Zanna Buckland

The mechanical response of packed rice could be leveraged for soft robotics, adaptive structures, or protective gear.

An international research team, led by the University of Birmingham, UK, finds that rice becomes weaker when compressed quickly and stronger under slow pressure. They have harnessed this effect to design a new metamaterial.

According to Dr Mingchao Liu from the university, weakening at higher speed, known as ‘rate softening’, is due to ‘the friction between the grains dropping at higher speeds, which weakens the internal force network that carries loads in the material’. This is not a common property in materials, but the team says it could enable lighter, safer and more adaptable machines than traditional metal robots.

They have created a ‘granular-based, rate-responsive metamaterial that switches mechanical function with loading speed’, as described in the paper Rate dependence in granular matter with application to tunable metamaterials, published in Matter.

Liu says, ‘What makes this interesting from an engineering perspective is that we can use this behaviour as a design principle. By combining materials with different rate responses, we created simple mechanical structures that change how they deform depending on how fast they are loaded. In other words, the structure can ‘decide’ how to respond purely through physics.’

They have combined rice-based granular units with materials such as silica sand that strengthen under fast loading. The paper reads, ‘We developed a tuneable granular metamaterial based on a bi-beam structure that integrates both rate-softening and rate-strengthening components, aiming to illustrate the functional significance of rate-dependent granular behaviour.’

Liu says this enabled them ‘to create a material that can bend, buckle, or stiffen differently under slow movements versus sudden impacts – without electronics, sensors, or active control’.

He also shares that granular materials can be noisy and sensitive, making them difficult to study, so a challenge was ensuring the rate-dependent behaviour was real and reproducible. Repeated experiments and numerical simulations confirm the behaviour is caused by changes in friction between the grains.

Liu says the research initially stemmed from a ‘general curiosity about granular materials, like sand, coffee powders, or glass beads’, and could ‘form the basis of a new class of functional materials’, but is ‘mainly a proof-of-concept’ for now.

He adds, ‘Rice itself is not necessarily the fi nal material we want to use. Instead, it provided a simple and accessible way to demonstrate the underlying physics and broader idea that granular materials can be used to design adaptive mechanical metamaterials.

‘Granular systems can be relatively low cost, scaleable and easy to manufacture, which could make them attractive for large-scale applications. [They] also have advantages because they can oft en be reused, recycled, or made from natural or bio-based particles.

‘We hope this work encourages people to think of granular materials not just as passive matter, but as building blocks for programmable mechanical materials.’

These metamaterials could be ideal for working with humans, exploring harsh environments and performing delicate tasks such as assisting with surgery.

Liu thinks the work opens several interesting directions, including ‘designing particles with tailored surface properties to achieve specific rate-dependent behaviours’ and ‘integrating granular systems with other architected materials or soft robotic structures to create more complex adaptive mechanical systems’.

Published Date: April 8, 2026