Objects that are kept underwater eventually succumb to the inevitable decay associated with being submerged – metal rusts, wood rots and human hands go all wrinkly. These effects could be delayed in the future by a new type of rough coating that "deflects" water.
The material uses the same strategy as water-walking insects such as water striders. Both bug and material take advantage of protrusions on their rough surfaces to resist water and stay dry. As long as the gaps between the grooves on the material (or hairs on the insect) are less than a micrometer apart, then it has the ability to stay dry when in contact with water.
The team exploited the fact that water can be prompted to evaporate using the right combination of surface roughness and chemistry. The little pockets of water vapor that form are trapped in the framework of the material. They act as a first line of defense against liquid water drops, deflecting them to keep the surface dry. This effect is shown in the image below.
In fact, the rough surface kept material samples dry for up to four months when constantly submerged in water. It could be longer, but the test will have to run for a longer period of time to know for sure what the time limit is. The research can be found in Scientific Reports.
"When we looked at the rough surfaces under the microscope, we could see clearly the vacant gaps – where the protective water vapor is," Neelesh A. Patankar from McCormick School of Engineering and Applied Science said in a statement.
"When the valleys are less than one micron wide, pockets of water vapor or gas accumulate in them by underwater evaporation or effervescence, just like a drop of water evaporates without having to boil it. These gas pockets deflect water, keeping the surface dry," he added.
For comparison, a human hair is about 100 micrometers wide, so these vapor pockets are too small to even see. So far, a range of materials have been tested for their water vapor-trapping properties. Materials that were effective in the right configuration included zinc oxide, silicon and a polymer called HFS.
The team tested the material samples in a few different conditions. The control, without the rough surface, became predictably wet. The samples with a rough surface remained dry for the four-month submersion. They also tested the material in water that had all of the ambient gases removed to ensure that it was indeed water vapor keeping the material dry, and not a fluke gas hanging around in the water. Even in these conditions, the material remained dry.
This rough surface has the potential to save an enormous amount of money. It could increase the lifetime and maintenance demands of boats, bridges, submarines, and maybe even scuba gear. It could also be used to coat the insides of water pipes to decrease drag as water flows through them.
Image in text: Frozen water on a silicon substrate demonstrating the water vapor pockets formed by the material. Konrad Rykaczewski/McCormick Northwestern University.