Friday, February 03, 2006

Pass it through nanotube, it'll be quick

When I started working on ceramic membrane I was fascinated by the science. Especially working with the unsupported alumina membrane where you control the pore size at the nanometer level. Its quite tricky playing with these membranes. The problem is associated with filtering. It can filter out very small molecules easily but as the channels becomes smaller the wall friction affects the flow (Hagen-Poisullie equation), thus quite high pressure is required to pass the fluid through filter. Duration of the filtering process due to low fluid velocity also becomes extremely long; certainly from application point of view this is disadvantageous. There is a brighter side of the story! Recently my undergrad seniour, Mainak has published a paper in Nature couple of months back on carbon nanotube membranes, this paper addressed very similar problems. This group from University of Kentucky has made multi walled carbon nanotubes with graphite as the inner core; the average channel diameter is around 7 nm. When water is filtered through these membranes, the fluid velocity was observed to be few orders of magnitude higher compared to the theoretical prediction. The experiment was carried out on few other fluids and the results were very similar, they also observed that the flow rate does not decrease with velocity of the fluid, that’s really an interesting observation. The reason cited for this enhanced flow is the hydrophobic nature of graphite which makes the wall almost friction less to flow. There is also a possibility of forming ordered hydrogen bonds inside the tube. This behaviour is analogous to the biological cells where water passes through the protein wall at considerable pace. Following this claim carbon nanotube will have a huge application coming in, mostly as sensors and filters against corrosive gases. The important part of the question which I felt quite interesting is: if the conventional concept fluid flow in a tube breaks in case of MWCNT, will it hold for other systems? Does nano has any effect here? I f you remember I wrote something about nanomechanics by John Pethica, where he mentioned an increase in viscosity of water molecule when put between very thinly spaced walls. Although these distances are smaller compared to nanochannels, the flow rate must get affected when the pore size is reduced more. That technology is yet to come; we will probably witness subnano effects influencing flow.

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