# Physicists Construct Frictionless “Supersolid”—in 2D

two-dimensional quantum phenomenon produced by scientists

## A newly-made, two-dimensional material defies laws of physics such as friction. The so-called “supersolid” exists as a quantum mechanical state of matter, with rigidly-structured but delocalized atoms. Friction otherwise exists between any two objects in contact.

A 50-year search for a theoretical material has come to a successful end. Austrian physicists have built a crystal-like structure of matter known as a “supersolid.” Supersolids have a rigid atomic structure like most solids, yet the atoms are magnetized and delocalized, leading to the ability for them to flow without friction. One physicist described it as similar to an ice cube sitting in liquid water, with water flowing through the cube of ice without any friction.

Anyone who’s ever gotten a rugburn can appreciate the laws of friction. In his video series Physics in Your Life, Dr. Richard Wolfson, the Benjamin F. Wissler Professor of Physics at Middlebury College, explained how friction works.

#### There’s the Rub…

Friction is a force that acts any time we have two surfaces in contact, trying to move relative to each other, whether they are moving, or in fact are stationary,” Dr. Wolfson said. “There is a force of friction acting between those surfaces, and that force requires us to apply a different force, another force, in order to keep an object moving.”

When we try to pull a heavy object across a floor, for example, two forces are being exerted on the object: the force we apply to get the object moving, and the frictional force that opposes our efforts. There are also two kinds of friction, depending on an object’s state of rest.

“There is so-called static friction, the friction that exists between two objects when they are at rest with respect to each other,” Dr. Wolfson said. “And that force can actually be bigger than the force of ‘sliding friction,’ the force of friction that exists when two objects are sliding with respect to each other.”

Why the difference? According to Dr. Wolfson, when two objects are at rest with respect to each other, there’s time for the molecules on the two surfaces to bond or lock together a bit. If we wish to move one of the objects, we have to break those bonds. If the two objects are already moving with respect to each other, the friction is less because those atoms have no time to form their bonds.

#### …And Them’s the Brakes

One of the most common, everyday, life-saving examples of friction happens at every moment on every driven road on the planet: automobile brakes.

“What the brakes do in your car is not stop your car,” Dr. Wolfson said. “If you are a good driver, that is something that you keep in the back of your mind: The brakes do not stop the car; they simply stop the wheels from turning.”

Brakes have frictional pads that grab onto the spinning metal discs known as disc brakes. This stops the wheels from turning, but it’s more accurate to say that the frictional force between the wheels themselves and the road is what actually stops the car from moving. The best way to make sense of it is to look at the original wheel, invented long ago.

“When a wheel rolls, the very bottom point of the wheel […] is actually instantaneously at rest,” Dr. Wolfson said. “It’s a little bit easier to see for something like the tread of a bulldozer, or a snowmobile—or if you want to get militaristic, a tank. If you watch—it has a belt-like device instead of a wheel that serves basically the same purpose—you’ll see that the part in contact with the ground is at rest, and it gets clunked up by the mechanisms that are driving it.”

Compared to this, a wheel is just a rounder version of a tread.

In the living world around us, it can be difficult to imagine anything achieving a state of being truly frictionless. However, this theory is becoming a reality.

Edited by Angela Shoemaker, Wondrium Daily