In a groundbreaking advancement poised to redefine the future of adaptive materials, researchers from an international consortium led by the University of Birmingham have unveiled a new class of metamaterials whose mechanical properties shift dynamically in response to loading speed. This novel discovery originates from an intriguing phenomenon observed in everyday rice grains: when compressed slowly, they sustain strength and resist deformation, but under rapid compression, they weaken significantly. This rate-dependent mechanical behavior, known as rate softening, inspired the design of artificial granular composites capable of autonomously toggling their stiffness without the need for embedded electronics or external control systems.
Granular materials like rice traditionally fall outside the scope of advanced engineering materials, yet exploiting their intrinsic micro-mechanics has paved the way for functional metamaterials that harness frictional interactions at the grain level. The research team meticulously quantified how friction between compressed rice grains diminishes sharply as loading speed increases, resulting in the weakening of internal force networks that confer structural rigidity. This counterintuitive response contrasts with most natural and synthetic solids, which generally become stronger or stiffer under faster stresses.
By ingeniously combining rice grains with other granular media such as sand—known to exhibit a rate strengthening response—the scientists fabricated a composite granular metamaterial exhibiting dual behavior. This material can buckle, bend, or stiffen selectively depending on whether it is subjected to slow or rapid mechanical loads. Crucially, this dualism arises purely from the physics of grain-to-grain interactions, obviating the need for sensors, power sources, or active feedback mechanisms traditionally required for tunable stiffness.
This discovery holds profound implications for the rapidly growing field of soft robotics, where creating machines with adaptable stiffness is paramount. Unlike conventional rigid robots built from metals and hard plastics, soft robots rely on compliant materials to achieve delicate manipulation and safe physical interactions with humans. The newly engineered metamaterial could enable soft robotic components that stiffen instantaneously during high-impact scenarios and relax during gentle motions, thereby enhancing safety and functional versatility in robotic assistants and exploratory devices alike.
Moreover, the potential extends to personal protective equipment where adaptive response to impact velocity is critical. Helmets, body armor, and other safety gear composed of this metamaterial could intelligently absorb and dissipate kinetic energy when subjected to sudden shocks, such as falls or collisions, while remaining flexible and comfortable during routine movements. This could mark a dramatic leap forward in injury prevention and comfort, as the material autonomously modulates its mechanical response in real-time.
Dr. Mingchao Liu, the lead researcher from the University of Birmingham, emphasized the significance of rediscovering a commonplace substance like rice in an engineering context. “Rice’s well-established role as a staple food belies its untapped potential as a building block for responsive materials. By embracing its inherent rate-dependent mechanical behavior, we have transformed a curiosity into a practical design principle,” he notes. This approach avoids the complexities of electronic actuation and permits the physics itself to dictate the metamaterial’s behavior, enabling inherently robust and scalable systems.
Beyond robotics and protective applications, the researchers anticipate broader impacts in fields where speed-sensitive mechanical behavior is desired. This includes bio-inspired devices mimicking living tissues that vary stiffness dynamically, non-electronic safety systems, and reconfigurable structures capable of self-adapting to changing environmental forces. Importantly, the research showcases how common granular systems, when engineered thoughtfully, can transcend their ordinary nature to exhibit complex, programmable responses.
The international collaboration, which included contributions from Nanyang Technological University in Singapore, the Hong Kong University of Science and Technology, and the University of Sydney, underscores the global interest and interdisciplinary effort invested in this cutting-edge material science breakthrough. Through meticulous experimentation and analysis detailed in their publication in the journal Matter, the team elucidated the underlying mechanisms and demonstrated practical proof-of-concept prototypes.
Technically, the metamaterial leverages the contrasting rate-dependent frictional characteristics of different granular constituents. The rice grains’ friction coefficient drops markedly at strain rates above a critical threshold, promoting network weakening, while sand grains exhibit increased friction and force chain strengthening under rapid loading. The aggregate effect produces a composite whose macroscopic stiffness inverts its dependence on loading speed, effectively tuning the modulus and energy absorption properties in situ.
The implications of eliminating electronic controls in smart materials are profound, reducing complexity, cost, and potential failure points. This paves the way for lightweight, durable, and inherently adaptive structures that autonomously respond to external mechanical stimuli. It also aligns with sustainability goals by exploiting readily available natural materials and straightforward engineering techniques rather than sophisticated electronics or rare components.
Looking forward, the research team envisions further exploration of granular mixtures incorporating other natural and synthetic particles with diverse frictional and mechanical profiles, enabling a broad spectrum of tunable responses. The fundamental principle of physics-driven adaptation demonstrated through rice-based composites may well catalyze innovation in multiple sectors, including aerospace, biomedical devices, and consumer products.
This landmark study challenges traditional paradigms in materials science by showcasing that complex, controllable behavior need not arise from active control systems but can emerge from intrinsic granular mechanics. It underscores the readily available potential lying in granular matter’s microstructural interactions and paves a new path towards intelligent, responsive materials fabricated from seemingly simple constituents.
Subject of Research:
Not applicable
Article Title:
Rate Dependence in Granular Matter with Application to Tunable Metamaterials
News Publication Date:
18-Dec-2025
Web References:
https://www.cell.com/action/showPdf?pii=S2590-2385%2825%2900605-8
References:
Mingchao Liu, Weining Mao, Yiqiu Zhao, Qin Xu, Yixiang Gan, Yifan Wang, and K. Jimmy Hsia, “Rate Dependence in Granular Matter with Application to Tunable Metamaterials,” Matter, December 2025.
Image Credits:
Not provided.
Keywords
Materials engineering, Engineering, Robotics, Materials science
Tags: active smart materials technologyadaptive metamaterials designautonomous mechanical property switchingbioinspired material engineeringdynamic stiffness modulationfrictional interactions in granular mediagranular metamaterials applicationsload-speed responsive materialsrate softening phenomenon in solidsrate-dependent mechanical behaviorrice-inspired granular compositesUniversity of Birmingham material research
