A droplet sitting on a silica surface containing the novel omniphobic, bio-inspired microtexture. Image: KAUST 2018.
A droplet sitting on a silica surface containing the novel omniphobic, bio-inspired microtexture. Image: KAUST 2018.

Researchers have developed a novel eco-friendly, coating-free strategy for making solid surfaces liquid repellent, which is crucial for transporting large quantities of liquids through pipes.

Himanshu Mishra and his colleagues in the Water Desalination and Reuse Center at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia have engineered nature-inspired surfaces that help to decrease frictional drag at the interface between liquids and pipe surfaces. They report their work in a paper in Nature Communications.

Piping networks are ubiquitous to many industrial processes ranging from the transport of crude and refined petroleum to irrigation and water desalination. However, frictional drag at the liquid-solid interface tends to reduce the efficiency of these processes.

Conventional methods for reducing drag rely solely on chemical coatings, which generally consist of perfluorinated compounds. When applied to rough surfaces, these coatings tend to trap air at the liquid-solid interface, which reduces contact between the liquid and the solid surface. This enhances the surface omniphobicity, or ability to repel both water- and oil-based liquids.

"But if the coatings get damaged, then you are in trouble," says Mishra, noting that the coatings can break down under abrasive and high temperature conditions.

So Mishra's team developed microtextured surfaces that do not require coatings to trap air when immersed in wetting liquids. They did this by imitating the omniphobic skins of springtails, or Collembola, which are insect-like organisms found in moist soils. The researchers worked at the KAUST Nanofabrication Core Laboratory to carve arrays of microscopic cavities with mushroom-shaped edges, termed doubly re-entrant cavities (DRC), on smooth silica surfaces.

"Through the DRC architecture, we could entrap air under wetting liquids for extended periods without using coatings," says co-author Sankara Arunachalam. Unlike simple cylindrical cavities, which were filled in less than 0.1 seconds on immersion in the solvent hexadecane, the biomimetic cavities retained the trapped air for more than 10,000,000 seconds (115 days).

To learn more about the long-term entrapment of air, the researchers systematically compared the wetting behavior of circular, square and hexagonal DRCs. They found that circular DRCs were the best at sustaining the trapped air.

The researchers also discovered that the vapor pressure of the liquids influences this entrapment. For low-vapor pressure liquids, such as hexadecane, the trapped gas was intact for months. For liquids with higher vapor pressure, such as water, capillary condensation inside the cavities disrupted long-term entrapment.

Using these design principles, Mishra's team is now exploring scalable approaches for generating the mushroom-shaped cavities on the surface of inexpensive materials such as polyethylene terephthalate for frictional drag reduction and desalination. "This work has opened several exciting avenues for fundamental and applied research!" Mishra concludes.

This story is adapted from material from KAUST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.