Friction is a part of everyday life. Most of us do not think about friction at all, except perhaps when we are trying to push something heavy along the floor, cursing the friction that it is making it more difficult.
Friction has its useful aspects, such as when you put your foot on the brake to stop the car. At that point, you are actually extremely grateful for friction, since it allows the car to slow down and come to a stop. However, friction is extremely costly for many industries and companies around the world. Energy is lost every moment of every day, costing companies millions of dollars per year. Statistics say that friction uses up to approximately 23% of the world’s energy consumption.
So, while friction may not be at the top of your list to think about on a daily basis, industries, researchers, and scientists feel very differently. They continue to spend a great deal of time and energy on the search for better ways to reduce friction, thereby increasing the length of usable life in the various machines and systems necessary to operate in their respective industry.
Lately, researchers look at friction at the nanoscale level, working to determine exactly how friction works and what are the best ways to reduce, or even eliminate, friction and the damage it causes.
In order to picture friction, imagine looking at what appears to the naked eye to be a smooth surface. Looking at the same surface under a powerful microscope shows you that the surface is really not smooth but is actually very bumpy. Scientists take the concept of the bumpy surface into account in their calculations when determining friction.
Now imagine those bumps are atoms. The friction occurs from the contact between the atoms. The Prandtl-Tomlinson model for friction describes friction as the force required to pull a tip across a bumpy atomic surface. As the tip is pulled across the surface, it can stick at one point and then suddenly slip.
One major problem holding back researchers in their search for a solution in friction control is the fact that describing friction using a model is very difficult. The Prandtl-Thomlinson model, proposed in 1929, remains one of the most popular models used today as it is quite general in scope. However, the model fails when looking at more detailed situations. David Andersson and Astrid S. de Wijn have proposed an adjustment to the Prandtl-Thomlinson model in the article “Understanding the Friction of Atomically Thin Layered Materials”.
Layered materials like graphene sheets have been used as an additive to lubricants in order to reduce friction for over two hundred years. Graphene, which is a layer of carbon, is only one atom thick. It is an extremely slippery material, which why it has been added to lubricants over the years. However, researchers only started studying graphene and other two-dimensional materials in detail in the last ten years. Andersson and Wijn’s model looks at the effect of friction on these thin sheets of graphene.
One surprising result, not predicted by the Prandtl-Tomlinson model, is that friction is at its highest when dealing with single-layer graphene sheets. In turn, as the number of layers of graphene increases, the level of friction decreases. This surprised Andersson and Wijn, and they decided to continue their research to see if it would be possible to develop a model that would explain this phenomenon.
Over the years, a variety of reasons have been proposed regarding this unexpected behavior. Those reasons have been investigated experimentally and with simulations. However, nothing conclusive came about as different experiments and simulations by different groups of researches produced different findings and reasons for the strange behavior. Suggestions explaining the experimental results included an out-of-plane bending of the sheet such as puckering or wrinkling. Other suggestions to explain the results brought up the idea of delamination.
Andersson and Wijn continued their research by reading through past research papers and noting the different contradictory findings. They realized that the contradictions could be explained by adding a variable to the measurement process. They realized that the Prandtl-Tomlinson model required the addition of another variable that describes the deformation of the layered materials, since the original model only took into account the force required to move a tip across a surface. This new model resolved many of the contradictory findings written about in research papers and can hopefully serve as the basis for understanding friction on thin sheets of graphene. Andersson and Wijn hope that this model will open up new possibilities for other researchers in solving the problem of friction and wear and how to best prevent it.
More information: David Andersson et al. Understanding the friction of atomically thin layered materials, Nature Communications (2020). DOI: 10.1038/s41467-019-14239-2