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Ultra-water repellant coatings: A lesson from nature

Water droplets on a glass sheet coated with an ultra-water repellant nano-textured thin film.

Water droplets on a glass sheet coated with an ultra-water repellant nano-textured thin film.

Nature has devised many schemes for creating surfaces that completely repel water. Such surfaces can be found on many plants as well as insect wings. Water droplets on these surfaces simply roll-off and collect dirt, thereby providing a simple mechanism for self-cleaning. The unique properties of these so-called superhydrophobic surfaces are a direct consequence of a complex surface texture that includes micron-scale hills and valleys that are decorated with nanoscale particles of hydrophobic material.

Professors Cohen and Rubner of the MIT MRSEC have developed a superhydrophobic coating with a structure that mimics that of the water-repellant lotus-leaf (Zhai et al., Nano Letters 4, 1349-1353, 2004). Using a combination of microporous polyelectrolyte multilayers and silica nanoparticles, they were able to form molecularly assembled coatings with highly stable superhydrophobic properties much like those encountered in nature (see image below). These coatings can be easily applied to most surfaces and may find application as, for example, self-cleaning surfaces, low flow resistance coatings for microfluidic devices, pipes and boats, and bacteria adhesion resistant coatings (since water essentially does not contact the surface, bacteria in the water cannot attach to the surface).

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