Aug. 5 () –
Rice University researchers have developed a simple method to make almost any surface water-repellent. without the chemicals often used in such processes.
His technique involves sandpaper, a selection of powders including teflon and grapheneand some effort.
The labs of Rice C. Professors Fred Higgs III and James Tour, co-corresponding authors of a paper in the journal ACS Applied Materials and Interfaces of the American Chemical Society, showed that sanding a surface increases its ability to shed water without getting wet. But grinding powder on it at the same time gives it hydrophobic superpowers.
Better yet, their superhydrophobic surfaces also have excellent anti-icing properties. They found that it took 2.6 times longer for water to freeze on treated surfaces compared to untreated materials. They also noted that the ice lost 40% of its adhesive strength, even in temperatures as low as -35 degrees Celsius.
How well a surface absorbs or repels water can be measured by analyzing the contact angle of 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 greater the drop formation, the greater the angle. An angle of zero degrees is a puddle, while a maximum angle of 180 degrees is a sphere that barely touches the surface.
To achieve their superstate, hydrophobic materials have a low surface energy and a rough surface. The best materials from Rice’s team 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 rough surface 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 forms,” Higgs said. it’s a statement. “That gives the added bonus of functionalizing the surface to repel water more and more.”
A tribofilm forms in a chemical reaction on surfaces that slide past each other. An engine piston surface is a good example, he said.
Higgs said sanding roughens softer surfaces and allows powders to adhere through van der Waals forces. “These forces are greatest when the surfaces are in close contact,” he said. “Therefore, dust particles can adhere even after the sanding process is complete.”
The structural changes and the transfer of mass and electrons appear to reduce the surface energy of materials that, before treatment, were already slightly hydrophobic or hydrophilic, according to the researchers.
Rice’s team applied the technique on a variety of surfaces (Teflon, polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polydimethylsiloxane) with a variety of powder additives. These included laser-induced graphene fiber, turbostratic flash graphene, molybdenum disulfide, Teflon, and boron nitride. A variety of aluminum oxide sandpapers, from 180 to 2,000 grit, were used.
The resistant materials proved to be robust, as neither heating to 130 degrees Celsius (266 degrees Fahrenheit) nor 18 months in the Houston sun degraded them. Sticking clear tape to the surface and peeling it off 100 times didn’t degrade them either. But even when the materials began to fail, the labs found that re-sanding them could easily restore their hydrophobicity.
The team also found that by changing sanding conditions and powder additives, the materials can also become hydrophilic, or water absorbent.
Tour said simplifying the manufacture of superhydrophobic and antifreeze materials should attract industry interest. “It’s hard to make these materials,” she said. “Superhydrophobic surfaces do not allow the accumulation of water. Water drips and rolls away if there is even the slightest angle or light wind.
“Now almost any surface can be made superhydrophobic in seconds,” said Tour. “Powders can be as simple as Teflon or molybdenum disulfide, both of which are readily available, or newer graphene materials. Many industries could take advantage of this, from aircraft and ship builders to skyscrapers, where thin ice grip is essential. “
“Aircraft manufacturers don’t want ice to form on their wings, ship captains don’t want ocean water drag to slow them down, and biomedical devices need to prevent biofouling, where bacteria accumulate on wet surfaces,” Higgs said. “The robust and durable superhydrophobic surfaces produced from this one-step sanding method can alleviate many of these problems.
“A limitation of other techniques for generating hydrophobic surfaces is that they don’t scale up to large surface areas, such as those on airplanes and ships,” he said. “Simple application techniques like the one developed here should be scalable.”