A team from Rice University have developed a one-step way to make surfaces very water-repellent, known as superhydrophobic, without using chemicals. Their new approach, which involves sandpaper, some powders and a bit of toil, offers the best properties for superhydrophobicity as well as providing materials with excellent anti-icing properties.
As reported in ACS Applied Materials and Interfaces [Chen et al. ACS Appl. Mater. Interfaces (2022) DOI: 10.1021/acsami.2c05076], the labs of co-corresponding authors Rice professors C. Fred Higgs III and James Tour showed that sanding works to enhance a surface’s ability to shed water without getting wet, but that grinding in a powder at the same time then brings superhydrophobic qualities.
For a material to be superhydrophobic, the angle at which the surface of the water meets the surface of the material, the water contact angle, should be more than 150 degrees. To be superhydrophobic, hydrophobic materials must have low surface energy as well as a rough surface, and here the best materials were demonstrated as having a contact angle of about 164 degrees.
Some types of sandpaper offer surface roughness that promotes the desired water-repelling or hydrophobic behavior. However, here the concept of tribology – the study of surfaces in sliding contact – was key, as select powder materials being introduced between the rubbing surfaces during the sand-in process means a tribofilm is formed, which has the advantage of functionalizing the surface to repel water even more.
The technique was applied to a range of surfaces, such as Teflon, polyethylene and polypropylene, with different powder additives, including laser-induced graphene fiber, molybdenum disulfide and boron nitride, and a variety of aluminum oxide sandpapers being used. The resistant materials were robust since neither heating to 130oC nor 18 months under a hot sun managed to degrade them. Transparent tape stuck to the surface and peeled off numerous times didn’t degrade them either. When the materials did start to fail, re-sanding easily revived their hydrophobicity.
The process should be scalable, something that other techniques for generating hydrophobic surfaces cannot manage. It also gives materials enhanced anti-icing properties, with ice taking longer to freeze on treated surfaces and also losing its adhesion strength.
Applications include stopping ice forming on the wings of airplanes, preventing ice from forming on ships, and avoiding biofouling from bacteria building up on wet surfaces in biomedical devices. As James Tour points out: “Now, almost any surface can be made superhydrophobic in seconds… Many industries could take advantage of this, from builders of aircraft and boats to skyscrapers, where low-ice adhesion is essential.” The researchers now plan to explore the sand-in method for other substrates and applications, such as various metal surfaces for rechargeable metal batteries.
“Now, almost any surface can be made superhydrophobic in seconds… Many industries could take advantage of this, from builders of aircraft and boats to skyscrapers, where low-ice adhesion is essential”James Tour
Sand-in process for superhydrophobicity