Vertically rolling ball ‘challenges our basic understanding of physics’

Screenshot of small orb rolling vertically down glass surface — Soft spheres like these may be useful in space exploration, inspecting pipes or exploring cave systems. Credit: University of Waterloo

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Gravity seems like a predictable, even mundane, aspect of existence. The physics dictating one of the universe’s four fundamental forces is relatively straightforward to understand and calculate (most of the time, at least). Even so, the relationships between objects with mass and energy continues to surprise physical engineers. Take recent observations made by a team at the University of Waterloo, for example. Under a very specific set of conditions, these experts achieved something previously thought impossible under gravity’s constraints: they documented a sphere not falling or sliding, but rolling down a vertical surface.

“When we first saw it happening, we were frankly in disbelief,” mechanical and mechatronics engineer Sushanta Mitra said in a recent university profile. “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.”

GIF of orb rolling vertically down glass surface — *The soft sphere is shown rolling down the vertical surface during laboratory tests. Credit: University of Waterloo*

The surreal display of physics relied on a pea-sized soft gel sphere’s finely tuned elasticity and its relationship to a vertical surface—in this case, a glass microscope slide. If researchers crafted a polymer orb that was too soft, then the sphere inevitably either stuck to the slide or slid down it. If the object was made too rigid, then gravity caused it to simply fall straight down.

However, a perfect combination of elasticity and texture made the unique behavior possible. According to the University of Waterloo team, their winning orb possessed a consistency similar to a gummy bear with an exterior reminiscent of a mouse pad. As they explain in their study recently published in the journal Soft Matter, these attributes produce a “dynamically changing contact diameter and a unique contact asymmetry.” This enables the advancing edge to behave like a closing crack as the receding edge acts like a reopening fissure. The repeating asymmetry thereby generates the necessary grip and friction to roll down a 90-degree surface without sliding or falling.

“The key is that as it rolls, the sphere slightly changes shape at the contact point,” Mitra explained.

Just don’t expect it to get anywhere too quickly. The team’s orb rolled vertically at a rate of about 0.5 millimeters per second.

Regardless, Mitra and colleagues wrote that their observation “challenges our basic understanding of physics,” with practical effects extending far beyond a neat lab trick. Harnessing the physics of vertical rolling could one day be applied across soft robotics to create new machines capable of inspecting pipe interiors, exploring difficult-to-reach cave systems, and future devices destined for the moon or Mars.

“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.”