By Liz Sheeley
Since scientists were first able to manufacture graphene, a flexible, single-layer sheet of carbon atoms, its unique properties have made it a highly sought-after and attractive material. Graphene is not only strong, it also conducts heat and electricity extremely well, and some research shows that it is almost frictionless. But researchers are not reporting consistent data on just how slippery graphene is, especially on the nanoscale level. In a recent paper in Nature Nanotechnology, Assistant Professor Chuanhua Duan (ME, MSE) and his team, including graduate student Quan Xie, have quantified water slippages in single graphene nanochannels for the first time and detailed why those data are inconsistent.
Earlier this year, Duan was awarded a National Science Foundation (NSF) Faculty Early Career Development (CAREER) grant to study the underlying mechanisms that lead to fast mass transport in graphitic nanoconduits like graphene nanochannels and carbon nanotubes. This paper is one of the few projects that are currently supported by the NSF CAREER award. “With new understanding learned from this paper, scientists can manufacture better graphitic nanoconduits and their membrane forms for revolutionary energy-efficient technologies in water desalination or purification, chemical separation and energy harvesting,” said Duan. “Better graphitic nanoconduits will also have major implications for lab-on-a-chip, which are a cheap, efficient and accurate mini-platforms for biochemical analysis and disease diagnostics.”
When water flows over a surface, how fast it can flow depends on the properties of that surface. Take for example water flowing through a channel; to the naked eye it appears as though it simply glides through it. But on a molecular level, water molecules are reacting with the molecules of the channel, and that reaction slows down the flow of the water. Theoretically, graphene should not react with water, which would make it an ideal material for water transport—the easier water can flow through a channel, the less power it takes to push it through that channel.
But when researchers across many labs measured how well membranes made of graphitic nanoconduits were able to transport water (or how much better they are than, say, membranes made of glass nanocoduits), they were getting inconsistent results (with differences up to several orders of magnitude) without being able to pinpoint why. A factor known as slip length is the benchmark scientists use to determine how well a liquid can flow through a channel—this is the inconsistently reported factor.
In the paper, Duan and his team develop a new method that can accurately measure water flow rates and the corresponding pressure in individual graphene nanochannels. “Our method allows us to unambiguously extract the slip lengths of graphene nanochannels for the first time,” said Duan. They show how surface charge is the biggest factor when determining how well a particular graphene channel transports water. The second factor affecting flow is the material that is directly underneath the graphene. When a solid has a surface charge, its molecules can react with the molecules within liquid, water in this case. But theoretically graphene should not have a surface charge and the water molecules should not react with it, but with this research, Duan showed that the reason behind the inconsistent slip length values is the differing amounts of surface charge that these graphene channels developed.