Credit: University of Pennsylvania
A group out of University of Pennsylvania’s Department of Mechanical Engineering and Applied Mechanics thinks it knows why, at nanoscales, the friction encountered by, say, an atomic force microscope, increases as the number of layers decrease: The AFM tip pushes material in front into sort of a wrinkle or wave in front, and stretches it in the back. This pucker in the material creates a force that pushes back on the AFM tip.
The effect is sort of like what would happen if you quickly tried to push an area rug on a slick floor. In the case of nanoscale materials, the fewer the layers, the easier it is for it to bunch up in front of the AFM.
The researchers, whose work is published in Science, say that after testing atomically thin samples of four materials they think this may be a universal characteristic for any material at this scale. The materials tested were graphene, molybdenum disulfide, hexagonal-boron nitride and niobium diselenide. The researchers test a range of thickness, from several atomic layers all the down to a single layer, and then compared the friction to that found in bulk quantities of these materials.
Compared to the bulk material, the researchers found that friction progressively increased as the number of layers is decreased.
Research co-leader Robert Carpick says in a Penn release that, “We call this mechanism, which leads to higher friction on thinner sheets, the ‘puckering effect.’ Interatomic forces, like the van der Waals force, cause attraction between the atomic sheet and the nanoscale tip of the atomic force microscope which measures friction at the nanometer scale.”
Here is a brief animation of what goes on:
[flash https://ceramics.org/ceramictechtoday/wp-content/video/nanotechnolo.flv mode=1 f={image=/ceramictechtoday/wp-content/video/nanotechnolo.jpg}]
He says that thicker sheets cannot deflect as easily because they are much stiffer. The material can’t bunch up as easily so the increase in friction is less pronounced.
The researchers also found a logical to prevent the increase in friction. Continuing with my analogy, an area rug won’t bunch up if the special Home Depot double-sided tape is used. Likewise, if the atomic sheets are strongly bound to a substrate, such as mica, the problem disappears.
Carprick and his group say that this will have practical implications for the design of nanomechanical devices that use graphene, and will will shed more light on the macroscopic behavior of common lubricants, such as graphite, MoS2 and BN.
