Waterproof warriors
Adapting super-water-repellent materials for national security
Nature often exhibits the very characteristics that scientists are trying to create. For instance, a lotus leaf found in the swamps of southeast Asia has a nanoscale and microscale structure that, combined with its waxy chemistry, makes water bead up and roll off its surface. These naturally occurring features are exactly what John Simpson aimed for when he fabricated a super-water repellent, or superhydrophobic, material.
Rather than starting with a hydrophobicmaterial, making it superhydrophobic, and then improving upon the results, John Simpson, an ORNL materials scientist, and Brian D’Urso, of the University of Pittsburgh, decided to make the most superhydrophobic material theoretically possible and then use that as a basis for a set of water repellent materials with desirable characteristics, including self-cleaning, anti-icing, thermally insulating, anti-biofouling and anti-corrosive properties.
Superhydrophobic materials are
being considered for national security applications ranging from unmanned aerial vehicles to battleships. Photo: Jason Richards
Wide-ranging applications
Suggested civilian uses for these versatile materials have ranged from clothing to electronics to solar panels. Now they’re being considered for national security applications as well.
Unmanned aerial vehicles – Simpson and his colleagues originally developed superhydrophobic coatings that could be sprayed or painted onto high-tension power lines to reduce ice accumulation during freezing rain events and ice storms. However, the US Air Force has also expressed an interest in applying these coatings to the wings of unmanned aerial vehicles to prevent ice from forming. Having ice-free wings means UAVs can fly faster, longer and more fuel-efficient missions.
Water-repellent clothing and equipment– Dry clothing and equipment is a boon to servicemen and women in every branch of the military and extends and improves combat readiness, especially in cold, damp conditions. When ORNL researchers apply nanotextured superhydrophobic coatings to clothing, the coated cloth exhibits remarkable water-repelling properties. These coatings amplify the effects of water’s surface tension, causing water and water-based solutions to roll or bounce off treated fabric. This effect is so strong that a layer of air is maintained on treated clothing, even while it is submerged in water. Because superhydrophobic coatings rely on a layer of air to repel water, instead of the impermeable polymer barrier used in traditional waterproofing, air is free to move through the fabric, making treated clothing “breathable” as well.
Drinkable water – Evaporation is a simple way to convert salt water or contaminated water into fresh water, but this approach creates salt deposits on the desalination equipment that are corrosive and expensive to remove. As a result, evaporative desalination has all but been abandoned commercially, although interest in a workable desalination process remains high in several regions of the world facing an increasing scarcity of fresh water. “People are predicting that the next war will be over water,” Simpson says. “In 10 or 20 years, we could be fighting over water, so we need an inexpensive and efficient way to turn relatively abundant saltwater into drinkable water.”
When applied to desalination equipment, the superhydrophobic coatings produced by Simpson and his colleagues can greatly reduce the amount of energy and fresh water needed to remove salt deposits, while also reducing their corrosive effects. Initial, small scale tests have shown these superhydrophobic coatings to be effective at mitigating the effects of salt because of their ability to pin a layer of air on the coating’s surface. This pinned air layer reduces the number of evaporated salt crystals that can bond to or corrode the coated surface. Successfully demonstrating this effect on a large scale would fundamentally change the commercial viability of evaporative desalination. Simpson notes that this technology has the potential to substantially reduce the energy needed to convert salt water to fresh water.
In the area of national security, this technology is being considered for portable desalination units that could be used by the military to provide drinking water in areas where fresh water is scarce and where transporting water is dangerous, difficult or expensive. “Imagine where soldiers could go if they knew they would have drinkable water available,” Simpson says.
Corrosion resistance – Corrosion due to salt costs the US Navy millions of dollars every year. Currently the best way to prevent or reduce corrosion on buildings and ships is to paint vulnerable surfaces. The problem with paint is that it eventually forms microcracks that allow moisture to seep under it and accelerate the corrosion process.
As noted earlier, superhydrophobic coatings have a unique ability to pin a layer of air on treated surfaces. This phenomenon can limit corrosion by maintaining a layer of air between the surface and surrounding water. Simpson notes that, while the treatment slows the interaction between surface and water, it doesn’t eliminate it. Treated surfaces can still be reached by water vapor and condensation.
To solve this problem, Simpson and his colleagues developed a superhydrophobic coating that can pin oil or air on its surface. Pinning oil on a surface not only slows down corrosion, but it also reduces “biofouling,” the accumulation of plants and algae on wet surfaces.
“Pinning oil on a superhydrophobic surface blocks condensation by acting as a barrier to both liquid water and water vapor,” Simpson says. “The oil can be pinned so well that these surfaces won’t even look or feel oily.”
The upshot of this development could be that, instead of taking ships out of service for 18 months every five years to strip barnacles off their hulls and repaint them, the US Navy could conceivably save tens of millions of dollars by applying an anticorrosive superhydrophobic coating to the undersides of its vessels.
Wide-ranging potential
By analyzing, enhancing and applying the natural water-shedding abilities of a plant that thrives in a soggy environment, Simpson and D’Urso have developed materials and treatments that enable people and machines to operate more safely, effectively and efficiently under similarly sodden conditions. In a national security setting, these advantages could mean the difference between life and death.
The potential applications of these water-repellent materials and surface treatments are wide ranging, both inside and outside the national security realm, and are just beginning to be explored. Simpson predicts that any one of these materials could have considerable impact on manufacturing if applied on a large scale. —Emma MacMillan