Odd companions create unusual environment.

Despite their generally incompatible natures, oil and water have found common ground. On a surface with an unusual structure, each substance spreads out evenly, report Akira Fujishima of the University of Tokyo and his colleagues in the July 31 Nature.

This finding reveals "a fundamentally new physical and chemical phenomenon," according to Adam Heller of the University of Texas at Austin, who considers the results "nothing short of revolutionary." Some materials, when perfectly clean, can bind to either oil or water but become committed to whichever one they adhere to first. On the surface described in the recent study, however, water and oil can displace each other, Fujishima says, a commercially valuable characteristic. Although the surface itself is not new, the possibility that it could behave in this unusual way was "simply not examined previously."

The scientists coated a piece of glass with titanium dioxide--a multipurpose material commonly used as a pigment in paint. After irradiating the surface with ultraviolet light, the researchers placed drops of water or oil on it. All of the drops flattened out completely, indicating that both fluids adhere to the coating, the team reports.

Using friction force microscopy to examine the surface on an atomic scale, the researchers discovered a mosaic of tiny regions 30 to 80 nanometers across, each with a strong affinity for either oil or water. The researchers suggest that each area binds the substance for which it has an affinity, so together the regions imbue the surface with this unique property.

The exceptional spreading behavior adds to what scientists previously knew about the self-cleaning properties of this coating (SN: 9/2/95, p. 157). Specifically, it explains how the surfaces get rid of particular types of dirt. Because water adheres strongly to the surface and forms so thin a film, it creeps under oily spots to displace them, Fujishima says. Similarly, a sheet of oil slips under a drop of water. The researchers have not yet explored how the surface would cope with equal amounts of water and oil.

Because the liquids spread extraordinarily evenly, the coating can be used to circumvent everyday problems associated with fluid globs. For example, normal glass and mirrors cloud up because water collects in droplets, which scatter light. On the titanium dioxide surface, beads of water flatten out so uniformly that they become transparent. The classic method of cutting through fog is to use water repellents; a surface that strongly attracts water represents a novel approach to the problem, says Heller.

The researchers' industrial collaborator, TOTO of Chigasaki, is already producing new antifogging products with this coating, Fujishima says.