Omniphobic objects

The design of materials that repel both water and nonpolar molecules can be led by the tuning of parameters related to a surface's physical and microstructual properties

Credit: © 2008 National Academy of Sciences of the USA

Materials that resist getting wet have many possible applications, from self-cleaning windows to liquid-repellent textiles, and surfaces that repel water are easily made and well-understood. Creating materials that repel organic, nonpolar liquids such as alkanes — which have lower surface tension than water — has proved problematic. Now, a team led by Robert Cohen from MIT has shown¹ that materials that resist wetting by both water and nonpolar liquids can be made, and that their design can be guided by considering several well-defined design parameters.

A significant factor in the superhydrophobicity of a surface is its roughness. The texture of a material can aid its non-wetting properties by trapping pockets of air between its surface and the water, thus reducing their contact and generating what is known as a 'composite' solid–liquid–air interface. Water thus forms droplets that can easily roll off, rather than wetting, the surface. This is usually not enough to achieve the same behaviour with nonpolar liquids, as they don't form droplets as easily.

After studying superhydrophobic duck feathers, Cohen and colleagues showed that by tuning certain surface parameters — related to, for example, the size, shape and spacing of topographical features and the surface chemical properties — a surface could be improved in order to create omniphobicity, that is, resistance to both polar and nonpolar liquids. This was achieved by coating a surface with electrospun fibres of blends of fluorodecyl-polyhedral oligomeric silsesquioxanes and poly(methyl methacrylate), which resulted in an omniphobic material that can resist wetting by both water and nonpolar liquids such as octane.