This story is a supplement to the feature "Self-Cleaning Materials: Lotus Leaf-Inspired Nanotechnology" which was printed in the August 2008 issue of Scientific American.
Thin films of titania have the very opposite property to the lotus—superhydrophilicity—yet they, too, shrug off dirt, and they are also antimicrobial.
What the Water Does
Water on a superhydrophilic material forms a sheet across the surface and easily dislodges and removes dirt as it flows. Superhydrophilicity also prevents a surface from fogging because water spreads instead of forming the innumerable tiny droplets that constitute a fog.
Ultraviolet rays (such as in sunshine) excite electrons and holes (positively charged “absences” of electrons) in the titania (1). The electrons combine with oxygen molecules to form negatively charged superoxide radical anions (2a), and the holes combine with hydroxide anions from water to form neutral hydroxyl radicals (2b). These highly reactive species kill microbes and break down organic materials on the surface (3). The ultraviolet light also changes the structure of the titania film, making it superhydrophilic (4), which allows water to wash off dirt (5).
Illustration Credit: Ann Sanderson