An illustration shows the sand technique developed at Rice to make superhydrophobic materials. The one-step method involving sandpaper and powder also gives the materials improved anti-freeze properties. Credit: Weiyin Chen
Do you want a surface that doesn’t get wet? Grab some sandpaper.
Rice University researchers have developed a simple method to make surfaces superhydrophobic, meaning highly water-repellent, without the chemicals often used in these processes.
Her technique involves sandpaper, a selection of powders and a little elbow grease.
The laboratories of Rice Professors C. Fred Higgs III and James Tour, co-corresponding authors of an article in the American Chemical Society ACS applied materials and interfaces, showed that polishing a surface increases its ability to shed water without getting wet. But grinding into powder at the same time gives it hydrophobic superpowers.
Better still, their superhydrophobic surfaces also have excellent anti-freeze properties. They found that water took 2.6 times longer to freeze on treated surfaces compared to untreated materials. They also noticed that the ice lost 40% of its adhesive strength, even at temperatures as low as -31 degrees Fahrenheit.
How well a surface absorbs or repels water can be measured by analyzing the contact angle of the droplets that settle on it. To be superhydrophobic, a material must have a water contact angle (the angle at which the surface of the water meets the surface of the material) greater than 150 degrees. The larger the cord, the larger the angle. An angle of zero degrees is a puddle, while a maximum angle of 180 degrees is a sphere that just touches the surface.
To achieve their super state, hydrophobic materials have a low surface energy and a rough surface. The Rice team’s best materials showed a contact angle of about 164 degrees.
Higgs, whose lab specializes in tribology, the study of surfaces in sliding contact, said certain types of sandpaper can provide a surface roughness that promotes the desired hydrophobic or water-repellent behavior.
“However, the Tour group’s idea of introducing selected powder materials between the rubbing surfaces during the sanding process means that a tribofilm is formed,” Higgs said. “This gives the added benefit of functionalizing the surface to repel water more and more.”
A tribofilm is formed in a chemical reaction on surfaces that slide against each other. The surface of an engine piston is a good example, he said.
A video shows Rice’s student Winston Wang polishing laser-induced graphene fibers on a polytetrafluoroethylene plate to make it superhydrophobic and accelerating a drop of water freezing onto the treated plate. The superhydrophobic process developed at Rice slows ice formation on treated surfaces by about 2.6 times. Credit: Courtesy of Tour Group
Higgs said the polishing hardens the smoother surfaces and allows the powders to stick together through van der Waals forces. “These forces are maximal when the surfaces come into close contact,” he said. “Thus, dust particles can stick even after the sandblasting process is complete.”
Structural changes and mass and electron transfer appear to lower the surface energy of materials that, before treatment, were already slightly hydrophobic or hydrophilic, the researchers say.
Rice’s team applied the technique to a variety of surfaces (Teflon, polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polydimethylsiloxane) with a variety of powder additives. These include laser-induced graphene fiber, turbostratic flash graphene, molybdenum disulfide, Teflon, and boron nitride. A variety of aluminum oxide sandpapers were used, from 180 to 2,000 grit.
The tough materials proved to be robust, as neither heating to 130 degrees Celsius (266 degrees Fahrenheit) nor 18 months under the hot Houston sun degraded them. Sticking clear tape on the surface and peeling it off 100 times didn’t degrade them either. But even as the materials began to fail, the labs discovered that polishing them again could easily refresh their hydrophobicity.
The team also discovered that by changing the sand conditions and powder additives, the materials can also be made hydrophilic, or water absorbent.
Tour said that simplifying the manufacturing of superhydrophobic and antifreeze materials should pique industry interest. “It’s hard to make these materials,” he said. “Superhydrophobic surfaces don’t allow water to pool. Water curls up and rolls off if there’s even the slightest angle or gentle wind.
“Now, almost any surface can be made superhydrophobic in seconds,” Tour said. “The powders can be as simple as Teflon or molybdenum disulfide, both readily available, or newer graphene materials. Many industries could take advantage of this, from aircraft and ship builders to skyscrapers, where the gel adhesion is essential.”
“Aircraft manufacturers don’t want ice forming on their wings, ship captains don’t want the drag of ocean water to slow them down, and biomedical devices must avoid biological contamination, where bacteria build up on wet surfaces,” Higgs said. “Robust, long-lasting superhydrophobic surfaces produced from this one-step sandblasting method can alleviate many of these problems.
“A limitation of other techniques for generating hydrophobic surfaces is that they don’t scale to large surfaces like those on planes and ships,” he said. “Simple application techniques like the one developed here should be scalable.”
Rice graduate student Weiyin Chen, co-senior author of the new paper, said the Tour lab has also applied its sandblasting technique to various metal surfaces, as reported in another recent paper, Sheets of lithium and sodium for metal batteries.
“Spontaneous chemical reactions lead to the formation of tribofilms, in this case, the artificial solid electrolyte interface,” Chen said. “Modified metals can be used as anodes for rechargeable metal batteries.”
Gas Gives Laser-Induced Graphene Super Properties More information: Weiyin Chen et al, Robust Superhydrophobic Surfaces Using the Sand-In Method, ACS applied materials and interfaces (2022). DOI: 10.1021/acsami.2c05076 Provided by Rice University
Citation: Water can’t touch this polished, powdery surface (2022, August 4) Retrieved August 4, 2022, from
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