Lotus Effect and Superhydrophobic Coatings using Carbon Nano Tubes

Since the discovery of the lotus effect by Barthlott and Neinhuis [1], there have been efforts to develop materials that mimic the ultrahydrophobic self-cleaning action of the lotus leaf. The super or ultra-hydrophobicity of the leaf is primarily because of nano-scale 3D patterns on the lotus leaf surface.There have been many efforts in the past few years to develop nanostructured and microstructured super-hydrophobic coatings.

But superhydrophobicity results not only from the 3D-patterns as Scientists at National Institute forInterdisciplinary Science and Technology have realized [2]. Dr. Ajayaghosh and his coworkers at thePhotosciences and Photonics group have mimicked the intricate natural structure of the surface of lotus leaves, through a structure of micron-level-3-D pattern coated with nanometeric layer of wax.

In contrast to earlier research, the suggested technique utilizes multi-walled carbon nano-tubes (MWCNT). It is exciting to observe that the research demonstrates that irregular micro-structure created by a nano structured material is sufficient to exhibit the super-hydrophobic character of lotus leaves.Earlier only regular micro- and nano structured topography of natural systems have been mimicked.

Further, use of MWCNT instead of the earlier single walled CNTs to prepare the super-hydrophobic surfaces could reduce the cost of preparation dramatically. SWCNT costs 500 US $ per gram, while MWCNT costs just 10 US $ per gram [3].

They have patented a nano-composite coating comprising carbon nano tubes with an organic oligomer that confer not just hydrophobic, but super-hydrophobic characteristics to the surface it is coated on.

Liquids wet surfaces when their surface tension is low, resulting in smaller contact angles for the liquid droplet with the surface. Lotus leaf surface have high contact angles with water - as high as 140 to 170degrees from the horizontal plane. Compared to this human skin, which is also slightly hydrophobic,has a contact angle of about 90. Compared to pure CNT and the oligomer alone, the composite prepared by Dr. Ajayaghosh's team has been found to have strong hydrophobicity, with contact angles as high as 160 deg, not only for water but for various water-based acidic and basic solution.

References

[1] Barthlott, W., and Neinhuis, C., Planta(1997) 202, 1

[2]Srinivasan, S., Praveen, V., Philip, R., & Ajayaghosh, A. (2008). Bioinspired Superhydrophobic Coatings of Carbon Nanotubes and Linear π Systems Based on the “Bottom-up” Self-Assembly Approach Angewandte Chemie International Edition DOI: 10.1002/anie.200802097]

[3] Carbon Nanotubes, by Michael J. O'Connell, page 22

---------------

I sent the above write-up as my 'expert opinion' about the paper in [2], when a Telegraph Science section writer approached me last week. I am no expert in CNTs, but since I had written earlier about the lotus effect and omniphobic surfaces at this site, the reporter after consulting with Abi approached me, with a deadline for finishing his article fast approaching. So I obliged.

The research work by Dr. Ajayaghosh and team is very interesting. Take a look at the well written paper in [2].

Lotus Effect, the Telegraph article by Mr. T. V. Jayan appeared today. It includes some portions from the above write-up.

Related posts:

1. Lotus Leaves, Hydrophobic and Omniphobic Surfaces
2. Nano-aluminium and Rocket Science
3. Coanda Effect
4. Effect of Temperature-Dependent Viscosity in Porous Medium Flows
5. On the Effect of Chennai Summer on Shaving Cream Containers