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New technique accurately measures the water slippage in single graphene nanofluidic channels

(Nanowerk Spotlight) For most viscous liquids, the solid surface of surrounding channel walls poses friction resistance to the flowing liquid, causing a – sometimes complete – loss of velocity at the liquid/solid interface.
As a result, for regular surfaces the energy required for nanofluidic applications is enormous because of the tremendous hydraulic resistance inside nanoscale conduits (the hydraulic resistance is reversely proportional to the fourth power of the conduit dimension).
Simply put, the smaller the channel, the more pressure you need to drive the liquid. Imaging drinking a beverage with a straw: the thinner the straw is, the more difficult this becomes.
Graphitic nanoconduits, for example carbon nanotubes (CNTs) and graphene nanochannels and their membrane forms, may provide a solution. It has been discovered that superfast water transport occurs in these structures and that has inspired great interest for numerous nanofluidic applications (read more: "Fast or superfast water transport with carbon nanotubes?"; "Surfing water molecules on graphene could hold the key to fast and controllable water transport"; and "Carbon nanotubes worth their salt").
Further development and applications of these graphitic nanoconduits require accurate measurement and understanding of water flow enhancement and surface slippage in single nanoconduits. Although great effort have been made to explore water transport in single CNTs, water transport in single graphene nanochannels, has yet to be unambiguously studied.
"There are a number of challenges for water transport studies in single graphene nanochannels, which to some extent are more difficult to overcome than those in single carbon nanotubes," says Chuanhua Duan, an assistant professor in the Department of Mechanical Engineering at Boston University. "First, single graphene nanochannels with well-controlled dimensions and atomically smooth graphene surfaces need to be fabricated on the target substrate. Second, ultra-low flow rate due to nanoconfinement and the corresponding pressure difference across the single graphene nanochannel need to be precisely measured. So far, there have been only limited efforts to resolve these two challenges."
Most previous studies have focused on flowrate measurement across carbon membrane structures, which consist of numerous individual conduits. The feature size and quantities of conduits on the membrane structures are based entirely on statistical estimation. This leads to inaccuracies as the flow behavior could be strongly affected by the dimensions of individual conduits. This motivated Xie and his collaborators to understand the flow behavior inside single nanoscale conduits.
In a paper in Nature Nanotechnology ("Fast water transport in graphene nanofluidic channels"), first-authored by Quan Xie, Duanís PhD student, an international team of researchers addresses these two issues and developed a technique to accurately measure the hydraulic resistance inside graphene nanofluidic channels.
With their technique, the team experimentally quantified the water flow enhancement inside graphene-coated nanofluidic channels. This observation makes increasing the efficiencies of most nanofluidic applications possible.
Hybrid nanochannel design for water transport measurement in single graphene nanochannels
Hybrid nanochannel design for water transport measurement in single graphene nanochannels. a,b, Schematic of the hybrid nanochannel design. The graphene nanochannel to be studied is connected to a silica nanochannel with the same height and a known permeability. The capping layer of the nanochannel is not shown for clearer visualization. Water fills the graphene nanochannel from the graphene side and the capillary flow constant A is calculated based on the meniscus movement (a). Next, water fills the same graphene nanochannel from the silica side (b). The preceding silica nanochannel is filled before the meniscus arrives at the graphene nanochannel. The mass flow resistance ratio, β, between silica nanochannel and graphene nanochannel is calculated based on the meniscus movement in the graphene nanochannel and the capillary flow constant A extracted from the previous filling experiment. c,d, Microscope images of a fabricated hybrid nanochannel after anodic bonding. 30 nanochannels, which are 100 µm apart, bridge two microchannels (c). A close view of c is shown in d. e, AFM characterization of a hybrid nanochannel before anodic bonding. The channel height for both the graphene nanochannel and silica nanochannel is 49 nm. The roughness for the silica/graphene nanochannel area is 0.47/0.98 nm. (© Nature Publishing Group) (click on image to enlarge)
"In this present work, we employ our recently developed method to fabricate graphene nanochannels with three-side graphene coverage to overcome the channel fabrication challenge (Nanoscale, "Ion transport in graphene nanofluidic channels")," comments Xie. "For the measurement challenge, we use capillary flow to optically measure the flow rate in individual graphene nanochannels, while employing a hybrid nanochannel design to avoid inaccurate estimation of the driving pressure."
In their experiments, the scientists connected another nanoscale conduit to their graphene-coated conduit and measured how fast the water can flow inside this hybrid system. This method is akin to weighing something with known weights.
This way, the hydraulic resistance ratio of the two conduits is determined. Since the resistance of one conduit is known, it is possible to calculate the hydraulic resistance of the other (graphene-coated) conduit. That then allows to calculate how fast the water can flow and how slippery the graphene surface is.
This is the first time it has become possible to experimentally quantify the water slippage in single graphene nanofluidic channels.
According to the team's results, coating the surface with graphene will greatly enhance the flow inside of nanofluidic conduits and can lead to exceptional performance of nanofluidic applications in areas such as seawater desalination, energy harvesting, nanofiltration, and lab-on-a-chip technology.
"Before applying this technique to industry, we need to have a better control of the graphene qualities and properties," cautions Xie. "Also, further investigations of the flow enhancement dependence on surface charge density, roughness, stress, and defects of graphene would be very beneficial."
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk
 

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