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October 22nd, 2007

'Nailing' superlyophobic surfaces with nanotechnology

Abstract:
When raindrops splash against your window you probably get frustrated because the weather has turned bad again. Physicists and material engineers, on the other hand, are quite fascinated by the process of 'wetting.' What happens when a fluid is brought in contact with a solid surface is much more complex than you might guess from just looking at your wet window. In physical terms, the process of wetting is driven by the minimum free energy principle - the liquid tends to wet the solid because this decreases the free energy of the system (in this case the system consists of a liquid plus solid). For low-surface-tension liquids the minimum free energy is achieved only when the liquid completely wets the solid. Understanding these mechanics, and using nanotechnology to structure surfaces to control wetting, has a far-reaching impact for many objects and products in our daily lives - by preventing wear on engine parts or fabricating more comfortable contact lenses, better prosthetics, and self-cleaning materials. The primary measurement to determine wettability is the angle between the solid surface and the surface of a liquid droplet on the solid's surface. For example, a droplet of water on a hydrophobic surface would have a high contact angle, but a liquid spread out on a hydrophilic surface would have a small one. Surfaces where the contact angle is approaching 180° are called superhydrophobic and surfaces where the contact angle is approaching 0° are called superhydrophilic. Advanced material engineering techniques can structure surfaces that allow dynamic tuning of their wettability all the way from superhydrophobic behavior to almost complete wetting - but these surfaces only work with high-surface-tension liquids. Unfortunately, almost all organic liquids that are ubiquitous in human environment such as oils, solvents, detergents, etc. have fairly low surface tensions and thus readily wet even superhydrophobic surfaces. Researchers are now about to create surfaces that would extend superhydrophobic behavior to all liquids, no matter what the surface tension.

Source:
nanowerk.com

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