Redefining physics to roll a ball vertically

Researchers from the University of Waterloo have achieved a feat previously thought to be impossible - getting a sphere to roll down a totally vertical surface without applying any external force.

The spontaneous rolling motion, captured by high-speed cameras, was an unexpected observation after months of trial, error, and theoretical calculations by two Waterloo research teams.

"When we first saw it happening, we were frankly in disbelief," said Dr.  Sushanta Mitra , a professor of mechanical and mechatronics engineering and executive director of the  Waterloo Institute for Nanotechnology.

"We double-checked everything because it seemed to defy common sense. There was excitement in the lab when we confirmed it wasn’t a fluke and that this was real vertical rolling."

The discovery hinged on finding just the right balance of elasticity, or softness, between a pea-sized sphere and a vertical slide about the size of a cellphone screen that was used in experiments.

If the spheres were too soft, they stuck to the surface or slid down it. If they were too rigid, the spheres fell straight down through the force of gravity.

But when researchers hit on just the right combination of elasticity - with a sphere the approximate consistency of a gummy bear and a surface that would feel like a spongy mouse pad - the sphere slowly rolled down at a rate of about one millimetre every two seconds.

"The key is that as it rolls, the sphere slightly changes shape at the contact point," Mitra said. "The front edge acts as a closing zipper, while the back edge acts like opening it. This asymmetry creates just enough torque, or grip, to maintain rolling without either sticking or completely falling off."

The academic journal Soft Matter, which published a study on the work,  Spontaneous rolling of a soft sphere on a vertical soft substrate , said in a summary that the discovery "challenges our basic understanding of physics."

In practical terms, Mitra said, the finding could be used in the development of soft robots to inspect pipes, explore cave systems, or assist in the manufacturing of better equipment for missions to Mars or other space exploration endeavors.

"This opens up a whole new way of thinking about movement on vertical surfaces," said Mitra. "Currently, robots and vehicles are limited to horizontal or slightly inclined surfaces. This discovery could change that."

The research teams also included Dr.  Boxin Zhao , a professor of chemical engineering at Waterloo, and post-doctoral fellows Dr.  Surjyasish Mitra  and Dr.  A-Reum Kim.