Physicists can now take control of ‘hidden’ friction in devices

Parts of devices that are perfectly smooth can still experience friction because of the electrons within them, but a new method may enable researchers to turn it down or fully turn it off. Controlling this electronic friction could help build more efficient and long-lasting devices.

The force of friction opposes motion, dissipates energy and exists everywhere around us, enabling us to walk without slipping, for instance, and to light matches. Within machines such as engines, friction wastes energy and causes wear, so it must be fought with lubricants and surface engineering. Yet, some friction can persist regardless of those methods because objects are full of electrons, which interact with each other.

Now, Zhiping Xu at Tsinghua University in China and his colleagues have devised a way to control this “electronic friction”. They made a device composed of two layers: a piece of graphite and a semiconductor made from either molybdenum and sulphur or from boron and nitrogen.

All three materials are good solid lubricants, which means that mechanical friction from them sliding against each other was nearly zero. This enabled the researchers to focus on the more “hidden” mechanism of electronic friction wasting energy when the device’s layers moved, says Xu. “Even when surfaces slide perfectly, mechanical motion can still stir up the ‘sea’ of electrons within the materials,” he says.

The researchers first studied how electronic states in the semiconductor layer corresponded to how energy was lost during sliding to confirm that they were really looking at electronic friction. Then, they tested several ways of controlling it.

They managed to fully turn it off by adding pressure to the device, which made electrons between the layers share states instead of interacting in energetically costly ways, and by adding a “bias voltage” to the device, which controlled how stirred-up the electron sea could get.

Changing the voltage along two different parts of the device, which affected how easy it was for electrons to flow within it, allowed the researchers to weaken electronic friction – it served as a control dial rather than an on-off switch.

Jacqueline Krim at North Carolina State University says the first studies of electronic friction date to 1998, when her team used a material that conducts electricity perfectly at extremely low temperatures – a superconductor – to see it disappear in this special state. Ever since, researchers have been developing new ways to control it without having to fully switch out materials or add new lubricants into their devices, she says.

Krim says the ideal situation would be analogous to using a smartphone app to adjust the friction of the soles of your shoes as you walk, for example, from an icy sidewalk into a carpeted room. “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,” she says.

Xu says managing all types of friction present in a device is difficult, in part, because researchers have not yet developed a mathematical model that would rigorously relate all of them to each other. However, in cases where electronic friction is the dominant cause of energy waste or wear, his team’s findings could already be promising, he says.