Magnetene could have useful applications as a lubricant in implantable devices or other micro-electro-mechanical systems.
A team of researchers from the University of Toronto Engineering and Rice University have reported the first measurements of the very low friction behavior of a material known as magnetene. The results pave the way for strategies to design similar low friction materials for use in a variety of fields, including tiny implantable devices.
Magnetene is a 2D material, which means that it is made up of a single layer of atoms. In this respect, it is akin to graphene, a material that has been the subject of intensive studies for its unusual properties, including ultra-low friction, since its discovery in 2004.
“Most 2D materials are formed as flat sheets,” explains doctoral student Peter Serles, who is the lead author of the new article published on November 17, 2021 in Scientists progress.
“The theory was that these graphene sheets exhibit low friction behavior because they are only very loosely bonded and slide against each other very easily. You can imagine it like rolling out a deck of playing cards: it doesn’t take a lot of effort to spread out the deck because the friction between the cards is really low.
The team, which includes Professors Tobin Filleter and Chandra Veer Singh, post-doctoral fellow Shwetank Yadav, and several current and graduate students from their lab groups, wanted to test this theory by comparing graphene to other 2D materials.
While graphene is made of carbon, magnetene is made of magnetite, a form of iron oxide, which normally exists as a 3D lattice. Collaborators from the Rice University team processed 3D magnetite using high-frequency sound waves to carefully separate a layer consisting of just a few sheets of 2D magnetene.
The engineering team at the University of Toronto then placed the magnetene sheets into an atomic force microscope. In this apparatus, a sharp tip probe is dragged over the top of the magnetene sheet to measure the friction. The process is comparable to how the stylus of a record player is dragged across the surface of a vinyl record.
“The bonds between the layers of magnetene are much stronger than they would be between a stack of graphene sheets,” says Serles. “They don’t slide over each other. What surprised us was the friction between the tip of the probe and the upper slice of magnetene: it was as weak as in graphene.
Until now, scientists have attributed the low friction of graphene and other 2D materials to the theory that sheets can slip because they are only linked by weak forces known as Van der Waals forces. But the low friction behavior of magnetene, which lacks these forces due to its structure, suggests that something else is happening.
“When you switch from a 3D material to a 2D material, a lot of unusual things start to happen due to the effects of quantum physics,” says Serles. “Depending on the angle at which you cut the slice, it can be very smooth or very rough. Atoms are not so restricted in this third dimension anymore, so they can vibrate in different ways. And the electronic structure changes too. We have found that all of these affect friction.
The team confirmed the role of these quantum phenomena by comparing their experimental results to those predicted by computer simulations. Yadav and Singh built mathematical models based on functional density theory to simulate the behavior of the probe tip sliding on 2D material. Models that incorporated quantum effects were the best predictors of experimental observations.
Serles says the practical result of the team’s findings is that they offer new information for scientists and engineers who want to intentionally design ultra-low friction materials. Such substances could be useful as lubricants in various small scale applications, including implantable devices.
For example, one could imagine a small pump that delivers a controlled amount of a given drug to a certain part of the body. Other types of microelectromechanical systems could harvest energy from a beating heart to power a sensor, or power a tiny robotic manipulator capable of sorting one type of cell from another in a Petri dish.
“When you’re dealing with such tiny moving parts, the ratio of area to mass is really high,” says Filleter, corresponding author of the new study. “It means things are much more likely to get stuck. What we have shown in this work is that it is precisely because of their small scale that these 2D materials have such low friction. These quantum effects would not apply to larger 3D materials.
Serles says these scale-dependent effects, combined with the fact that iron oxide is non-toxic and inexpensive, make magnetene very attractive for use in implantable mechanical devices. But he adds that there is still work to be done before quantum behaviors are fully understood.
“We’ve tried this with other types of iron-based 2D materials, such as hematene or chromiteen, and we’re not seeing the same quantum signatures or low friction behavior,” he says. “So we need to focus on why these quantum effects occur, which could help us be more intentional in designing new types of low friction materials.”
Reference: “Friction of magnetene, a non – van der Waals 2D material” by Peter Serles, Taib Arif, Anand B. Puthirath, Shwetank Yadav, Guorui Wang, Teng Cui, Aravind Puthirath Balan, Thakur Prasad Yadav, Prasankumar Thibeorchews, Nithya Chakingal , Gelu Costin, Chandra Veer Singh, Pulickel M. Ajayan and Tobin Filleter, November 17, 2021, Scientists progress.
DOI: 10.1126 / sciadv.abk2041