Rate dependence in granular matter with application to tunable metamaterials

Despite its significance, the rate dependence of granular materials has not been purposefully incorporated into applications. Here, we reveal a pronounced rate softening driven by frictional weakening in common rice grains and use experiments and simulations to study how this rate dependence can be tuned. We then fabricate a granular-based, rate-responsive metamaterial that switches mechanical function with loading speed. This exploration of granular rate effects—and their translation into a device concept—opens a new path beyond the prevailing paradigm of metamaterials built only from solids. While our proof of concept uses natural granules, manufacturable media with strongly rate-dependent friction (e.g., rubberlike particles or engineered coatings) can enable more controllable, scalable granular metamaterials, potentially yielding unique combinations of adaptivity, protection, and energy management.