Unlike on other surfaces, a water droplet adopts a serrated three phase contact line with graphene.  This unique property pins the droplet to the surface. Image courtesy of H. E. N’Guessan and A. Leh. Click image to enlarge. 

Researchers at Lamar University, in collaboration with Bridgeport and Rice Universities, have discovered yet another unique property of graphene: Contrary to other materials, the force required for a droplet to slide on a graphene surface remains constant over time; the interaction between the droplet and the surface doesn’t change.  This, according to principal investigator Rafael Tadmor, “demonstrates a unique tribological property. The sliding mechanism of water drops on graphene is different than the sliding mechanism on other surfaces.”

The study of frictional forces of interacting surfaces, known as tribology, is useful, not only in friction and lubrication-dependent machining processes, but in any process where two surfaces come in contact.  Even lipstick and other cosmetics that smoothly slide over skin are designed with tribology in mind.  “How graphene behaves mechanically, especially its friction and wear properties, can be a key factor determining the usability and reliability of the nano-electronic devices,” says Qunyang Li, professor at Tsinghua University and expert on frictional properties of nanomaterials, who was not involved in this study.

As described in Nature Communications, the researchers used a centrifugal adhesion balance to measure the lateral adhesion forces—akin to friction between two solid surfaces—of a water droplet sliding on a graphene surface.  Before interaction with a liquid droplet, functional groups on a solid surface are oriented to maximize their interaction with air.  Once a droplet touches the surface, these functional groups reorient over time to interact with the liquid and hold the droplet to the surface.  This process is most pronounced at the three phase contact line (the edge of the drop where the liquid, solid, and air meet) resulting in higher static lateral force and added affinity of the droplet to the surface.  However, “graphene has only one functional group along the surface,” says Tadmor.  “The higher stresses at the three phase contact line can deform it, but this will not result in molecular reorientation on the solid surface, and cannot enhance or weaken the intermolecular interaction with the liquid.”  So reorientation of functional groups over time doesn’t happen.  The friction needed to slide the droplet stays the same.

Since there is only one functional group along the surface, there is no difference between two adjacent locations, nothing to hold the droplet to the surface; the slightest tilt will cause the droplet to slide off.  But with graphene there is still a retention force.  Something is holding the droplet to the surface. 

After detailed discussions with co-author Robert Vajtai from Rice University, Tadmor started to get an idea of what pins a droplet to the surface.  “Before talking to Robert, I imagined there were a few centimeters of surface, all graphene,” Tadmor explains, “but that’s not quite true.  There are micrometric patches of graphene bordered by each other.   It’s more of a tessellation of graphene rather than a single layer.”  As it turns out, these borders are what hold droplets and stop them from quickly rolling off a tilted surface.  Researchers checked the surfaces under a microscope and, indeed, saw serrated borders at the three phase contact line.

As of now, graphene is the only material known to possess this property.  Tadmor hopes to investigate other materials with unique frictional properties.  “One thing I’m very curious about is diamond,” says Tadmor.  “Diamond is different, because when you break [a natural diamond] you expect to create a high energy surface.  On the other hand, when you grow a diamond in the lab, you are not breaking anything.  Is this a uniform functional group, and if so what would be the tribological properties of diamond?”

Read the abstract in Nature Communications  here.