and diameter of the tube and the pressure difference between the entry and exit of the fluid at the respective tube ends. Experiments have yielded water-transport rates for CNTs that are supposedly 100,000 times greater than the macroscale theoretically calculated limit that would apply to water transport through such nanotubes. The scale is pivotal for ultra-fast transport processes. On a nano-scale, it has been argued that water molecules literally fly through the carbon nanotubes without touching the hydrophobic walls, hence their reduced friction and enhanced transport rates. And the narrower the CNTs are, the higher the water transport rates.

Read more at: http://phys.org