There is virtually no change in the way water interacts with materials including copper, gold, and silicon after they are coated with a single layer ofgraphene。虽然石墨烯是令人难以置信的，但这些结果仍然令人惊讶，因为它是不可渗透的。利用这种能力石墨烯可用于保护材料免受氧化的，而不会影响它们与水相互作用的方式。
石墨烯.is the thinnest material known to science. The nanomaterial is so thin, in fact, water often doesn’t even know it’s there.
Engineering researchers at Rensselaer Polytechnic Institute and Rice University coated pieces of gold, copper, and silicon with a single layer of graphene, and then placed a drop of water on the coated surfaces. Surprisingly, the layer of graphene proved to have virtually no impact on the manner in which water spreads on the surfaces.
Results of the study were published Sunday in the journal Nature Materials. The findings could help inform a new generation of graphene-based flexible electronic devices. Additionally, the research suggests a new type of heat pipe that uses graphene-coated copper to cool computer chips.
该发现源于Rensselaer教授Nikhil Koratkar和Rice教授Pulickel Ajayan领导的跨大学合作。
“We coated several different surfaces with graphene, and then put a drop of water on them to see what would happen. What we saw was a big surprise—nothing changed. The graphene was completely transparent to the water,” said Koratkar, a faculty member in the Department of Mechanical, Aerospace, and Nuclear Engineering and the Department of Materials Science and Engineering at Rensselaer. “The single layer of graphene was so thin that it did not significantly disrupt the non-bonding van der Waals forces that control the interaction of water with the solid surface. It’s an exciting discovery, and is another example of the unique and extraordinary characteristics of graphene.”
The spreading of water on a solid surface is called wetting. Calculating wettability involves placing a drop of water on a surface, and then measuring the angle at which the droplet meets the surface. The droplet will ball up and have a high contact angle on a hydrophobic surface. Inversely, the droplet will spread out and have a low contact angle on a hydrophilic surface.
The contact angle of gold is about 77 degrees. Koratkar and Ajayan found that after coating a gold surface with a single layer of graphene, the contact angle became about 78 degrees. Similarly, the contact angle of silicon rose from roughly 32 degrees to roughly 33 degrees, and copper increased from around 85 degrees to around 86 degrees, after adding a layer of graphene.
As the researchers increased the number of layers of graphene, however, it became less transparent to the water and the contact angles jumped significantly. After adding six layers of graphene, the water no longer saw the gold, copper, or silicon and instead behaved as if it was sitting on graphite.
这种令人困惑的行为的原因是微妙的。水形成具有某些表面的化学或氢键，而水与其他表面的吸引力由名为van der WaaS力的非粘结相互作用决定。Koratkar表示，这些非粘合力与纳米级版的重力不同。类似于重力决定地球和太阳之间的相互作用，范德沃尔斯力决定了原子和分子之间的相互作用。
在金色，铜，硅等材料的情况下，表面和水滴之间的范德瓦尔斯力将水的吸引力决定在表面上，并规定了水的差距在固体表面上。通常，这些力具有至少几纳米的范围。由于长距离，这些力量不会被单一的存在破坏atom- 在表面和水之间的石墨烯层。换句话说，克拉特卡尔说，van der Waals力能够“通过”超薄石墨烯涂料。
If you continue to add additional layers of graphene, however, the van der Waals forces increasingly “see” the carbon coating on top of the material instead of the underlying surface material. After stacking six layers of graphene, the separation between the graphene and the surface is sufficiently large to ensure that the van der Waals forces can now no longer sense the presence of the underlying surface and instead only see the graphene coating. On surfaces where water forms hydrogen bonds with the surface, the wetting transparency effect described above does not hold because such chemical bonds cannot form through the graphene layer.
Along with conducting physical experiments, the researchers verified their findings with molecular dynamics modeling as well as classical theoretical modeling.
“我们发现van der Waals力量不会被石墨烯扰乱。Koratkar表示，这种效果是石墨烯的极端薄度的伪像 - 这仅是约0.3纳米，“Koratkar表示。“没有什么可以媲美石墨烯的薄弱。因此，石墨烯是用于润湿角度透明度的理想材料。“
A practical application of this new discovery is to coat copper surfaces used in dehumidifiers. Because of its exposure to water, copper in dehumidifier systems oxidizes, which in turn decreases its ability to transfer heat and makes the entire device less efficient. Coating the copper with graphene prevents oxidation, the researchers said, and the operation of the device is unaffected because graphene does not change the way water interacts with copper. This same concept may be applied to improve the ability of heat pipes to dissipate heat from computer chips, Koratkar said.
“It’s an interesting idea. The graphene doesn’t cause any significant change to the wettability of copper, and at the same time it passivates the copper surface and prevents it from oxidizing,” he said.
Along with Koratkar and Ajayan, co-authors of the paper are Yunfeng Shi, assistant professor in the Department of Materials Science and Engineering at Rensselaer; Rensselaer mechanical engineering graduate students Javad Rafiee, Abhay Thomas, and Fazel Yavari; Rensselaer physics graduate student Xi Mi; and Rice mechanical and materials engineering graduate student Hemtej Gullapalli.