On a roll: soft materials breakthrough

This behaviour is described in ‘Soft Matter’, and it flips the basic ideas of physics on its head and adds a new layer of understanding to how soft materials interact with surfaces.

The research

In the experiment, a tiny polyacrylamide (PAAm) sphere – a soft, gel-like material – was gently placed against a vertical surface coated with a stiffer type of silicone rubber called PDMS. Instead of falling, sticking, or sliding, the 1mm-radius sphere started to roll down slowly.

Rolling without slipping normally requires torque, and torque typically needs friction. But on a vertical wall, there’s no ‘normal’ force between the object and the surface – so there's no grip, no usual way to make the object spin. On rigid surfaces, even sticky ones, objects tend to either fall straight down or stick in place. Rolling isn’t on the menu.

So why did this ball roll?

This unexpected rolling only happened when the right combination of softness and stiffness was used:

• The sphere had a stiffness of around 170kPa
• The wall (PDMS coating) was more than 2,000kPa

Too soft, and the sphere would stick or spread like a droplet. Too stiff, and it would just fall. But at the sweet spot, the contact area between the sphere and surface became dynamic and slightly uneven – and this imbalance did something important, it generated a small but consistent internal torque which allowed the sphere to roll on its own down a vertical surface.

The front edge of the rolling sphere behaves like a closing crack, and the rear edge behaves like an opening crack. This continual cycle of sticking and peeling generated enough friction and torque to sustain the rolling motion.

Even more interestingly, the contact area changes shape and size throughout the motion. This creates an asymmetry that behaves like a small internal engine, helping the sphere roll – very slowly – without falling or slipping away entirely.

How it moves

The motion isn’t like a rigid wheel. It’s not pure rolling, but rolling with some slipping. The centre of the sphere moves at about 0.5mm per second, and the rolling motion continues steadily until the sphere reaches the bottom of the surface or until some irregularity interrupts it.

High-speed cameras and image analysis showed that the friction force produced is almost exactly equal to the gravitational force pulling the sphere down. The motion also matches energy conservation rules: the sphere's potential energy is gradually lost to friction – not stored as speed or spin.

Why it matters

This discovery matters not just because it’s unusual, but because it helps fill gaps in how we understand the behaviour of soft materials.

According to the paper, firstly it shows that softness alone can drive motion, even on surfaces where rolling should not happen.

Secondly, it introduces a useful model of how contact asymmetry and internal energy changes can mimic external forces – something that may be relevant in:

• Soft robotics, where movement through confined or vertical spaces is a challenge
• Micro-scale rovers for exploring planetary surfaces or medical devices for navigating the body
• Studies of how dust particles move and stick in early planet formation – a process thought to rely heavily on rolling contacts between soft particles
• How we can use material combinations, not just motors or magnets, to enable and control motion in miniature systems

A self-perpetuating future

The spontaneous rolling of a soft sphere on a vertical soft wall adds a new piece to the puzzle of motion and adhesion in soft systems. It may seem like a small and slow effect, but it could have larger consequences for how materials and machines are designed to move in new ways – all without needing to be pushed.

Image created using AI, June 2025.