Researchers discovered that packed rice grains behave in an unusual way under pressure. When compressed slowly, the grains remain relatively strong. But when squeezed quickly, they actually become weaker. This surprising behavior has allowed scientists to create a new material that could one day be used in soft robots that automatically adjust their stiffness and protective equipment that responds differently depending on the force of an impact.

The international research team, led by the University of Birmingham, reported its findings in the journal Matter.

Rice's Unusual Response to Pressure

Experiments showed that tightly packed rice grains respond very differently depending on how quickly a load is applied. At higher loading speeds, the material weakens significantly.

This phenomenon, known as "rate softening," is uncommon in most materials. Researchers found that it happens because friction between individual rice grains drops sharply when forces are applied rapidly. As a result, the internal networks of forces that normally help support the load become weaker.

The team used this unusual property to develop a new metamaterial, an engineered composite structure designed to exhibit behaviors not found in naturally occurring materials.

Creating a Self-Adapting Metamaterial

To build the new material, researchers combined rice-based granular units with materials such as sand, which become stronger when subjected to rapid loading. The result was a granular metamaterial capable of responding differently to slow movements and sudden impacts.

Depending on the situation, the material can bend, buckle, or stiffen in different ways, all without electronics, sensors, or active control systems.

Dr. Mingchao Liu, from the University of Birmingham, said: "Rice might be best known as a staple food globally, but it's rarely associated with advanced engineering. Our research shows that it can form the basis of a new class of functional materials.

"Rather than treating this phenomenon as curiosity, we turned it into a design principle. This approach enabled us to create a material that can bend, buckle, or stiffen differently under slow movements versus sudden impacts -- without electronics, sensors, or active control. Instead of telling a structure how to respond, we let physics decide: fast loads trigger one behavior, slow loads another."

The researchers say the work highlights how common granular materials can be transformed into engineered systems that respond intelligently through their own mechanical properties.

Potential Applications in Robotics and Safety Gear

The speed-sensitive metamaterial could open new possibilities in soft robotics. Unlike traditional metal robots, future systems built with these materials could be lighter, safer, and more adaptable.

Such robots could be especially useful for working alongside people, operating in challenging environments, and performing delicate tasks, including assisting with surgery.

The material may also have applications in protective equipment. Because it can respond differently depending on the speed of an impact, it could absorb energy or deform in a controlled way during a collision, helping reduce the risk of injury.

Importantly, these responses occur without the need for electronics, external power, or sensors, allowing the material itself to automatically adapt to changing conditions.