A team of nanomaterial scientists and mechanical engineers at MIT have created a superhydrophobic material that is 10,000 times more slippery than existing hydrophobic surface. This could seriously boost the efficiency of fossil fuel power plants, and many other technologies that include cooling towers, such as desalination plants.
Hydrophobic, as you have probably guessed, literally means “water fear.” There are hydrophobic substances that resolutely refuse to mix with water (such as oils and fats), and hydrophobic materials and coatings that prevent water from pooling on its surface. In scientific terms, hybrophobicity is caused by surfaces that disrupt the hydrogen bonding in water. So as to minimize the disruption to its molecular makeup, the water droplet pushes itself away from the surface to minimize its contact area, becoming a very tight droplet.
It is very easy to identify a hydrophobic material because water forms into droplets that rapidly roll around — such as on your car windshield, or on a freshly waxed hood/roof. The more hydrophobic the material, the stronger this effect is, until the water effectively floats or skims across the surface with very low friction.
This is obviously very useful on car windshields, but it’s also very useful in cooling towers. In a cooling tower, water vapor comes into contact with the cool surface of the tower, condenses, and then dribbles back down. Fossil fuel and nuclear power plants (some 80% of the world’s power production) use steam turbines to create electricity — and then cooling towers to condense the steam back into water. The problem, though, is that the inside of the cooling tower isn’t hydrophobic, so the water has a tendency to hang around, significantly reducing the tower’s efficiency. This new material from MIT could change that.
There are two ways to create a hydrophobic material: You either coat it with some kind of wax (oil, grease, or some other special, hydrophobic substance); or you use nanoengineering to create a special, nanopatterned textured surface. These nanopatterns, which are hydrophobic, take the form of little bumps or posts that are around 10 micrometers (10 micron, 10,000 nanometers) across. This kind of hydrophobic material is fairly well understood. The MIT breakthrough being discussed today starts with a nanopatterned hydrophobic material — and then coats it in a very fine layer of lubricant, massively increasing its hydrophobicity.
It turns out that the small gaps between the bumps/posts are capable of exerting just enough capillary force to hold an oil lubricant in place. The scientists simply had to dunk the nanopatterned material into a vat of lubricant, pull it out, and the lubricant remains fixed in the material. The nanopattern, plus the lubricant, results in a material that is 10,000 times more hybrophobic than the non-lubricated version. The pits are so small that it takes just half a teaspoon of lubricant to cover a square yard (0.8sqm) of the material. “Drops can glide on the surface,” Kripa Varanasi, the lead researcher, says. “These are just crazy velocities.”
Varanasi and his team now want to find out how much efficiency can actually be gained by using their new material in power station cooling towers. Because almost all of the world’s power is generated by steam turbines, “even if it saves 1 percent, that’s huge,” says Varanasi. Desalination plants also use cooling towers — and over the next few decades, with naturally occurring fresh water reserves struggling to keep up with demand, desalination will become very important indeed.