Material engineers at Aalto University are claiming to have made a breakthrough in designing surfaces that can repel oil when wet, and yet can also repel moisture when oily. Whilst full verification of the results has yet to be carried out, the discovery may be the first in a new wave of materials that could revolutionise both the surfactant and coatings industries.
The new surface materials are able to function in this way because they don’t need isolating air to stay trapped between the droplet and rough surface to prevent wetting. As a result, these ‘novel dual superlyophobic surfaces’ are able to repel oil when submerged or covered in water, as well as repel water when covered in oil. Something that the researchers believe has, “So far, been regarded as contradictory to each other and not expected to be present on the same surface.”
The online scientific journal ScienceDaily, describes how, “[The] Researchers propose two design criteria for new surfaces: the liquid filling criterion and the steady composite interface criterion. The design criteria lead to steady trapping of airless oil and water films within surface texture. Such liquid films enable both underoil superhydrophobicity, repelling water under oil, and underwater superoleophobicity, repelling oil under water.
Surfaces are prepared by the combination of re-entrant topography and delicately matched surface chemistry.”
The researchers have published their results in the journal Advanced Materials, where they describe the breakthrough in more detail as follows: “We consider the surfaces need to meet two design criteria i) the microstructures must be readily filled by water and oil without trapped air layer when submerged in one of the two liquids; ii) the submerged microstructures can support steady oil–water interfaces when the second liquid is introduced, or in other words, the second liquid can be suspended by the texture rather than that it intrudes into the texture.
The first criterion would create a water or oil film trapped within the texture and the second one guarantees the trapped liquid film not to be displaced by the other suspended liquid. Under the above two criteria, Cassie-type composite interfaces can be obtained in both water-in-oil and oil-in-water situations, and moreover large apparent CAs are able to be induced, both of which are important for liquid repellency — repelling of the second liquid by the first liquid in oil–water systems.”
This image was taken from the researchers’ publication, and shows how the dual superlyophobic surface keeps the oil droplet separate in water when it is against the surface.
As co-author in the study, Assistant Professor Robin Ras, explains, “Such surfaces can be regarded as an environment-responsive material which means its surface wettability changes with the environmental liquid it contacts. Unlike other responsive surfaces, the new surface does not rely on reconfigurable organic molecular modification, and thus offers a new strategy to make smart materials. When processed in the form of porous materials, it 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.”
If the discovery holds true, then it may allow for major design alternatives for many chemical manufacturers. For example, marine industries often rely on chemical products that give underwater protection from organic fouling or oil contamination. By repelling oils, these materials may provide alternatives to surfactant products.
Similarly, new coatings designed to repel water in order to inhibit corrosion may be developed based on the discovery.
Xuelin Tian, who was a postdoctoral researcher in Aalto University before recently becoming a professor at Sun Yat-sen University, China, can also foresee many practical applications for the new materials. Explaining how, “The competitive interaction with surfaces between oil and water plays an essential role in various technological applications. Our new design strategy of surfaces can be used in many ways from self-cleaning to dirt-repellency.”
Like many ground breaking technologies the full impact of these ‘novel dual superlyophobic surfaces’ is not yet known, but it is certain to attract the interest of coating manufacturers, surfactant producers and chemical feedstock traders in numerous industries. Whilst only time will show the full scope of applications, we can ask ourselves, ‘What is the best use of a material that repels oil when wet, and wetness when oily?’