First they held the glass slide over a heart-shaped candle (though any candle will do). This led to the deposition of soot on the slide—spheres of soot that were 30 to 40 nanometers in diameter, stacked loosely and producing the right kind of surface texture: about 80 percent empty and 20 percent spheres.
To protect the soot from washing away, they coated it with a silica shell 25 nanometers thick; to get rid of the black color of the soot, they baked the slide at 600 ºC, making it transparent. Afterward, they sprayed various oils—peanut oil and solvents—and took micrographs of these liquid droplets bouncing up and down like ping-pong balls.
The coating sticks to aluminum, steel, and copper, too. And because it has both oil- and water-repelling qualities, the material is said to be "superamphiphobic."
'It is difficult to say whether this particular work will yield practical applications, but in principle there are many areas where such non-sticky self-cleaning surfaces are needed,' says Michael Nosonovsky, who works at the University of Wisconsin-Milwaukee in the US on omniphobic surfaces. Examples include optical devices and the windows of skyscrapers.
But Vollmer doesn't expect a conveyer belt full of candles to be used to make self-cleaning surfaces any time soon. Instead, she says, she thinks the method is a useful way to investigate how to make omniphobic surfaces. 'Now we're trying to improve it further and get particles that are larger, with better mechanical stability,' she adds 'but still keeping the idea the particles are spherical.'