[Image above] Credit: Clive; Flickr; CC BY-NC-ND 2.0
Adjusting the surface properties of materials is kind of a big deal—and a big business. In addition to self-cleaning coatings for cars, solar cells, glass, screens, and more, repellent surfaces have the potential to reduce friction and drag to make boats, planes, trains, and other fast things more energy-efficient.
Scientists have for years tried to create coatings, patterns, and schemes to make surfaces shed liquids as well as many biomaterials do. While there have been many successes in that arena, even the best engineered surfaces still couldn’t stand up and stay dry against low-surface-tension liquids like solvents—until now.
New research published in Science shows how a couple of University of California, Los Angeles researchers have devised a patterned surface that resembles a bed of nails and is superrepellent against all liquid assaults—a true superomniphobic surface.
“There are numerous superhydrophobic surfaces. Recently some groups reported superoleophobic surfaces that can super-repel oils and many solvents, but fluorinated solvents, like 3M FC-72, have been out of reach,” says coauthor CJ Kim in an email. “We broke this final barrier, so our surface superrepels ‘all’ available liquids at standard conditions—thus superomniphobic.”
Most superhydrophobic and superoleophobic surfaces are created by patterning surfaces with microstructures or by applying a polymer coating.
Back to Kim: “All existing superrepellent surfaces were based on the maximizing the liquid repellent property of a hydrophobic material by microstructuring its surface. This approach worked down to superoleophobic surfaces. However, extreme liquids like FC-72 perfectly wet the most hydrophobic material, so we concluded the existing approach wouldn’t work.
“We set out to create surface microstructures that would super-repel FC-72 purely by their geometric effect—regardless of the chemical property of the material. First, we analyzed to quantitatively predict what kind of microstructures would successfully super-repel FC-72. Second, we developed fabrication to obtain such microstructures.”
Kim and colleague Leo Liu etched patterns of nails onto the surface of silica, a “completely wettable material,” using photolithography and reactive ion etching. The duo found that the trick was not to only fabricate simple posts or posts with table tops, but to create create posts with table tops that have an overhanging lip.
This lip prevents the liquid from seeping into the space below the nailheads, preventing the material’s surface from getting wet. The team showed that their novel structures worked brilliantly to keep 14 different liquids at bay. (Watch the drops bounce off the surface in the short video below.) They also showed that it worked for the rest of the materials science world, with successful tests on metals and polymers, too.
Credit: Science Magazine; YouTube
Because the super surface requires no coatings or layers on top of the material, it’s also stable when up against temperatures, weather, and time periods. The surface is only limited by the material it’s composed of. According to the paper’s abstract, the superomniphobic silica surface is stable at temperatures over 1000°C.
“Without the need to apply a hydrophobic material coating, which is almost always a polymer, now one can make extremely repellent surfaces using the most durable materials,” Kim says. “Also, since the change of material surface properties does not affect this ‘mechanical’ surface, there is a good possibility one can develop surfaces that overcome biofouling, too.”
The paper is “Turning a surface superrepellent even to completely wetting liquids” (DOI: 10.1126/science.1254787).