Researchers at Aalto University, in Finland, have developed new surface materials that are extremely difficult to wet by both water and oil.

Liquid-repellent surfaces are very important for a number of industries. Extreme oil repellency under water can be utilized in the maritime industry to deter fouling, including oil contamination and biofouling by organisms. On the other hand, extreme water repellency of surfaces when covered by oil is a great help against corrosion: moisture is effectively inhibited and oxidation can be reduced.

Dual superlyophobic surface keeps an oil droplet separate in water when it is against the surface (l) and a water droplet separate in oil when it is against the surface (r). Image credit: Xuelin Tian, Robin Ras.To date, however, the ability to repel water when covered by oil, and oil when covered by water, has been regarded as contradictory. According to the scientists—led by Xuelin Tian, a postdoctoral researcher at the time of the study—their new materials maintain their "unwettable" property even when doused with the other liquid because they do not need isolating air to stay trapped between the droplet and the surface to prevent wetting.

"We consider [such] surfaces need to meet two design criteria: the microstructures must be readily filled by water and oil without a trapped air layer when submerged in one of the two liquids, and the submerged microstructures [must] support steady oil-water interfaces when the second liquid is introduced," the researchers explain in their study.

Such surfaces can be regarded as environment-responsive, meaning that their wettability changes with the liquid they contact, says Robin Ras, assistant professor of experimental condensed matter physics. However, unlike other responsive surfaces, the new materials do not rely on reconfigurable organic molecular modification and thus offer a new strategy to make smart materials, he adds.

"When processed in the form of porous materials, [they] may be used for separation of both water-in-oil and oil-in-water emulsions, whereas common oil/water separation materials work for one type of emulsion only," Ras says.