Even perfectly smooth surfaces can exhibit friction due to the interactions of electrons within them. A novel technique, however, may provide a means to reduce or entirely eliminate this “electronic friction,” potentially leading to more efficient and durable devices.
Friction, a force that opposes motion and dissipates energy, is a ubiquitous phenomenon. It allows us to walk without slipping and enables actions like lighting a match. Within mechanical systems, such as engines, friction represents a significant source of wasted energy and wear, necessitating the use of lubricants and specialized surface treatments. Yet, even with these measures, some level of friction can persist. This is largely because materials are composed of electrons, which, in turn, interact with one another.
Researchers at Tsinghua University in China, led by Zhiping Xu, have now developed a method to manage this electronic friction. Their experimental setup involved a device constructed from two distinct layers: a segment of graphite and a semiconductor material. This semiconductor was fabricated from either molybdenum and sulfur, or alternatively, from boron and nitrogen.
Significantly, all three materials employed—graphite, the molybdenum-sulfur compound, and the boron-nitrogen compound—function as effective solid lubricants. This property meant that the mechanical friction generated as these layers slid against each other was virtually negligible. This allowed the research team to concentrate their investigation on the less apparent mechanism of electronic friction, which contributes to energy loss as the device’s layers move, according to Xu. He explained that “even when surfaces slide perfectly, mechanical motion can still stir up the ‘sea’ of electrons within the materials.”
The initial phase of their research involved examining the relationship between the electronic states in the semiconductor layer and the energy lost during the sliding process. This step was crucial to confirm that they were indeed observing and measuring electronic friction. Following this, the team proceeded to test various approaches for controlling this phenomenon.
Methods for Controlling Electronic Friction
Their experiments demonstrated two key methods for completely eliminating electronic friction. One involved applying external pressure to the device. This pressure encouraged electrons between the layers to occupy shared states, thus diminishing energetically costly interactions. The second method involved applying a “bias voltage” to the device. This voltage helped regulate the extent to which the electron sea could become agitated.
Furthermore, by adjusting the voltage across different sections of the device, which in turn influenced the ease with which electrons could flow internally, the researchers were able to fine-tune the level of electronic friction. This approach offered a more nuanced control, acting more like an adjustable dial than a simple on-off switch.
Historical Context and Future Aspirations
Jacqueline Krim, from North Carolina State University, noted that early investigations into electronic friction began in 1998. Her own research group at the time utilized a superconductor—a material that conducts electricity with zero resistance at very low temperatures—to observe the disappearance of friction in this unique state. Since then, scientists have been actively seeking new methods to control electronic friction without resorting to replacing materials entirely or incorporating additional lubricants into their devices.
Krim envisions an ideal scenario akin to using a smartphone application to adjust the friction on the soles of one’s shoes, adapting them for different surfaces, such as transitioning from an icy sidewalk to a carpeted room. She stated, “The goal is this real-time remote control with no down time or material waste. To achieve this, one needs a material that responds to external fields in a way that yields the desired friction level.”
Xu acknowledges the complexity involved in managing all forms of friction within a device. A significant challenge, he points out, is the current lack of a comprehensive mathematical model that can rigorously correlate these different friction types. Nevertheless, he suggests that for situations where electronic friction is the primary contributor to energy loss or wear, the findings from his team’s research could prove highly beneficial.