Dutch nanotechnologists have developed a new material that is not only extremely water-repellent but also extremely oil-repellent.
Water-repellent surfaces can be used as a coating for windows, obviating the need to clean them ever again. These surfaces have an orderly arrangement of tiny pillars less than one-hundredth of a millimeter high – similar to a bed of nails but on an extremely small scale. Water droplets stay on the tips of the pillars, retaining the shape of perfectly round tiny pearls. As a result they can roll off the surface like marbles, taking all the dirt with them.
The nanotechnologists at the University of Twente have created a silicon surface that retains not only water droplets but also oil droplets like tiny pearls. What makes the material unique is that the droplets stay on top even when they evaporate. The material has been described in a paper in the journal Soft Matter.
With existing materials, evaporating droplets drop down between the pillars onto the surface after a while, changing in shape to hemispheres which can no longer simply roll off the surface. The surface can therefore still get dirty. By modifying the edges and the roughness of the minuscule pillars the scientists have managed to create a surface on which the droplets do not drop down even when they evaporate but stay neatly on top.
Surfaces that repel both water and oil are currently used among other things as a dirt-resistant coating on screens of mobile devices. In practice the screens still get dirty, however, showing greasy fingerprints. The new material brings screens that really never get dirty a lot closer.
Bibliographic information: Susarrey-Arce A. et al. 2012. Absence of an evaporation-driven wetting transition on omniphobic surfaces. Soft Matter 8, 9765-9770; doi: 10.1039/C2SM25746G