Tough New Surfaces
In the race to meet burgeoning global energy and water demands, MIT mechanical engineer Kripa K. Varanasi believes there’s a lot to be gained from boosting the efficiency of existing systems such as steam, nuclear power, and desalination plants by creating surfaces that shed water like feathers on a duck’s back.
Droplets are shaped by surface tension, which helps water stick to most surfaces like dew on a spider web. Some machines, aircraft, power, and desalination plants are forced to waste energy dealing with inefficient condensation and excess water before they slow them down or ice them up.
Working for General Electric’s energy, propulsion and nanotechnology research laboratories for five years, “I got a lot of my inspiration for what can be done to dramatically change the efficiency of these systems,” says Varanasi, d’Arbeloff Assistant Professor of Mechanical Engineering. “We need to change the paradigm for systems that have been around so long that people take them — and their limitations — for granted.”
Varanasi is designing tough new nanoengineered surfaces and coatings that could fundamentally change the thermal-fluid-surface interactions pervasive in energy, water, agriculture, transportation and related fields. “If we can control how water condenses on a surface, we can develop a new class of surfaces with very high heat transfer capabilities,” he says. These could be used in applications ranging from energy, electronics — the extreme heat created by packing many components in a small space has been a bottleneck in the development of next-generation devices — to water purification systems, which are becoming increasingly important in the Middle East, India, and Africa. “I’m really passionate about water because of its scarcity. Water will be a more precious commodity than energy,” he says. “Without energy, there is no water and without water, there is no agriculture.”
BUBBLES AND CLOUDS
Nanotechnology is revealing the dynamics of condensation at the microscale. Water undergoes a phase-changing phenomenon called nucleation that we see in action when bubbles in soda pop or in boiling water form on the sides of their containers and when clouds are seeded to produce rain. Suspended particles or minute bubbles provide sites for nucleation, which plays an important role in industrial processes such as semiconductor manufacture.
“Though nucleation is a ubiquitous everyday experience, spatial control of this phenomenon is extremely difficult,” Varanasi says. He and his colleagues accomplished this for the first time by creating a surface patterned with orderly rows of hydrophobic, or water-repelling, and hydrophilic, or water-attracting, regions. These rows control where droplets form so they can be shed more easily.
Varanasi’s surfaces make water drops recoil like ping-pong balls off a table. By shedding droplets, the surface is then available for more nucleation, which leads to a corresponding increase in heat transfer.
Power and desalination plants could undergo big boosts in efficiency from Varanasi’s materials. In steam turbine plants, which produce up to half our energy, steam makes the turbines rotate. When droplets of water nucleate out of the steam, they hinder the system’s efficiency. The nucleated droplets that stick to the rotating turbine blades shed as much bigger droplets, forcing the turbine to move the droplets through the whole system, resulting in up to 40 percent of the efficiency losses in a steam power plant. To counter the effect, engineers have designed blades with grooves and units called moisture separators that are not effective. Varanasi’s materials can overcome these fundamental limitations.
Deposits of organic materials on airfoils, turbine blades, the hulls of ships, and solar panels also take a toll on efficiency. Varanasi has achieved scientists’ long-term quest to mimic the structure of the lotus leaf, revered in Asian religions for its ability to shed water and dirt. “If you can help materials shed debris as well as water as well as a lotus leaf can, you can make a big dent” in the problem, he says.
Previously, new materials with these properties have not been manufactured at high enough volume, low enough cost and sufficient robustness to withstand the harsh environments — inside airplane engines, on ships’ hulls, deep inside gas turbine plants — in which they need to work. Varanasi’s materials can handle these tasks.
“We are fundamentally changing the physics of thermal-fluid-surface interactions that have constrained engineers in the past,” he says. “If we can improve the efficiency of power plants by 5 or 10 percent, that’s a really big number.”
What’s more, Varanasi believes he has just scratched the surface of what can be accomplished with nanoengineered ceramics and metals. His next step is to explore quantum mechanical and atomistic tools to make as yet unimagined new materials. “This,” he says, “is only the beginning.”