But at a certain length, the researchers found that resistance to water flow comes primarily from a molecule's probability of bouncing. In other words, in very short pores, the flow of water is constrained by the chance of water molecules bouncing off the liquid surface, rather than their traveling across the nanopores. When the researchers quantified this effect, they found that only 20 to 30 percent of water vapor molecules hitting the liquid surface actually condense, with the majority bouncing away.
A no-bounce design
They also found that a molecule's bouncing probability depends on temperature: 64 percent of molecules will bounce at 90 degrees Fahrenheit, while 82 percent of molecules will bounce at 140 degrees. The group charted water's probability of bouncing in relation to temperature, producing a graph that Karnik says researchers can refer to in computing nanoscale flows in many systems.
"This probability tells us how different pore structures will perform in terms of flux," Karnik says. "How short do we have to make the pore and what flow rates will we get? This parameter directly impacts the design considerations of our filtration membrane."
Lee says that knowing the bouncing probability of water may also help control moisture levels in fuel cells.
"One of the problems with proton exchange membrane fuel cells is, after hydrogen and oxygen react, water is generated. But if you have poor control of the flow of water, you'll flood the fuel cell itself," Lee says. "That kind of fuel cell involves nanoscale membranes and structures. If you understand the correct behavior of water condensation or evaporation at the nanoscale, you can control the humidity of the fuel cell and maintain good performance all the time."
|Contact: Abby Abazorius|
Massachusetts Institute of Technology